IMT Institutional Repository: No conditions. Results ordered -Date Deposited. 2020-02-25T06:52:39ZEPrintshttp://eprints.imtlucca.it/images/logowhite.pnghttp://eprints.imtlucca.it/2018-03-28T13:20:52Z2018-03-28T13:20:52Zhttp://eprints.imtlucca.it/id/eprint/4065This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/40652018-03-28T13:20:52ZPartitioned coupling of advection–diffusion–reaction systems and Brinkman flowsWe present a partitioned algorithm aimed at extending the capabilities of existing solvers for the simulation of coupled advection–diffusion–reaction systems and incompressible, viscous flow. The space discretisation of the governing equations is based on mixed finite element methods defined on unstructured meshes, whereas the time integration hinges on an operator splitting strategy that exploits the differences in scales between the reaction, advection, and diffusion processes, considering the global system as a number of sequentially linked sets of partial differential, and algebraic equations. The flow solver presents the advantage that all unknowns in the system (here vorticity, velocity, and pressure) can be fully decoupled and thus turn the overall scheme very attractive from the computational perspective. The robustness of the proposed method is illustrated with a series of numerical tests in 2D and 3D, relevant in the modelling of bacterial bioconvection and Boussinesq systems.Pietro LenardaMarco Paggimarco.paggi@imtlucca.itR. Ruiz Baier2018-03-28T13:15:58Z2018-03-28T13:15:58Zhttp://eprints.imtlucca.it/id/eprint/4064This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/40642018-03-28T13:15:58ZA 3D finite strain model for intralayer and interlayer crack simulation coupling the phase field approach and cohesive zone modelIn this study, a new 3D finite element formulation which enables simulating the interaction between brittle crack propagation and interface delamination in heterogeneous materials is presented. The Phase Field (PF) model for brittle fracture has been coupled with the Cohesive Zone Model (CZM) within the framework of the large deformation analysis. These numerical techniques have been implemented within a 8-node locking-free solid shell element, relying on the enhanced assumed strain concept, and a 8-node interface finite element, respectively. The predictive capabilities of the proposed formulation have been assessed through the simulation of cracking in flat and curved geometries under in-plane and out-ofplane loading conditions. The results show the ability of the model to predict complex crack paths where intralayer crack propagation and delamination occur simultaneously and interact. The proposed formulation provides a powerful modeling tool for the simulation of fracture phenomena in heterogeneous materials and laminate structures, which are characterized by the existence of numerous interfaces, such as in photovoltaic laminates.Valerio Carollovalerio.carollo@imtlucca.itJosé ReinosoMarco Paggimarco.paggi@imtlucca.it2018-03-28T13:09:05Z2018-03-28T13:09:05Zhttp://eprints.imtlucca.it/id/eprint/4062This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/40622018-03-28T13:09:05ZInkjet printed 2D-crystal based strain gauges on paperWe present an investigation of inkjet printed strain gauges based on two-dimensional (2D) materials. The technology leverages water-based and biocompatible inks to fabricate strain measurement devices on flexible substrates such as paper. We demonstrate that the device performance and sensitivity are strongly dependent on the printing parameter (i.e., drop-spacing, number of printing passes, etc.). We show that values of the Gauge Factor up to 125 can be obtained, with large sensitivity (>20) even when small strains (0.3) are applied. Furthermore, we provide preliminary examples of heterostructure-based strain sensors, enabled by the inkjet printing technology.C. CasiraghiM. MacucciK. ParvezR. WorsleyY. ShinF. BronteClaudia Borriclaudia.borri@imtlucca.itMarco Paggimarco.paggi@imtlucca.itG. Fiori2018-03-28T13:00:53Z2018-03-28T13:00:53Zhttp://eprints.imtlucca.it/id/eprint/4061This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/40612018-03-28T13:00:53ZLong-term EVA degradation simulation: Climatic zones comparison and possible revision of accelerated testsThe increasing demand of photovoltaics installations, also in harsh climatic conditions, requires the accurate comprehension of module lifetime and durability. Accelerated environmental tests (damp heat, thermal cycling, and humidity freeze) are performed as pass/fail criteria to determine whether modules are suitable for sale, while do not accurate predict durability in all possible climates. Recently, we proposed a computational model to study the thermo-oxidative degradation of EVA encapsulant. This model was suitable to describe effects of temperature fluctuations on degradation, while neglecting dramatic changes of outdoor exposure in different climatic zones. To investigate the correlation between climatic zones and EVA degradation, we completed the existing degradation model by adding the UV exposure dependency. This model, for the first time, simulates EVA thermo-photo-oxidation in accelerated and environmental conditions. We compared results of simulated standard accelerated tests and outdoor exposure, observing a significant mismatch of results. The low prediction capability of standard tests pushed us to analyze modified accelerated tests, by adding an internal UV source. Modified test simulations show a better matching with outdoor long-term weathering. The modified setup will enable novel accelerated tests with predictive behavior of long-term EVA degradation and a more accurate PV module lifetime.Mariacristina Gagliardimariacristina.gagliardi@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2018-01-16T10:10:51Z2018-01-16T10:10:51Zhttp://eprints.imtlucca.it/id/eprint/3862This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/38622018-01-16T10:10:51ZMulti-parametric sensitivity analysis of the band structure for tetrachiral acoustic metamaterialsTetrachiral materials are characterized by a cellular microstructure made by a periodic pattern of stiff rings and flexible ligaments. Their mechanical behaviour can be described by a planar lattice of rigid massive bodies and elastic massless beams. The periodic cell dynamics is governed by a monoatomic structural model, conveniently reduced to the only active degrees-of-freedom. The paper presents an explicit parametric description of the band structure governing the free propagation of elastic waves. By virtue of multiparametric perturbation techniques, sensitivity analyses are performed to achieve an analytical asymptotic approximation of the dispersion functions. The parametric conditions for the existence of full band gaps in the low-frequency range are established. Furthermore, the band gap amplitude is analytically assessed in the admissible parameter range. In tetrachiral acoustic metamaterials, stop bands can be opened by the introduction of intra-ring resonators. Perturbation methods can efficiently deal with the consequent enlargement of the mechanical parameter space. Indeed high-accuracy parametric approximations are achieved for the band structure, enriched by the new optical branches related to the resonator frequencies. In particular, target stop bands in the metamaterial spectrum are analytically designed through the asymptotic solution of inverse spectral problems.Marco LepidiAndrea Bacigalupoandrea.bacigalupo@imtlucca.it2017-09-29T09:27:58Z2017-09-29T09:27:58Zhttp://eprints.imtlucca.it/id/eprint/3816This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/38162017-09-29T09:27:58Z(a cura di) Atti del Convegno GIMC-GMA 2016Marco Paggimarco.paggi@imtlucca.itAndrea Bacigalupoandrea.bacigalupo@imtlucca.itStefano BennatiClaudia Borriclaudia.borri@imtlucca.itMauro Corradomauro.corrado@polito.itAndrea GizziPaolo Sebastiano Valvo2017-09-29T09:03:06Z2017-09-29T09:03:06Zhttp://eprints.imtlucca.it/id/eprint/3817This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/38172017-09-29T09:03:06ZPercolation properties of the free volume generated by two rough surfaces in contactThe mechanism of fluid leakage trough the free volume between rough surfaces in
contact is relevant in physics and in many engineering applications. In the present study, the
normal contact problem between randomly generated fractal rough surfaces is solved using the
boundary element method. Then, an algorithm for the evaluation of the network involved in
the percolation of fluid is proposed. Numerical results are synthetically collected in diagrams
relating the free volume involved in the percolation to the dimensionless statistical parameters
of the rough surfacePaolo Cinatpaolo.cinat@imtlucca.itMarco Paggimarco.paggi@imtlucca.itClaudia Borriclaudia.borri@imtlucca.it2017-09-29T09:00:45Z2017-09-29T09:00:45Zhttp://eprints.imtlucca.it/id/eprint/3818This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/38182017-09-29T09:00:45ZSimulazione numerica e confronto con misure sperimentali del processo di raffreddamento di barriere termiche.Claudia Borriclaudia.borri@imtlucca.itAlessio FossatiAlessandro LavacchiTiberio BacciUgo Bardi2017-09-29T08:56:06Z2017-09-29T08:56:06Zhttp://eprints.imtlucca.it/id/eprint/3819This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/38192017-09-29T08:56:06ZNet energy return (EROEI) of Thermal Barrier Coatings in turbine enginesClaudia Borriclaudia.borri@imtlucca.itUgo BardiCarlo GiolliAndrea GiorgettiS. MeneghettiJ. NocivelliA. Scrivani2017-09-29T08:54:43Z2017-09-29T08:54:43Zhttp://eprints.imtlucca.it/id/eprint/3820This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/38202017-09-29T08:54:43ZCurrent Density Simulations in the Electrodeposition from Ionic Liquids: Effects of the Conductivity.The overall goal of this work is the
use of COMSOL Multiphysics in the modelling
of the current density distributions for the
electrodeposition of Aluminium coatings from
Ionic Liquids. The local current distribution is
strongly dependant on the conductivity and on
the geometry of the galvanic cell and can only be
performed by the numerical solution of the
PDE’s governing the system. The ability to
predict the local current density on an electrode
is crucial to eventually evidence portions where
the deposition may be invalidate.I. PerissiClaudia Borriclaudia.borri@imtlucca.itStefano CaporaliAlessandro Lavacchi2017-09-29T08:52:40Z2017-09-29T08:52:40Zhttp://eprints.imtlucca.it/id/eprint/3821This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/38212017-09-29T08:52:40ZFinite element analysis of thermal fatigue in thermal barrier coatings (TBC)A Finite element model of plasma sprayed
TBC’s was developed to estimate the stress
induced by thermal cycling experiments. A heat
transfer analysis was performed to evaluate the
temperature distribution on the specimen during the cooling under an impinging air jet; temperature measurements performed with an infrared pyrometer on the cooled samples show good agreement with the evaluated data. These results were then integrated in a structural
mechanic model as thermal load. The COMSOL Multiphysics® Thermal-Structural interaction model allowed to determine the dependence of the stress on the temperature fieldsClaudia Borriclaudia.borri@imtlucca.itAlessandro LavacchiAlessio FossatiIlaria PerissiUgo Bardi2017-09-28T15:30:32Z2017-09-28T15:30:32Zhttp://eprints.imtlucca.it/id/eprint/3812This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/38122017-09-28T15:30:32ZTopology simulation and contact mechanics of bifractal rough surfaces*A numerical method to generate bifractal surfaces due to a modification of the slope of the power spectral density function in the low- or high-frequency range is proposed. The method has been applied to simulate real surfaces of Ginkgo Biloba leaf scanned at two different magnifications by matching the corresponding experimental power spectral densities. Slight differences have been found in the statistical distributions of the asperity heights and curvatures for the lowest magnification that had marginal influence on the frictionless normal contact response of the surface. For highest magnification, however, the statistics of the simulated numerical surface were quite different from those of the real one, leading also to a significant difference in the normal contact results.Claudia Borriclaudia.borri@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2017-09-28T15:29:25Z2017-09-28T15:29:25Zhttp://eprints.imtlucca.it/id/eprint/3813This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/38132017-09-28T15:29:25ZA two-scale constitutive parameters identification procedure for elasto-plastic fractureConstitutive parameters identification for elasto-plastic fracture is a complex problem due to the interplay between two forms of material nonlinearity, viz. plasticity and cohesive fracture. In the present study we examine this problem in relation to Copper specimens covered by Silver used in photovoltaic modules as electrical conductors. Uniaxial tensile tests on un-notched and notched specimens are performed with a tensile stage inside a scanning electron microscope, monitoring crack growth for each imposed far-field displacement. Parameters identification is then performed by considering an elasto-plastic constitutive relation with isotropic hardening for the continuum and a polynomial cohesive zone model (CZM) with two free parameters. For a better numerical-experimental fitting, a four-parameter CZM should be used to independently control the CZM stiffness and the fracture energy. To do so effectively, a constrained optimization procedure with a two-scale objective function is outlined.Valerio Carollovalerio.carollo@imtlucca.itClaudia Borriclaudia.borri@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2017-09-26T08:00:27Z2017-09-26T08:00:27Zhttp://eprints.imtlucca.it/id/eprint/3797This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37972017-09-26T08:00:27ZPhase field modelling of brittle fracture for enhanced assumed strain shells at large deformations: formulation and finite element implementationFracture of technological thin-walled components
can notably limit the performance of their corresponding
engineering systems. With the aim of achieving reliable
fracture predictions of thin structures, this work presents a new phase field model of brittle fracture for large deformation analysis of shells relying on a mixed enhanced assumed strain (EAS) formulation. The kinematic description of the shell body is constructed according to the solid shell concept. This enables the use of fully three-dimensional constitutive models for the material. The proposed phase field formulation integrates the use of the (EAS) method to alleviate locking pathologies, especially Poisson thickness and volumetric locking. This technique is further combined with the assumed natural strain method to efficiently derive a locking-free solid shell element. On the computational side, a fully coupled monolithic framework is consistently formulated.
Specific details regarding the corresponding finite
element formulation and the main aspects associated with
its implementation in the general purpose packages FEAP
and ABAQUS are addressed. Finally, the applicability of the current strategy is demonstrated through several numerical examples involving different loading conditions, and including linear and nonlinear hyperelastic constitutive models.José ReinosoMarco Paggimarco.paggi@imtlucca.itChristian Linder2017-09-26T07:48:16Z2017-09-26T08:01:46Zhttp://eprints.imtlucca.it/id/eprint/3795This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37952017-09-26T07:48:16ZSimulated hail impacts on flexible photovoltaic laminates: testing and modellingThe problem of simulated low-velocity
hail impacts on flexible photovoltaic (PV) modules
resting on a substrate with variable stiffness is investigated.
For this type of PV module it is shown that the
prescriptions of the IEC 61215 International Standard
for quality control used for rigid (glass-covered) PV
modules should be augmented by taking into account
their real mounting condition and the stiffness of the
substrate in the simulated hail impact tests. Moreover,
electroluminescence inspection of the crack pattern
should be made in addition to electric power output
measurements.An implicit finite element simulation of
the contact problem in dynamics is also proposed, with
two different degrees of accuracy, to interpret the
experimentally observed extension of cracking.
Results pinpoint the important role of stress wave
propagation and reflection in the case of soft substrates.Mauro Corradomauro.corrado@polito.itAndrea InfusoMarco Paggimarco.paggi@imtlucca.it2017-09-18T12:42:34Z2017-09-18T12:42:34Zhttp://eprints.imtlucca.it/id/eprint/3790This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37902017-09-18T12:42:34ZA lumped mass beam model for the wave propagation in anti-tetrachiral periodic latticesThe engineered class of periodic anti-tetrachiral materials is mainly characterized by
the unusual macroscopic property of a negative Poisson’s ratio. The auxetic behavior of the material
depends on the geometric and elastic features of the microstructure. In particular, the material symmetries
of the periodic cell govern the quadratic or orthotropic symmetry of the first-order elastic
tensor (i.e. auxetic quadratic or auxetic orthotropy). Under the assumption of uniform mass density
and elastic properties, one or the other case can be realized by a square or rectangular microstructure,
respectively. A beam lattice model with lumped masses is employed to analyse the effects
of different, usually small-valued, geometric and elastic parameters of the high- and low-frequency
dispersion curves and band gaps characterizing the free wave propagation.Andrea Bacigalupoandrea.bacigalupo@imtlucca.itMarco Lepidi2017-09-18T12:26:07Z2017-09-18T12:26:07Zhttp://eprints.imtlucca.it/id/eprint/3789This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37892017-09-18T12:26:07ZPassive control of wave propagation in periodic anti-tetrachiral meta-materialsPeriodic anti-tetrachiral materials are strongly characterized by a marked auxeticity,
the unusual and fascinating mechanical property mathematically expressed by negative values
of the Poisson’s ratio. The auxetic behavior is primarily provided by pervasive rolling-up mechanisms
developed by the doubly-symmetric micro-structure of the periodic cell, composed by a
regular pattern of rigid rings connected by tangent flexible ligaments. Adopting a beam-lattice
model to describe the linear free dynamics of the elementary cell, the planar wave propagation
along the bi-dimensional material domain can be studied according to the Floquet-Bloch
theory. Parametric analyses of the dispersion curves, carried out with numerical or asymptotic
tools, typically reveal a highly-dense spectrum, with persistent absence of total band-gaps in the
low-frequency range. The paper analyses the wave propagation in the meta-material developed
by introducing rigid massive inserts, locally housed by all the rings and working as undamped
linear oscillators with assigned inertia and/or stiffness properties. The elastic coupling between
the cell microstructure and the oscillators, if properly tuned (inertial resonators), is found to
significantly modify the Floquet-Bloch spectrum of the material. The effects of the resonator
parameters (tuning frequency and mass ratio) on the low-frequency band structure of the metamaterial
are discussed, with focus on the valuable possibility to (i) open total band gaps, by
either the widening of an existing partial band gap or the avoidance of a crossing point between
adjacent dispersion curves, (ii) finely control the total band-gap amplification, in order to assess
the maximum achievable performance of the meta-material against the vibration propagationMarco LepidiAndrea Bacigalupoandrea.bacigalupo@imtlucca.it2017-09-18T12:18:33Z2017-09-18T12:18:33Zhttp://eprints.imtlucca.it/id/eprint/3788This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37882017-09-18T12:18:33ZAnalytical and computational methods for modeling mechanical filters against bloch wave propagationThe free propagation of elastic waves through periodic microstructured materials
can be studied by the analytical formulation of beam lattice models for the elementary cell, in
combination with the Floquet-Bloch theory. Within this framework, the present paper deals
with periodic tetrachiral materials characterized by a monoatomic cell. Alternative analytical
formulations can be developed by continualization-homogenization techniques in micropolar
equivalent continua, characterized by overall elastic and inertial tensors. Valid approaches
for the solution of the wave propagation problems are offered by perturbation methods, numerical
continuation techniques, and – finally – computational analyses, suited to account for
some mechanical updates or improvements that can hardly be included in synthetic formulations.
Based on these considerations, the dispersion curves achievable by different formulations
are compared and discussed. The major interest is focused on the spectral effects
determined by changes in the geometry, inertia, elasticity of the microstructural elements and,
finally, by variations in the cellular symmetry. Some attention is paid to the parameter combinations,
which might open band gaps in the low-frequency range, useful to filter undesired
dynamic signals for vibration shielding purposes.Francesca VadalàAndrea Bacigalupoandrea.bacigalupo@imtlucca.itMarco LepidiLuigi Gambarotta2017-09-18T10:54:34Z2017-09-18T10:54:34Zhttp://eprints.imtlucca.it/id/eprint/3787This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37872017-09-18T10:54:34ZAsymptotic approximation of the band structure for tetrachiral metamaterialsTetrachiral materials are characterized by a cellular microstructure made by a periodic pattern of stiff rings and flexible ligaments. Their mechanical behaviour can be described by a planar lattice of rigid massive bodies and elastic massless beams. Therefore, the periodic cell dynamics is governed by a monoatomic structural model, conveniently reduced to the only active degrees-of-freedom. The paper presents an explicit parametric description of the Floquet-Bloch spectrum (or band structure) governing the propagation of elastic waves through the tetrachiral material. By virtue of multiparametric perturbation techniques, an analytical asymptotic approximation is achieved for the dispersion surfaces in the Brillouin zone. Since different optimization strategies tend to fail in opening low-frequency band gaps in the material spectrum, this specific design purpose is commonly pursued by introducing interring inertial resonators. The paper demonstrates that multiparametric perturbation methods can efficiently deal with the consequent enlargement of the parameter space, necessary to describe the resulting inertial metamaterial. Indeed, paying due attention to the doubling of internal resonance conditions, an accurate parametric approximations of the enriched band structure can be achieved. From the applicative perspective, the research findings furnish suited analytical tools for the optimal design of pass and stop bands.Marco LepidiAndrea Bacigalupoandrea.bacigalupo@imtlucca.it2017-09-18T09:47:53Z2017-09-18T09:49:02Zhttp://eprints.imtlucca.it/id/eprint/3786This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37862017-09-18T09:47:53ZDamped Bloch Waves in Lattices Metamaterials with Inertial ResonatorsThe present paper is focused on the acoustic behaviour of periodic beam-lattices metamaterials containing inertial viscoelastic resonators connected with elastic slender ligaments. A simplified model is considered where the ligaments are considered as massless and the viscoelastic resonators are contained inside rigid rings located at the lattice nodes. Firstly, a Lagrangian model is formulated in order to assess the influence of the dynamic and viscoelastic properties of the resonators on the acoustic behaviour. An equivalent generalized micropolar model is obtained through a continualization of the discrete model and the constitutive tensors and the equation of motion are formulated. The propagation of harmonic waves is assumed and the Christoffel equation for both the discrete and the continuum model are obtained. It is shown that the hermitian matrix governing the Christoffel equation of the Lagrangian model is approximated by the corresponding one from the micropolar model with an error O (|k|3)Andrea Bacigalupoandrea.bacigalupo@imtlucca.itLuigi Gambarotta2017-09-18T09:41:05Z2017-09-18T09:41:05Zhttp://eprints.imtlucca.it/id/eprint/3785This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37852017-09-18T09:41:05ZOn the Statics of the Dome of the Basilica of S. Maria Assunta in Carignano, GenoaThe paper deals with the dome of the Basilica of S. Maria Assunta in Carignano in Genoa, designed by Galeazzo Alessi and built in the sixteenth century, for which meridian cracking, rather common in masonry domes, requires the assessment of the dome. In order to set a general procedure for the assessment this structures, limit analysis approaches are here discussed and compared. On the basis of classic limit analysis, local (dome only) and global (dome-drum system) collapse mechanisms are considered considering the different behaviour of several structural elements (lantern, shells of the dome, drum, colonnade). A static (safe theorem) and a kinematic approach are applied to the structure by means of equilibrium limit conditions and kinematically admissible collapse mechanisms. Comparisons between the obtained results are carried out so as to: (i) discuss a general approach to the assessment of dome-drum systems based on both numerical tools and standard limit analyses approaches; (ii) provide a first glance in the assessment of the dome.Andrea Bacigalupoandrea.bacigalupo@imtlucca.itAntonio BrencichLuigi Gambarotta2017-09-18T08:51:48Z2017-09-18T08:51:48Zhttp://eprints.imtlucca.it/id/eprint/3784This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37842017-09-18T08:51:48ZSimplified modelling of chiral lattice materials with local resonatorsA simplified model of periodic chiral beam-lattices containing local resonators has been formulated to obtain a better understanding of the influence of the chirality and of the dynamic characteristics of the local resonators on the acoustic behaviour. The beam-lattice models are made up of a periodic array of rigid heavy rings, each one connected to the others through elastic slender massless ligaments and containing an internal resonator made of a rigid disk in a soft elastic annulus. The band structure and the occurrence of low frequency band-gaps are analysed through a discrete Lagrangian model. For both the hexa- and the tetrachiral lattice, two acoustic modes and four optical modes are identified and the influence of the dynamic characteristics of the resonator on those branches is analysed together with some properties of the band structure. By approximating the ring displacements of the discrete Lagrangian model as a continuum field and through an application of the generalized macro-homogeneity condition, a generalized micropolar equivalent continuum has been derived, together with the overall equation of motion and the constitutive equation given in closed form. The validity limits of the dispersion functions provided by the micropolar model are assessed by a comparison with those obtained by the discrete model.Andrea Bacigalupoandrea.bacigalupo@imtlucca.itLuigi Gambarotta2017-08-04T10:05:21Z2017-08-04T10:05:21Zhttp://eprints.imtlucca.it/id/eprint/3743This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37432017-08-04T10:05:21ZMulti-field asymptotic homogenization of thermo-piezoelectric materials with periodic microstructureAbstract This study proposes a multi-field asymptotic homogenization for the analysis of thermo-piezoelectric materials with periodic microstructures. The effect of the microstructural heterogeneity is taken into account by means of periodic perturbation functions, which derive from the solution of nonhomogeneous recursive cell problems defined over the unit periodic cell. A strong coupling is present between the microdisplacement field and the microelectric potential field, since the mechanical and the electric problems are fully coupled in the asymptotically expanded microscale field equations. The microdisplacement, the electric potential, and the relative temperature fields have been related to the macroscopic quantities and to their gradients in the derived down-scaling relations. Average field equations of infinite order have been obtained and the closed form of the overall constitutive tensors has been determined for the equivalent first-order homogenized continuum. A formal solution of such equations has been derived by means of an asymptotic expansion of the macrofields. The accuracy of the proposed formulation is assessed in relation to illustrative examples of a bi-material periodic microstructure subjected to harmonic body forces, free charge densities, and heat sources, whose periodicity is much greater than the characteristic microstructural size. The good agreement obtained between the solution of the homogenized model and the finite element solution of the original heterogeneous material problem confirms the validity of the proposed formulation.Francesca FantoniAndrea Bacigalupoandrea.bacigalupo@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2017-08-04T10:04:05Z2017-08-04T10:04:05Zhttp://eprints.imtlucca.it/id/eprint/3742This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37422017-08-04T10:04:05ZDispersive wave propagation in two-dimensional rigid periodic blocky materials with elastic interfacesAbstract Dispersive waves in two-dimensional blocky materials with periodic microstructure made up of equal rigid units, having polygonal centro-symmetric shape with mass and gyroscopic inertia, connected with each other through homogeneous linear interfaces, have been analyzed. The acoustic behavior of the resulting discrete Lagrangian model has been obtained through a Floquet–Bloch approach. From the resulting eigenproblem derived by the Euler–Lagrange equations for harmonic wave propagation, two acoustic branches and an optical branch are obtained in the frequency spectrum. A micropolar continuum model to approximate the Lagrangian model has been derived based on a second-order Taylor expansion of the generalized macro-displacement field. The constitutive equations of the equivalent micropolar continuum have been obtained, with the peculiarity that the positive definiteness of the second-order symmetric tensor associated to the curvature vector is not guaranteed and depends both on the ratio between the local tangent and normal stiffness and on the block shape. The same results have been obtained through an extended Hamiltonian derivation of the equations of motion for the equivalent continuum that is related to the Hill-Mandel macro homogeneity condition. Moreover, it is shown that the hermitian matrix governing the eigenproblem of harmonic wave propagation in the micropolar model is exact up to the second order in the norm of the wave vector with respect to the same matrix from the discrete model. To appreciate the acoustic behavior of some relevant blocky materials and to understand the reliability and the validity limits of the micropolar continuum model, some blocky patterns have been analyzed: rhombic and hexagonal assemblages and running bond masonry. From the results obtained in the examples, the obtained micropolar model turns out to be particularly accurate to describe dispersive functions for wavelengths greater than 3-4 times the characteristic dimension of the block. Finally, in consideration that the positive definiteness of the second order elastic tensor of the micropolar model is not guaranteed, the hyperbolicity of the equation of motion has been investigated by considering the Legendre–Hadamard ellipticity conditions requiring real values for the wave velocity.Andrea Bacigalupoandrea.bacigalupo@imtlucca.itLuigi Gambarotta2017-08-04T09:59:14Z2017-08-04T09:59:14Zhttp://eprints.imtlucca.it/id/eprint/3741This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37412017-08-04T09:59:14ZMulti-parametric sensitivity analysis of the band structure for tetrachiral inertial metamaterialsTetrachiral materials are characterized by a cellular microstructure made by a periodic pattern of stiff rings and flexible ligaments. Their mechanical behaviour can be described by a planar lattice of rigid massive bodies and elastic massless beams. The periodic cell dynamics is governed by a monoatomic structural model, conveniently reduced to the only active degrees-of-freedom. The paper presents an explicit parametric description of the band structure governing the free propagation of elastic waves. By virtue of multiparametric perturbation techniques, sensitivity analyses are performed to achieve analytical asymptotic approximation of the dispersion functions. The parametric conditions for the existence of full band gaps in the low-frequency range are established. Furthermore, the band gap amplitude is analytically assessed in the admissible parameter range. In inertial tetrachiral metamaterials, stop bands can be opened by the introduction of intra-ring resonators. Perturbation methods can efficiently deal with the consequent enlargement of the mechanical parameter space. Indeed high-accuracy parametric approximations are achieved for the band structure, enriched by the new optical branches related to the resonator frequencies. In particular, target stop bands in the metamaterial spectrum are analytically designed through the asymptotic solution of inverse spectral problems.
Subjects: Materials Science (cond-mat.mtrl-sci)
Cite as: arXiv:1706.08754 [cond-mat.mtrl-sci]
(or arXiv:1706.08754v1 [cond-mat.mtrl-sci] for this version)Marco LepidiAndrea Bacigalupoandrea.bacigalupo@imtlucca.it2017-08-04T09:55:23Z2017-08-04T09:55:23Zhttp://eprints.imtlucca.it/id/eprint/3740This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/37402017-08-04T09:55:23ZIdentification of higher-order continua equivalent to a Cauchy elastic compositeA heterogeneous Cauchy elastic material may display micromechanical effects that can be modeled in a homogeneous equivalent material through the introduction of higher-order elastic continua. Asymptotic homogenization techniques provide an elegant and rigorous route to the evaluation of equivalent higher-order materials, but are often of difficult and awkward practical implementation. On the other hand, identification techniques, though relying on simplifying assumptions, are of straightforward use. A novel strategy for the identification of equivalent second-gradient Mindlin solids is proposed in an attempt to combine the accuracy of asymptotic techniques with the simplicity of identification approaches. Following the asymptotic homogenization scheme, the overall behaviour is defined via perturbation functions, which (differently from the asymptotic scheme) are evaluated on a finite domain obtained as the periodic repetition of cells and subject to quadratic displacement boundary conditions. As a consequence, the periodicity of the perturbation function is satisfied only in an approximate sense, nevertheless results from the proposed identification algorithm are shown to be reasonably accurate.Andrea Bacigalupoandrea.bacigalupo@imtlucca.itMarco Paggimarco.paggi@imtlucca.itF. Dal CorsoD. Bigoni2017-03-01T11:41:43Z2017-03-01T11:41:43Zhttp://eprints.imtlucca.it/id/eprint/3656This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36562017-03-01T11:41:43ZSimulation of reaction-diffusion systems to assess EVA degradation in accelerated and environmental ageing conditions: a tool to design novel accelerated climate testsMariacristina Gagliardimariacristina.gagliardi@imtlucca.itPietro Lenardapietro.lenarda@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2017-03-01T11:37:15Z2017-03-01T11:38:28Zhttp://eprints.imtlucca.it/id/eprint/3655This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36552017-03-01T11:37:15ZA reaction-diffusion formulation to simulate EVA polymer degradation in environmental and accelerated ageing conditionsAmong polymers used as encapsulant in photovoltaic (PV) modules, poly(ethylene-co-vinyl acetate), or EVA, is the most widely used, for its low cost and acceptable performances. When exposed to weather conditions, EVA undergoes degradation that affects overall PV performances. Durability prediction of EVA, and thus of the module, is a hot topic in PV process industry. To date, the literature lacks of long-term predictive computational models to study EVA aging. To fill this gap, a computational framework, based on the finite element method, is proposed to simulate chemical reactions and diffusion processes occurring in EVA. The developed computational framework is valid in either case of environmental or accelerated aging. The proposed framework enables the identification of a correspondence between induced degradation in accelerated tests and actual exposure in weathering conditions. The developed tool is useful for the prediction of the spatio-temporal evolution of the chemical species in EVA, affecting its optical properties. The obtained predictions, related to degradation kinetics and discoloration, show a very good correlation with experimental data taken from the literature, confirming the validity of the proposed formulation and computational approach. The framework has the potential to provide quantitative comparisons of degradation resulting from any environmental condition to that gained from accelerated aging tests, also providing a guideline to design new testing protocols tailored for specific climatic zones.Mariacristina Gagliardimariacristina.gagliardi@imtlucca.itPietro Lenardapietro.lenarda@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2016-12-27T09:12:55Z2016-12-27T09:12:55Zhttp://eprints.imtlucca.it/id/eprint/3611This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36112016-12-27T09:12:55ZA computational framework for the interplay between delamination and wrinkling in functionally graded thermal barrier coatingsStiff films bonded to compliant substrates are used in a wide range of technological applications and especially in thermal barrier coatings (TBC). Thin films can be made of Functionally Graded Materials (FGMs) with a heterogeneous composition that usually range from a metallic to a ceramic phase. Aiming at investigating the phenomenon of delamination of thin FGM layers from compressed elastic substrates, a fully 3D nonlinear computational framework combining nonlinear fracture mechanics based on a novel interface element formulation for large displacements and a solid shell finite element to model the thin film is proposed. A comprehensive numerical analysis of delamination in TBCs is carried out, paying a special attention to the interplay between fracture and wrinkling instabilities. Results of the computations are also compared with benchmark 2D semi-analytical results, showing good accuracy of the proposed method that can be applied to general 3D configurations that are difficult to address by semi-analytical approaches.José ReinosoMarco Paggimarco.paggi@imtlucca.itRaimund Rolfes2016-12-27T09:12:48Z2016-12-27T09:12:48Zhttp://eprints.imtlucca.it/id/eprint/3612This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36122016-12-27T09:12:48ZA finite element framework for the interplay between delamination and buckling of rubber-like bi-material systems and stretchable electronicsIn this study, a finite element (FE) framework for the analysis of the interplay between buckling and delamination of thin layers bonded to soft substrates is proposed. The current framework incorporates the following modeling features: (i) geometrically nonlinear solid shell elements, (ii) geometrically nonlinear cohesive interface elements, and (iii) hyperelastic material constitutive response for the bodies that compose the system. A fully implicit Newton–Raphson solution strategy is adopted to deal with the complex simultaneous presence of geometrical and material nonlinearities through the derivation of the consistent FE formulation. Applications to a rubber-like bi-material system under finite bending and to patterned stiff islands resting on soft substrate for stretchable solar cells subjected to tensile loading are proposed. The results obtained are in good agreement with benchmark results available in the literature, confirming the accuracy and the capabilities of the proposed numerical method for the analysis of complex three-dimensional fracture mechanics problems under finite deformations.José ReinosoMarco Paggimarco.paggi@imtlucca.itP Areias2016-12-27T09:12:41Z2016-12-27T09:12:41Zhttp://eprints.imtlucca.it/id/eprint/3613This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36132016-12-27T09:12:41ZA geometrical multi-scale numerical method for coupled hygro-thermo-mechanical problems in photovoltaic laminatesA comprehensive computational framework based on the finite element method for the simulation of coupled hygro-thermo-mechanical problems in photovoltaic laminates is herein proposed. While the thermo-mechanical problem takes place in the three-dimensional space of the laminate, moisture diffusion occurs in a two-dimensional domain represented by the polymeric layers and by the vertical channel cracks in the solar cells. Therefore, a geometrical multi-scale solution strategy is pursued by solving the partial differential equations governing heat transfer and thermo-elasticity in the three-dimensional space, and the partial differential equation for moisture diffusion in the two dimensional domains. By exploiting a staggered scheme, the thermo-mechanical problem is solved first via a fully implicit solution scheme in space and time, with a specific treatment of the polymeric layers as zero-thickness interfaces whose constitutive response is governed by a novel thermo-visco-elastic cohesive zone model based on fractional calculus. Temperature and relative displacements along the domains where moisture diffusion takes place are then projected to the finite element model of diffusion, coupled with the thermo-mechanical problem by the temperature and crack opening dependent diffusion coefficient. The application of the proposed method to photovoltaic modules pinpoints two important physical aspects: (i) moisture diffusion in humidity freeze tests with a temperature dependent diffusivity is a much slower process than in the case of a constant diffusion coefficient; (ii) channel cracks through Silicon solar cells significantly enhance moisture diffusion and electric degradation, as confirmed by experimental tests.Pietro Lenardapietro.lenarda@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2016-12-27T09:12:31Z2016-12-27T09:12:31Zhttp://eprints.imtlucca.it/id/eprint/3614This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36142016-12-27T09:12:31ZA global/local approach for the prediction of the electric response of cracked solar cells in photovoltaic modules under the action of mechanical loadsA numerical approach based on the finite element method to assess the impact of cracks in Silicon solar cells on the electric response of photovoltaic modules is proposed. A global coarse-scale finite element model of the composite laminate is used for carrying out the structural analysis. The computed displacements at the edges of each solar cell are passed via a projection scheme as boundary conditions to a 3D local fine-scale finite element model of the cells which accounts for cohesive cracks. The evaluated crack opening displacements along the crack faces are finally used as input to an electric model characterizing the grid line/solar cell ensemble. The identification of the relation between the localized electric resistance due to cracks and the crack opening, to be used as a constitutive model of cracks, is finally discussed in reference to experimental tests performed in the laboratory.Marco Paggimarco.paggi@imtlucca.itMauro Corradomauro.corrado@polito.itIrene Berardoneirene.berardone@polito.it2016-12-27T09:12:26Z2016-12-27T09:12:26Zhttp://eprints.imtlucca.it/id/eprint/3615This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36152016-12-27T09:12:26ZA two parameter elasto-plastic formulation for hardening pressure-dependent materialsA new pressure-dependent yield function is proposed by introducing a plastic Poisson's ratio within the theoretical formulation of the plastic potential. In analogy with other classical models, an equivalent stress and an equivalent plastic strain increment are defined. Then, according to these definitions, the equivalent stress–strain curve is derived and an exponential hardening law is introduced. The advantage of the proposed formulation over alternative approaches relies in explicit closed-form expressions of the flow rules and of the plastic multiplier.Valerio Carollovalerio.carollo@imtlucca.itMarco Paggimarco.paggi@imtlucca.itAlberto Rossani2016-12-27T09:12:15Z2016-12-27T09:12:15Zhttp://eprints.imtlucca.it/id/eprint/3616This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36162016-12-27T09:12:15ZAdhesive behaviour of bonded paper layers: Mechanical testing and statistical modellingIn this study, an experimental methodology based on micromechanical testing inside a scanning electron microscope is proposed to characterise bonding of paper layers connected by wet pressing. The peeling force–displacement evolution law that characterises the delamination of micromechanical double cantilever beam specimens of paper tissue have been extracted from such peeling tests. It is observed that the force–displacement evolution curve achieves a steady-state value related to the effective adhesive energy of the interface. This behaviour is explained by examining the complex load transfer mechanism between the layers exerted by cellulose fibrils. A statistical approach is used for the computation of the effective adhesive energy. It is argued that the observed force–displacement evolution law may be satisfactory described by a stochastic model that depends on the distribution function of the fibrils strength, and on two geometrical distribution functions related to the in-plane and out-of-plane fibrils angles with respect to the undeformed interface configuration. Some applications of the proposed model are demonstrated on examples.Claudia Borriclaudia.borri@imtlucca.itMarco Paggimarco.paggi@imtlucca.itJosé ReinosoFeodor M Borodich2016-12-27T09:06:58Z2016-12-27T09:06:58Zhttp://eprints.imtlucca.it/id/eprint/3617This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36172016-12-27T09:06:58ZMechanical properties of Graphene: Molecular dynamics simulations correlated to continuum based scaling lawsIn this paper, the combined effect of domain size, lattice orientation and crack length on the mechanical properties of Graphene, namely the yield strength and strain, are studied extensively based on molecular dynamics simulations. Numerical predictions are compared with the continuum-based laws of size effect and multifractal scaling. The yield strength is found to vary with the specimen size as ≈L^{−1/3}, which is in agreement with the multifractal scaling law, and with the inverse square of the initial crack length as ≈a^{-1/2}, according to the Griffith’s energy criterion for fracture.Brahmanandam JavvajiPattabhi R. Budarapupattabhi.budarapu@imtlucca.itV. K. SutrakarD. Roy MahapatraMarco Paggimarco.paggi@imtlucca.itGoangseup ZiTimon Rabczuk2016-12-27T09:05:28Z2016-12-27T09:05:28Zhttp://eprints.imtlucca.it/id/eprint/3610This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/36102016-12-27T09:05:28ZA 3D coupled thermo-visco-elastic shear-lag formulation for the prediction of residual stresses in photovoltaic modules after laminationEvaluation of the residual stress distribution arising from lamination of photovoltaic (PV) modules is important to address thermomechanically induced failure of PV modules during service. In view of the fact that PV modules contain several Silicon cells, modelling the thermo-mechanical response of PV laminates during cooling after lamination is computationally challenging. Due to the coupling between the thermal and the mechanical fields, the stress state experienced by each silicon cell in a module varies from one position to another. Here, a novel 3D coupled thermo-visco-elastic shear-lag model is proposed to determine the stress distribution in a PV module after lamination. To enhance the prediction of stress distribution in the laminate, viscoelastic properties of the EVA encapsulant are taken into account by using an asymptotic model which is stable for small and large time steps of strain increments. The results for a simulated mini-module show that residual stresses vary significantly from point to point inside the PV module.Saheed Olalekan Ojosaheed.ojo@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2016-04-04T09:28:35Z2016-04-04T09:28:35Zhttp://eprints.imtlucca.it/id/eprint/3352This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/33522016-04-04T09:28:35ZEffect of Punch Diameters on Shear Extrusion of 6063 Aluminium AlloyThis paper reports the effect of punch diameters on the
shear extrusion of 6063 Aluminium alloy. During the shear extrusion
process, Aluminium billets of considerable diameter 30 mm and
height 25 mm were inserted in a die hole and different punches of
diameter 12 mm, 14 mm, 16 mm and 18 mm respectively were
allowed to come in contact to perform the shear operation. The setup
took place under a hydraulic press with maximum capacity of 600
kN. This work is aimed at studying the selection of the optimum
punch diameter for shear extrusion using local groundnut oil as the
lubricant. Different extrusion pressures were measured and the punch
with a diameter of 18 mm gives the highest load of 77.7 kN while the
punch with a diameter of 12 mm gives the lowest load of 51.2 kN.
An indication shows that, an increase in the punch diameters led to
an increase in the height of the extrudates and this in turn reduces the
stress inducedMutiu F. ErinoshoSaheed Olalekan Ojosaheed.ojo@imtlucca.itJoseph S. AjiboyeEsther T. Akinlabi2016-04-04T09:20:10Z2016-04-05T11:19:16Zhttp://eprints.imtlucca.it/id/eprint/3351This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/33512016-04-04T09:20:10ZExperimental Analysis for Lubricant and Punch Selection in Shear Extrusion of Aa-6063Shear extrusion is a forming process which is based on combined backward cup-forward rod extrusion. This extrusion process is attractive due its potential to achieve severe plastic deformation thus enabling texture and microstructural control of materials. Furthermore, the economic potential of shear extrusion for mass production and production of complex shapes provides for numerous applications in automotive, transportation, aero-space and other industries. However, a trending challenge in the use of this method for complex shapes is the design and selection of tools to achieve a high quality product. This paper focuses on deep study of shear extrusion of AA-6063. The process was studied experimentally using variables which affect the forming load as well as the quality of the product. It is concluded from the load-displacement and stress plots that a punch with large diameter and small punch land is desirable for easy forming of the material during shear extrusion. Analysis of the effect of lubricants on deformation load and stress shows that palm oil lubricant remains the best lubricant of the four lubricants examined since its gives the minimum load obtained during shear extrusion.Saheed Olalekan Ojosaheed.ojo@imtlucca.itMutiu F. ErinoshoJoseph S. Ajiboye2016-04-04T09:07:50Z2016-04-04T09:07:50Zhttp://eprints.imtlucca.it/id/eprint/3350This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/33502016-04-04T09:07:50ZA thermo-visco-elastic shear-lag model for the prediction of residual stresses in photovoltaic modules after laminationAbstract The distribution of residual thermo-elastic stresses in encapsulated solar cells arising from lamination is relevant for the characterization of the long term performance of photovoltaic (PV) modules during service. Accurate modelling of the structural response of the laminate in the transient regime during cooling after lamination is a challenging task from the computational point of view. In this work we propose a semi-analytic model based on the Kirchhoff plate theory and the shear-lag approach for the treatment of the polymeric encapsulant layers and accounting for their time and temperature dependency according to a rheological model derived from fractional calculus considerations. Spatially uniform and non-uniform temperature distributions are compared to accurately assess the amount of the residual compressive stresses raised in the Silicon cells after lamination. The use of more realistic non-uniform temperature distributions leads to lower residual compressive stresses in Silicon as compared to the uniform case.Saheed Olalekan Ojosaheed.ojo@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2016-03-21T10:50:38Z2016-03-21T10:50:38Zhttp://eprints.imtlucca.it/id/eprint/3258This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32582016-03-21T10:50:38ZRelevance of Mesh Dimension Optimization, Geometry Simplification and Discretization Accuracy in the Study of Mechanical Behaviour of Bare Metal StentsIn this paper, a set of analyses on the deployment of coronary stents by using a nonlinear finite element method is proposed. The author proposes a convergence test able to select the appropriate mesh dimension and a methodology to perform the simplification of structures composed of cyclically repeated units to reduce the number of degree of freedom and the analysis run time. A systematic study, based on the analysis of seven meshes for each model, is performed, gradually reducing the element dimension. In addition, geometric models are simplified considering symmetries; adequate boundary conditions are applied and verified based on the results obtained from analysis of the whole model.Mariacristina Gagliardimariacristina.gagliardi@imtlucca.it2016-03-21T09:02:41Z2016-03-21T09:19:23Zhttp://eprints.imtlucca.it/id/eprint/3243This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32432016-03-21T09:02:41ZA poly(ether-ester) copolymer for the preparation of nanocarriers with improved degradation and drug delivery kineticsAbstract This paper reports the synthesis and the physicochemical, functional and biological characterisations of nanocarriers made of a novel di-block biodegradable poly(ether-ester) copolymer. This material presents tunable, fast biodegradation rates, but its products are less acidic than those of other biosorbable polymers like PLGA, thus presenting a better biocompatibility profile and the possibility to carry pH-sensitive payloads. A method for the production of monodisperse and spherical nanoparticles is proposed; drug delivery kinetics and blood protein adsorption were measured to evaluate the functional properties of these nanoparticles as drug carriers. The copolymer was labelled with a fluorescent dye for internalisation tests, and rhodamine B was used as a model cargo to study transport and release inside cultured cells. Biological tests demonstrated good cytocompatibility, significant cell internalisation and the possibility to vehiculate non-cell penetrating moieties into endothelial cells. Taken together, these results support the potential use of this nanoparticulate system for systemic administration of drugs.Mariacristina Gagliardimariacristina.gagliardi@imtlucca.itAlice BerteroGiuseppe BardiAngelo Bifone2016-03-15T10:30:26Z2016-03-15T10:30:26Zhttp://eprints.imtlucca.it/id/eprint/3238This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32382016-03-15T10:30:26ZInterfacial cracks in bi-materials solids: Stroh
formalism and skew-symmetric weight functionsLorenzo Morinilorenzo.morini@imtlucca.itEnrico RadiAlexander MovchanNatalia Movchan2016-03-15T10:15:54Z2016-03-15T10:15:54Zhttp://eprints.imtlucca.it/id/eprint/3237This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32372016-03-15T10:15:54ZLoading profile effects on dynamic crack
propagation in couple stress elastic materialsAmdrea PiccolroazLorenzo Morinilorenzo.morini@imtlucca.itGennady MishurisP.A. Gourgiotis2016-03-15T10:05:20Z2016-03-15T10:05:20Zhttp://eprints.imtlucca.it/id/eprint/3236This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32362016-03-15T10:05:20ZDynamic energy release rate in couple-stress elasticityThis paper is concerned with energy release rate for dynamic steady state crack problems in elastic materials with microstructures. A Mode III semi-infinite crack subject to loading applied on the crack surfaces is considered. The micropolar behaviour of the material is described by the theory of couple-stress elasticity developed by Koiter. A general expression for the dynamic J-integral including both traslational and micro-rotational inertial contributions is derived, and the conservation of this integral on a path surrounding the crack tip is demonstrated.Lorenzo Morinilorenzo.morini@imtlucca.itAmdrea PiccolroazGennady Mishuris2016-03-15T10:00:03Z2016-03-15T10:08:29Zhttp://eprints.imtlucca.it/id/eprint/3235This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32352016-03-15T10:00:03ZAnalysis of interfacial crack propagation under asymmetric loading in anisotropic materialsThis paper considers a steady-state crack propagating along an interface between dissimilar orthotropic materials under an asymmetric load. Although most of the known results so far deal with symmetric loading, it has been shown recently that a significant asymmetry in the applied loading may lead to a pronounced effect in terms of the values of the SIFs. The aim of the paper is to extend these results from the static case to a moving crack. In particular, we show the significance of the asymmetry of the loading for computing the energy release rate.Lewis PryceLorenzo Morinilorenzo.morini@imtlucca.itGennady Mishuris2016-03-15T09:31:37Z2016-03-15T09:31:37Zhttp://eprints.imtlucca.it/id/eprint/3234This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32342016-03-15T09:31:37ZInterfacial Cracks in Piezoelectric Bimaterials: an approach based on Weight Functions and Boundary Integral EquationsThe focus of this paper is on the analysis of a semi-infinite crack lying along a perfect interface in a piezoelectric bimaterial with arbitrary loading on the crack faces. Making use of the extended Stroh formalism for piezoelectric materials combined with Riemann-Hilbert formulation, general expressions are obtained for both symmetric and skew-symmetric weight functions associate with plane crack problems at the interface between dissimilar anisotropic piezoelectric media. The effect of the coupled electrical fields is incorporated in the derived original expressions for the weight function matrices. These matrices are used together with Betti's reciprocity identity in order to obtain singular integral equations relating the extended displacement and traction fields to the loading acting on the crack faces. In order to study the variation of the piezoelectric effect, two different poling directions are considered. Examples are shown for both poling directions with a number of mechanical and electrical loadings applied to the crack faces.Lewis PryceLorenzo Morinilorenzo.morini@imtlucca.itD. AndreevaA. Zagnetko2016-03-15T08:47:07Z2016-03-15T08:47:07Zhttp://eprints.imtlucca.it/id/eprint/3231This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32312016-03-15T08:47:07ZRF current drive and plasma fluctuationsThe role played by electron density fluctuations near the plasma edge on rf current drive in tokamaks is assessed quantitatively. For this purpose, a general framework for incorporating density fluctuations in existing modelling tools has been developed. It is valid when rf power absorption takes place far from the fluctuating region of the plasma. The ray-tracing formalism is modified in order to take into account time-dependent perturbations of the density, while the Fokker–Planck solver remains unchanged. The evolution of the electron distribution function in time and space under the competing effects of collisions and quasilinear diffusion by rf waves is determined consistently with the time scale of fluctuations described as a statistical process. Using the ray-tracing code C3PO and the 3D linearized relativistic bounce-averaged Fokker–Planck solver LUKE, the effect of electron density fluctuations on the current driven by the lower hybrid (LH) and the electron cyclotron (EC) waves is estimated quantitatively. A thin fluctuating layer characterized by electron drift wave turbulence at the plasma edge is considered. The effect of fluctuations on the LH wave propagation is equivalent to a random scattering process with a broadening of the poloidal mode spectrum proportional to the level of the perturbation. However, in the multipass regime, the LH current density profile remains sensitive to the ray chaotic behaviour, which is not averaged by fluctuations. The effect of large amplitude fluctuations on the EC driven current is found to be similar to an anomalous radial transport of the fast electrons. The resulting lower current drive efficiency and broader current profile are inYves PeyssonJoan DeckerLorenzo Morinilorenzo.morini@imtlucca.itS. Coda2016-03-15T08:44:33Z2016-03-18T10:58:52Zhttp://eprints.imtlucca.it/id/eprint/3230This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32302016-03-15T08:44:33ZA versatile ray-tracing code for studying rf wave propagation in toroidal magnetized plasmasA new ray-tracing code named C 3 PO has been developed to study the propagation of arbitrary electromagnetic radio-frequency (rf) waves in magnetized toroidal plasmas. Its structure is designed for maximum flexibility regarding the choice of coordinate system and dielectric model. The versatility of this code makes it particularly suitable for integrated modeling systems. Using a coordinate system that reflects the nested structure of magnetic flux surfaces in tokamaks, fast and accurate calculations inside the plasma separatrix can be performed using analytical derivatives of a spline-Fourier interpolation of the axisymmetric toroidal MHD equilibrium. Applications to reverse field pinch magnetic configuration are also included. The effects of 3D perturbations of the axisymmetric toroidal MHD equilibrium, due to the discreteness of the magnetic coil system or plasma fluctuations in an original quasi-optical approach, are also studied. Using a Runge–Kutta–Fehlberg method for solving the set of ordinary differential equations, the ray-tracing code is extensively benchmarked against analytical models and other codes for lower hybrid and electron cyclotron waves.Y. PeyssonJoan DeckerLorenzo Morinilorenzo.morini@imtlucca.it2016-03-14T14:23:16Z2016-03-14T14:23:16Zhttp://eprints.imtlucca.it/id/eprint/3229This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32292016-03-14T14:23:16ZStroh formalism in analysis of skew-symmetric and symmetric weight functions for interfacial cracksThe focus of the article is on analysis of skew-symmetric weight matrix functions for interfacial cracks in two dimensional anisotropic solids. It is shown that the Stroh formalism proves to be an efficient approach to this challenging task. Conventionally, the weight functions, both symmetric and skew-symmetric, can be identified as a non-trivial singular solutions of the homogeneous boundary value problem for a solid with a crack. For a semi-infinite crack, the problem can be reduced to solving a matrix Wiener-Hopf functional equation. Instead, the Stroh matrix representation of displacements and tractions, combined with a Riemann-Hilbert formulation, is used to obtain an algebraic eigenvalue problem, that is solved in a closed form. The proposed general method is applied to the case of a quasi-static semi-infinite crack propagation between two dissimilar orthotropic media: explicit expressions for the weight matrix functions are evaluated and then used in the computation of complex stress intensity factor corresponding to an asymmetric load acting on the crack faces.Lorenzo Morinilorenzo.morini@imtlucca.itEnrico RadiAlexander MovchanNatalia Movchan2016-03-14T14:17:09Z2016-03-14T14:17:09Zhttp://eprints.imtlucca.it/id/eprint/3227This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32272016-03-14T14:17:09ZIntegral identities for a semi-infinite interfacial crack in anisotropic elastic bimaterialsThe focus of the article is on the analysis of a semi-infinite crack at the interface between two dissimilar anisotropic elastic materials, loaded by a general asymmetrical system of forces acting on the crack faces. Recently derived symmetric and skew-symmetric weight function matrices are introduced for both plane strain and antiplane shear cracks, and used together with the fundamental reciprocal identity (Betti formula) in order to formulate the elastic fracture problem in terms of singular integral equations relating the applied loading and the resulting crack opening. The proposed compact formulation can be used to solve many problems in linear elastic fracture mechanics (for example various classic crack problems in homogeneous and heterogeneous anisotropic media, as piezoceramics or composite materials). This formulation is also fundamental in many multifield theories, where the elastic problem is coupled with other concurrent physical phenomena.Lorenzo Morinilorenzo.morini@imtlucca.itAmdrea PiccolroazGennady MishurisEnrico Radi2016-03-11T13:18:00Z2016-03-11T13:18:00Zhttp://eprints.imtlucca.it/id/eprint/3221This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32212016-03-11T13:18:00ZOn fracture criteria for dynamic crack propagation in elastic materials with couple stressesThe focus of the article is on fracture criteria for dynamic crack propagation in elastic materials with microstructures. Steady-state propagation of a Mode III semi-infinite crack subject to loading applied on the crack surfaces is considered. The micropolar behavior of the material is described by the theory of couple-stress elasticity developed by Koiter. This constitutive model includes the characteristic lengths in bending and torsion, and thus it is able to account for the underlying microstructures of the material. Both translational and micro-rotational inertial terms are included in the balance equations, and the behavior of the solution near to the crack tip is investigated by means of an asymptotic analysis. The asymptotic fields are used to evaluate the dynamic J-integral for a couple-stress material, and the energy release rate is derived by the corresponding conservation law. The propagation stability is studied according to the energy-based Griffith criterion and the obtained results are compared to those derived by the application of the maximum total shear stress criterion.Lorenzo Morinilorenzo.morini@imtlucca.itAmdrea PiccolroazGennady MishurisEnrico Radi2016-03-11T12:45:11Z2016-03-11T12:45:11Zhttp://eprints.imtlucca.it/id/eprint/3219This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32192016-03-11T12:45:11ZWeight function approach to study a crack propagating along a bimaterial interface under asymmetric loading in anisotropic solidsThe focus of this paper is the study of the dynamic steady-state propagation of interfacial cracks in anisotropic bimaterials under general, nonsymmetric loading conditions. Symmetric and skew-symmetric weight functions, defined as singular nontrivial solutions of a homogeneous traction-free crack problem, have been recently derived for a quasistatic semiinfinite crack at the interface between two dissimilar anisotropic materials. In this paper, the expressions for the weight functions are generalized to the case of a dynamic steady-state crack between two anisotropic media. A functional matrix equation, through which it is possible to evaluate the stress intensity factors and the energy release rate at the crack tip, is obtained. A general method for calculating the asymptotic coefficients of the displacement and traction fields, without any restrictions regarding the loading applied on the crack faces, is developed. The proposed approach is applied for the computing stress intensity factors and higher-order asymptotic terms corresponding to two different example loading configurations acting on the crack faces in an orthotropic bimaterial.Lewis PryceLorenzo Morinilorenzo.morini@imtlucca.itGennady Mishuris2016-03-11T12:26:37Z2016-04-06T09:07:16Zhttp://eprints.imtlucca.it/id/eprint/3217This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32172016-03-11T12:26:37ZRemarks on the energy release rate for an antiplane moving crack in couple stress elasticityThis paper is concerned with the steady-state propagation of an antiplane semi-infinite crack in couple stress elastic materials. A distributed loading applied at the crack faces and moving with the same velocity of the crack tip is considered, and the influence of the loading profile variations and microstructural effects on the dynamic energy release rate is investigated. The behavior of both energy release rate and maximum total shear stress when the crack tip speed approaches the critical speed (either that of the shear waves or that of the localized surface waves) is studied. The limit case corresponding to vanishing characteristic scale lengths is addressed both numerically and analytically by means of a comparison with classical elasticity results.Lorenzo Morinilorenzo.morini@imtlucca.itAmdrea PiccolroazGennady Mishuris2016-03-11T12:15:25Z2016-03-11T12:15:25Zhttp://eprints.imtlucca.it/id/eprint/3215This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32152016-03-11T12:15:25ZBoundary integral formulation for interfacial cracks in thermodiffusive bimaterialsAn original boundary integral formulation is proposed for the problem of a semi-infinite crack at the interface between two dissimilar elastic materials in the presence of heat flows and mass diffusion. Symmetric and skew-symmetric weight function matrices are used together with a generalized Betti’s reciprocity theorem in order to derive a system of integral equations that relate the applied loading, the temperature and mass concentration fields, the heat and mass fluxes on the fracture surfaces and the resulting crack opening. The obtained integral identities can have many relevant applications, such as for the modelling of crack and damage processes at the interface between different components in electrochemical energy devices characterized by multi-layered structures (solid oxide fuel cells and lithium ions batteries).Lorenzo Morinilorenzo.morini@imtlucca.itAmdrea Piccolroaz2016-03-11T12:10:25Z2016-05-04T09:55:02Zhttp://eprints.imtlucca.it/id/eprint/3214This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32142016-03-11T12:10:25ZConservation integrals for two circular holes kept at different temperatures in a thermoelastic solidAbstract An explicit analytic solution for thermal stresses in an infinite thermoelastic medium with two circular cylindrical holes of different sizes kept at different constant temperatures, under steady-state heat flux is presented. The solution is obtained by using the most general representation of a biharmonic function in bipolar coordinates. The stress field is decomposed into the sum of a particular stress field induced by the steady-state temperature distribution and an auxiliary isothermal stress field required to satisfy the boundary conditions on the holes. The variations of the stress concentration factor on the surface of the holes are determined for varying geometry of the holes. The concept of the conservation integrals Jk, M and L is extended to steady state thermoelasticity and the integrals are proved to be path-independent. These integrals are calculated on closed contours encircling one or both holes. The geometries of a hole in a half-space and an eccentric annular cylinder are considered as particular cases.Enrico RadiLorenzo Morinilorenzo.morini@imtlucca.itI. Sevostianov2016-03-11T12:02:01Z2016-05-04T09:53:37Zhttp://eprints.imtlucca.it/id/eprint/3212This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/32122016-03-11T12:02:01ZMultiscale asymptotic homogenization analysis of thermo-diffusive composite materialsIn this paper an asymptotic homogenization method for the analysis of composite materials with periodic microstructure in presence of thermodiffusion is described. Appropriate down-scaling relations correlating the microscopic fields to the macroscopic displacements, temperature and chemical potential are introduced. The effects of the material inhomogeneities are described by perturbation functions derived from the solution of recursive cell problems. Exact expressions for the overall elastic and thermodiffusive constants of the equivalent first order thermodiffusive continuum are derived. The proposed approach is applied to the case of a two-dimensional bi-phase orthotropic layered material, where the effective elastic and thermodiffusive properties can be determined analytically. Considering this illustrative example and assuming periodic body forces, heat and mass sources acting on the medium, the solution performed by the first order homogenization approach is compared with the numerical results obtained by the heterogeneous model.Andrea Bacigalupoandrea.bacigalupo@imtlucca.itLorenzo Morinilorenzo.morini@imtlucca.itAmdrea Piccolroaz2015-12-11T11:32:28Z2015-12-11T11:32:28Zhttp://eprints.imtlucca.it/id/eprint/2970This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/29702015-12-11T11:32:28ZSize-Scale Effects on the Friction Coefficient: From Weak Faults at the Planetary Scale to Superlubricity at the NanoscaleIn the present paper, two size-effect laws for the friction coefficient of rough surfaces are proposed and compared. The former is based on purely dimensional analysis arguments and is related to the fractality of the contact domains. This scaling law applies from the macro to the planetary scales, where contact is almost elastic. The latter, holding at the micro and nanoscales, is based on the adhesion theory of friction and assumes that the friction resistance is governed by the strong adhesive bonds at the asperities, caused by elasto-plastic deformations. Whereas the fractal scaling law suggests a friction coefficient decreasing with the size of the nominal contact area, the opposite trend is predicted by the adhesion theory. The application of these two scaling laws to Zircalloy (Zr-4), Stainless Steel (SS304) and Nickel (Ni200) permits to determine the scale range of validity of each scaling law and to show that they may coexist. Finally, it is found that the length scale which marks the transition between the two regimes is a function of the plasticity index proposed by Mikic.Marco Paggimarco.paggi@imtlucca.itAlberto Carpinteri2015-11-02T14:37:29Z2017-03-27T14:28:03Zhttp://eprints.imtlucca.it/id/eprint/2802This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/28022015-11-02T14:37:29ZTopological characterization of antireflective and hydrophobic rough surfaces: are random process theory and fractal modeling applicable?The random process theory (RPT) has been widely applied to predict the joint probability distribution functions (PDFs) of asperity heights and curvatures of rough surfaces. A check of the predictions of RPT against the actual statistics of numerically generated random fractal surfaces and of real rough surfaces has been only partially undertaken. The present experimental and numerical study provides a deep critical comparison on this matter, providing some insight into the capabilities and limitations in applying RPT and fractal modeling to antireflective and hydrophobic rough surfaces, two important types of textured surfaces. A multi-resolution experimental campaign using a confocal profilometer with different lenses is carried out and a comprehensive software for the statistical description of rough surfaces is developed. It is found that the topology of the analyzed textured surfaces cannot be fully described according to RPT and fractal modeling. The following complexities emerge: ( i ) the presence of cut-offs or bi-fractality in the power-law power-spectral density (PSD) functions; ( ii ) a more pronounced shift of the PSD by changing resolution as compared to what was expected from fractal modeling; ( iii ) inaccuracy of the RPT in describing the joint PDFs of asperity heights and curvatures of textured surfaces; ( iv ) lack of resolution-invariance of joint PDFs of textured surfaces in case of special surface treatments, not accounted for by fractal modeling.Claudia Borriclaudia.borri@imtlucca.itMarco Paggimarco.paggi@imtlucca.it2015-06-30T13:27:47Z2015-07-10T13:12:37Zhttp://eprints.imtlucca.it/id/eprint/2725This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/27252015-06-30T13:27:47ZThe ghosts of the past, present and future: the case of the Army Headquarters in Belgrade, Serbia AbstractWhen the construction of Dobrović's Army Headquarters in Belgrade, Serbia was finally finished in 1965, at a location continuously designated for the Army, it was thought that it would serve its purpose in a secured future, the socialist one. And it was thought that it would house the leadership of the Army, which was seen as the rightful heir of the most glorious examples of military tradition from the Second World War. With his building Dobrović filled the void left by the WWII, but he also left a true mystery – how to interpret it. Long after the date of inception, in 1960, he offered two clues, the philosophical one – through the Bergson's dynamic schemes and the void as the central dynamizing element of the composition and the symbolically appropriate one – through the story of the Sutjeska canyon. In this way he allowed everyone to find a reading suitable for them. But when the system changed, followed by a decrease in size of both the State and the Army, the question of the dual reading, which functioned so perfectly, suddenly became the cause of conflicts, conflicts of a more profound nature than ever before. Even in these changed circumstances the building performed its function, until the 1999 NATO aggression, when it was, although empty, bombed several times. The history repeated itself and this location once again experienced bombardment which left disturbing ruins, voids and shattered identities, in need of renegotiation. How to interpret a building from a socialist period in a society which is both post-socialist and post-conflict? How to find peace with the ghosts of the past, present and future, which permeate both the location and the building? How to approach different narratives surrounding the physical structure destroyed by war and considered as heritage even before those events, although officially listed only after the ruination and cessation of use. Those are the main subjects of this article. Srdjan Milosevicsrdjan.milosevic@imtlucca.it2015-06-29T14:12:27Z2015-06-29T14:12:27Zhttp://eprints.imtlucca.it/id/eprint/2724This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/27242015-06-29T14:12:27ZAuxetic anti-tetrachiral materials: equivalent elastic properties and frequency band-gaps A comprehensive characterization of the novel class of anti-tetrachiral cellular solids, both considering the static and the dynamic response, is provided in the paper. The heterogeneous material is characterized by a periodic microstructure made of equi-spaced rings each interconnected by four ligaments. In the most general case, rings and ligaments are surrounded by a softer matrix and the rings can be filled by a different material. First, the first order linear elastic homogenized constitutive response is estimated resorting to two different microstructural models: a discrete model, in which the ligaments are modeled as beams and the presence of the matrix is neglected and the equivalent elastic properties are evaluated through a simplified analytical approach, and a more detailed continuous model, where the actual properties of matrix, ligaments and rings, varying in the 2D domain, are considered and the first order computational homogenization is adopted. Special attention is given to the dependence of the 2D overall Cauchy-type elastic constants on the mechanical and geometrical parameters characterizing the microstructure. The results, indeed, show the existence of large variations in the linear elastic constants and degree of anisotropy. A comparison with available experimental results confirms the validity of the analytical and numerical approaches adopted. Finally, the rigorous Floquet–Bloch approach is applied to the periodic cell of the cellular solid to evaluate the dispersion of propagation waves along the orthotropic axes in the framework of elasticity and to detect band gaps characterizing the material. A numerical approach, based on the first order computational homogenization, is also adopted and the rigorous and approximate solutions are compared. Andrea Bacigalupoandrea.bacigalupo@imtlucca.itMaria Laura De Bellis2015-03-27T11:14:59Z2015-03-27T12:54:14Zhttp://eprints.imtlucca.it/id/eprint/2651This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/26512015-03-27T11:14:59ZAero-Elastic Analysis of Stiffened Composite Wing StructureThe interaction of the Elastic, Inertia and Aerodynamic forces is a dynamic phenomena resulting in flutter. Dynamic Aero elasticity is critical for a high-speed subsonic class of AerialVehicles. In the presentwork, dynamic Aero-elastic analysis of the Unmanned Aerial Vehicles (UAVs) has been studied for a stiffened composite structure. The flutter speed and the corresponding flutter frequencies are computed using the Velocity-Damping (V -g) method. The V -g method is employed to estimate the flutter speed and flutter frequencies, for a high-speed subsonic composite wing structure, designed and developed at ADE. Also the improvement of the flutter frequencies over the existing metallic wing structure is discussed. The work can be further extended to develop an optimized composite structure with higher margins in flutter speeds.
Keywords: Aeroelasticity, Flutter, Velocity-damping methodPattabhi R. Budarapupattabhi.budarapu@imtlucca.itB. RammohanS. Vijay KumarD. Satish Babu R. Raghuatnhan2015-03-27T10:54:35Z2016-03-18T10:43:50Zhttp://eprints.imtlucca.it/id/eprint/2650This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/26502015-03-27T10:54:35ZAn adaptive multiscale method for quasi-static crack growthThis paper proposes an adaptive atomistic- continuum numerical method for quasi-static crack growth. The phantom node method is used to model the crack in the continuum region and a molecular statics model is used near the crack tip. To ensure self-consistency in the bulk, a virtual atom cluster is used to model the material of the coarse scale. The coupling between the coarse scale and fine scale is realized through ghost atoms. The ghost atom positions are interpolated from the coarse scale solution and enforced as boundary conditions on the fine scale. The fine scale region is adaptively enlarged as the crack propagates and the region behind the crack tip is adaptively coarsened. An energy criterion is used to detect the crack tip location. The triangular lattice in the fine scale region corresponds to the lattice structure of the (111) plane of an FCC crystal. The Lennard-Jones potential is used to model the atom–atom interactions. The method is implemented in two dimensions. The results are compared to pure atomistic simulations; they show excellent agreement.Pattabhi R. Budarapupattabhi.budarapu@imtlucca.itRobert GracieStéphane P.A. BordasTimon Rabczuk2015-03-27T10:51:27Z2015-03-27T12:54:14Zhttp://eprints.imtlucca.it/id/eprint/2649This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/26492015-03-27T10:51:27ZEfficient coarse graining in multiscale modeling of fracture Abstract We propose a coarse-graining technique to reduce a given atomistic model into an equivalent coarse grained continuum model. The developed technique is tailored for problems involving complex crack patterns in 2D and 3D including crack branching and coalescence. Atoms on the crack surface are separated from the atoms not on the crack surface by employing the centro symmetry parameter. A rectangular grid is superimposed on the atomistic model. Atoms on the crack surface in each cell are used to estimate the equivalent coarse-scale crack surface of that particular cell. The crack path in the coarse model is produced by joining the approximated crack paths in each cell. The developed technique serves as a sound basis to study the crack propagation in multiscale methods for fracture. Pattabhi R. Budarapupattabhi.budarapu@imtlucca.itRobert GracieShih-Wei YangXiaoying ZhuangTimon Rabczuk2015-03-27T10:40:25Z2015-03-27T12:54:13Zhttp://eprints.imtlucca.it/id/eprint/2648This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/26482015-03-27T10:40:25ZA meshless adaptive multiscale method for fracture Abstract The paper presents a multiscale method for crack propagation. The coarse region is modelled by the differential reproducing kernel particle method. Fracture in the coarse scale region is modelled with the Phantom node method. A molecular statics approach is employed in the fine scale where crack propagation is modelled naturally by breaking of bonds. The triangular lattice corresponds to the lattice structure of the (111)plane of an {FCC} crystal in the fine scale region. The Lennard–Jones potential is used to model the atom–atom interactions. The coupling between the coarse scale and fine scale is realized through ghost atoms. The ghost atom positions are interpolated from the coarse scale solution and enforced as boundary conditions on the fine scale. The fine scale region is adaptively refined and coarsened as the crack propagates. The centro symmetry parameter is used to detect the crack tip location. The method is implemented in two dimensions. The results are compared to pure atomistic simulations and show excellent agreement. Shih-Wei YangPattabhi R. Budarapupattabhi.budarapu@imtlucca.itD. Roy MahapatraStéphane P.A. BordasGoangseup ZiTimon Rabczuk2015-03-27T10:38:14Z2016-04-06T08:49:47Zhttp://eprints.imtlucca.it/id/eprint/2647This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/26472015-03-27T10:38:14ZVibration analysis of multi-walled carbon nanotubes embedded in elastic mediumWe propose a method to estimate the natural frequencies of the multi-walled carbon nanotubes (MWCNTs) embedded in an elastic medium. Each of the nested tubes is treated as an individual bar interacting with the adjacent nanotubes through the inter-tube Van der Waals forces. The effect of the elastic medium is introduced through an elastic model. The mathematical model is finally reduced to an eigen value problem and the eigen value problem is solved to arrive at the inter-tube resonances of the MWCNTs. Variation of the natural frequencies with different parameters are studied. The estimated results from the present method are compared with the literature and results are observed to be in close agreement.Pattabhi R. Budarapupattabhi.budarapu@imtlucca.itSudhirSastry YbBrahmanandam JavvajiD. Roy Mahapatra2015-03-27T10:35:00Z2015-03-27T12:54:13Zhttp://eprints.imtlucca.it/id/eprint/2646This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/26462015-03-27T10:35:00ZParametric studies on buckling of thin walled channel beams Abstract The lateral buckling analysis of cold-formed thin walled beams subjected to pure bending moments has been performed. The critical buckling loads are estimated based an optimization criteria. The estimated critical buckling stresses are compared with the published results, they show excellent agreement. The effect of the beam length, radius and thickness of the flanges and the length of the extended open flanges, on the critical buckling stresses have been studied for several combinations of the geometric parameters of the beam. Among the three beams, the critical buckling moments for the beam with the extended open flanges are found to be the maximum. However, considering the material and manufacturing costs, beams with rounded cross section are efficient in resisting the buckling loads. Y.B. SudhirSastryY. KrishnaPattabhi R. Budarapupattabhi.budarapu@imtlucca.it2015-03-27T10:32:33Z2015-03-27T12:54:13Zhttp://eprints.imtlucca.it/id/eprint/2645This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/26452015-03-27T10:32:33ZBuckling analysis of thin wall stiffened composite panels Abstract We present the pre and post buckling analysis of stiffened composite panels based on the finite element models. Individual buckling studies are conducted on the stiffened composite panel madeup of woven fabric CFC/epoxy, E-glass/epoxy and the Kevlar/epoxy composites. Straight, T shaped and I shaped stiffeners are considered to stiffen the panel. The panel is fabricated with 8 layers and the stiffeners are madeup of 16 layers, of equal thickness arranged in different orientations. The panel is subjected to a uniform axial compression load of 10 kN. The distribution of the buckling stresses and the buckling loads with different panel and stiffener combinations are estimated for three different layup sequences. The variation of the buckling stresses and the buckling loads from the numerical model are compared with the experiment. The results are in excellent agreement. Y.B. SudhirSastryPattabhi R. Budarapupattabhi.budarapu@imtlucca.itN. MadhaviY. Krishna2015-03-27T10:27:08Z2015-03-27T12:54:14Zhttp://eprints.imtlucca.it/id/eprint/2644This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/26442015-03-27T10:27:08ZStudies on ballistic impact of the composite panels Abstract The ballistic impact of the composite materials is studied using the numerical models. Individual impact studies are conducted on the composite plate made-up of woven fabric CFRP, E-glass/epoxy and the Kevlar/epoxy composites. The plate is fabricated with 8 layers of equal thickness arranged in different orientations. A spherical steel projectile is considered for the high velocity impact. The projectile is placed very close to the plate, at the center and impacted with a velocity of 100 m/s. The displacement and the stress distribution in each layer are studied for the layup sequence +45/−45/+45/−45/−45/+45/−45/+45. The variation of the kinetic energy, the increase in the internal energy of the laminate and the decrease in velocity of the projectile with time are also studied. Based on the results, the best layup sequence for the ballistic impact of each material is suggested. Y.B. Sudhir SastryPattabhi R. Budarapupattabhi.budarapu@imtlucca.itY. KrishnaS. Devaraj2015-03-27T10:18:27Z2015-03-27T12:54:13Zhttp://eprints.imtlucca.it/id/eprint/2643This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/26432015-03-27T10:18:27ZDirectionality of sound radiation from rectangular panels Abstract In this paper, the directionality of sound radiated from a rectangular panel, attached with masses/springs, set in a baffle, is studied. The attachment of masses/springs is done based on the receptance method. The receptance method is used to generate new mode shapes and natural frequencies of the coupled system, in terms of the old mode shapes and natural frequencies. The Rayleigh integral is then used to compute the sound field. The point mass/spring locations are arbitrary, but chosen with the objective of attaining a unique directionality. The excitation frequency to a large degree decides the sound field variations. However, the size of the masses and the locations of the masses/springs do influence the new mode shapes and hence the sound field. The problem is more complex when the number of masses/springs are increased and/or their values are made different. The technique of receptance method is demonstrated through a steel plate with attached point masses in the first example. In the second and third examples, the present method is applied to estimate the sound field from a composite panel with attached springs and masses, respectively. The layup sequence of the composite panel considered in the examples corresponds to the multifunctional structure battery material system, used in the micro air vehicle (MAV) (Thomas and Qidwai, 2005). The demonstrated receptance method does give a reasonable estimate of the new modes. Pattabhi R. Budarapupattabhi.budarapu@imtlucca.itT.S.S. NarayanaB. RammohanTimon Rabczuk2015-01-28T13:35:19Z2016-04-04T09:05:08Zhttp://eprints.imtlucca.it/id/eprint/2547This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/25472015-01-28T13:35:19ZModel order reduction applied to heat conduction in photovoltaic modulesModeling of physical systems may be a challenging task when it requires solving large sets of numerical equations. This is the case of photovoltaic (PV) systems which contain many PV modules, each module containing several silicon cells. The determination of the temperature field in the modules leads to large scale systems, which may be computationally expensive to solve. In this context, Model Order Reduction (MOR) techniques can be used to approximate the full system dynamics with a compact model, that is much faster to solve. Among the several available MOR approaches, in this work we consider the Discrete Empirical Interpolation Method (DEIM), which we apply with a suitably modified formulation that is specifically designed for handling the nonlinear terms that are present in the equations governing the thermal behavior of PV modules. The results show that the proposed DEIM technique is able to reduce significantly the system size, by retaining a full control on the accuracy of the solution.Saheed Olalekan Ojosaheed.ojo@imtlucca.itStefano Grivet-TalociaMarco Paggimarco.paggi@imtlucca.it2015-01-28T13:31:29Z2015-01-28T13:31:29Zhttp://eprints.imtlucca.it/id/eprint/2546This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/25462015-01-28T13:31:29ZNonlinear fracture dynamics of laminates with finite thickness adhesivesFinite thickness interfaces, such as structural adhesives, are often simplified from the modeling point of view by introducing ideal cohesive zone models that do not take into account the finite thickness properties in the evaluation of the interface stiffness and inertia. In the present work, the nonlinear dynamic response of those layered systems is numerically investigated according to the finite element method. The weak form of the dynamic equilibrium is written by including not only the contribution of cohesive interfaces related to the virtual work exerted by the cohesive tractions for the corresponding relative displacements, but also considering the work done by the dynamic forces of the finite thickness interfaces resulting from their inertia properties. A fully implicit solution scheme both in space and in time is exploited and the numerical results for the double cantilever beam test show that the role of finite thickness properties is not negligible as far as the crack growth kinetics and the dynamic strength increase factor are concerned.Mauro Corradomauro.corrado@polito.itMarco Paggimarco.paggi@imtlucca.it2015-01-28T13:30:24Z2017-03-27T14:33:31Zhttp://eprints.imtlucca.it/id/eprint/2545This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/25452015-01-28T13:30:24ZA consistent interface element formulation for geometrical and material nonlinearitiesDecohesion undergoing large displacements takes place in a wide range of applications. In these problems, interface element formulations for large displacements should be used to accurately deal with coupled material and geometrical nonlinearities. The present work proposes a consistent derivation of a new interface element for large deformation analyses. The resulting compact derivation leads to an operational formulation that enables the accommodation of any order of kinematic interpolation and constitutive behavior of the interface. The derived interface element has been implemented into the finite element codes FEAP and ABAQUS by means of user-defined routines. The interplay between geometrical and material nonlinearities is investigated by considering two different constitutive models for the interface (tension cut-off and polynomial cohesive zone models) and small or finite deformation for the continuum. Numerical examples are proposed to assess the mesh independency of the new interface element and to demonstrate the robustness of the formulation. A comparison with experimental results for peeling confirms the predictive capabilities of the formulation.Marco Paggimarco.paggi@imtlucca.itJosé Reinoso2014-10-09T09:27:38Z2014-10-09T09:32:44Zhttp://eprints.imtlucca.it/id/eprint/2302This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/23022014-10-09T09:27:38ZImage analysis of polycrystalline solar cells and modeling of intergranular and transgranular crackingAn innovative image analysis technique is proposed to process real solar cell pictures, identify grains and grain boundaries in polycrystalline Silicon, and finally generate finite element meshes. Using a modified intrinsic cohesive zone model approach to avoid mesh dependency, nonlinear finite element simulations show how grain boundaries and Silicon bulk properties influence the crack pattern. Numerical results demonstrate a prevalence of transgranular over intergranular cracking for similar interface fracture properties of grains and grain boundaries, in general agreement with the experimental observation.Andrea Infusoandrea.infuso@polito.itMauro Corradomauro.corrado@polito.itMarco Paggimarco.paggi@imtlucca.it2014-07-03T09:06:15Z2014-07-03T09:36:26Zhttp://eprints.imtlucca.it/id/eprint/2235This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/22352014-07-03T09:06:15ZMorphological analysis of the left ventricular endocardial surface and its clinical implicationsThe complex morphological structure of the left ventricular endocardial surface and its relation to the severity of arterial stenosis has not yet been thoroughly investigated due to the limitations of conventional imaging techniques. By exploiting the recent developments in Multirow-Detector Computed Tomography (MDCT) scanner technology, the complex endocardial surface morphology of the left ventricle is studied and the cardiac segments affected by coronary arterial stenosis localized via analysis of Computed Tomography (CT) image data obtained from a 320-MDCT scanner. The non-rigid endocardial surface data is analyzed using an isometry-invariant Bag-of-Words (BOW) feature-based approach. The clinical significance of the analysis in identifying, localizing and quantifying the incidence and extent of coronary artery disease is investigated. Specifically, the association between the incidence and extent of coronary artery disease and the alterations in the endocardial surface morphology is studied. The results of the proposed approach on 15 normal data sets, and 12 abnormal data sets exhibiting coronary artery disease with varying levels of severity are presented. Based on the characterization of the endocardial surface morphology using the Bag-of-Words features, a neural network-based classifier is implemented to test the effectiveness of the proposed morphological analysis approach. Experiments performed on a strict leave-one-out basis are shown to exhibit a distinct pattern in terms of classification accuracy within the cardiac segments where the incidence of coronary arterial stenosis is localized.Anirban Mukhopadhyayanirban.mukhopadhyay@imtlucca.itZhen QianSuchendra M. BhandarkarTianming LiuSarah RinehartSzilard Voros2014-07-03T08:34:33Z2014-07-03T09:36:48Zhttp://eprints.imtlucca.it/id/eprint/2233This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/22332014-07-03T08:34:33ZShape analysis of the left ventricular endocardial surface and its application in detecting coronary artery diseaseCoronary artery disease is the leading cause of morbidity and mortality worldwide. The complex morphological structure of the ventricular endocardial surface has not yet been studied properly due to the limitations of conventional imaging techniques. With the recent developments in Multi-Detector Computed Tomography (MDCT) scanner technology, we propose to study, in this paper, the complex endocardial surface morphology of the left ventricle via analysis of Computed Tomography (CT) image data obtained from a 320 Multi-Detector CT scanner. The CT image data is analyzed using a 3D shape analysis approach and the clinical significance of the analysis in detecting coronary artery disease is investigated. Global and local 3D shape descriptors are adapted for the purpose of shape analysis of the left ventricular endocardial surface. In order to study the association between the incidence of coronary artery disease and the alteration of the endocardial surface structure, we present the results of our shape analysis approach on 5 normal data sets, and 6 abnormal data sets with obstructive coronary artery disease. Based on the morphological characteristics of the endocardial surface as quantified by the shape descriptors, we implement a Linear Discrimination Analysis (LDA)-based classification algorithm to test the effectiveness of our shape analysis approach. Experiments performed on a strict leave-one-out basis are shown to achieve a classification accuracy of 81.8%.Anirban Mukhopadhyayanirban.mukhopadhyay@imtlucca.itZhen QianSuchendra M. BhandarkarTianming LiuSzilard Voros2014-03-31T08:30:37Z2014-09-17T12:30:38Zhttp://eprints.imtlucca.it/id/eprint/2187This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/21872014-03-31T08:30:37ZFatigue degradation and electric recovery in Silicon solar cells embedded in photovoltaic modulesCracking in Silicon solar cells is an important factor for the electrical power-loss of photovoltaic modules. Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios. Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual thermo-elastic stresses, cracked regions can recover the electric conductivity during mechanical unloading due to crack closure. During cyclic bending, fatigue degradation is reported. This pinpoints the importance of reducing cyclic stresses caused by vibrations due to transportation and use, in order to limit the effect of cracking in Silicon cells.Marco Paggimarco.paggi@imtlucca.itIrene Berardoneirene.berardone@polito.itAndrea Infusoandrea.infuso@polito.it2014-03-03T13:05:06Z2014-03-03T14:07:02Zhttp://eprints.imtlucca.it/id/eprint/2165This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/21652014-03-03T13:05:06ZCyclic voltammetry simulation at microelectrode arrays with COMSOL MultiphysicsThe present paper reports the results obtained applying the general purpose software COMSOL Multiphysics® to the finite elements simulation of Cyclic Voltammetries (CV’s) at microelectrodes arrays (MEA). CV’s at inlaid micro disk electrode arrays have been simulated benchmarking our results with those obtained by Compton with the finite difference method. Then the influence of meshing on the quality of the simulated data have been investigated showing that bad meshing may provide shapes with no physical meaning. Simulations have also been performed on recessed micro disk arrays in order to show the effect of the depth of the recess on the voltammetric wave shape. We found that COMSOL Multiphysics® provides a flexible and straightforward route to the simulation of electrochemical systems with complex geometry.Alessandro LavacchiUgo BardiClaudia Borriclaudia.borri@imtlucca.itStefano CaporaliAlessio FossatiI. Perissi2013-10-25T08:56:20Z2014-06-12T10:09:34Zhttp://eprints.imtlucca.it/id/eprint/1844This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/18442013-10-25T08:56:20ZConstrained Model Predictive Control Based on Reduced-Order ModelsThe need for reduced-order approximations of dynamical systems emerges naturally in model-based control of very large-scale systems, such as those arising from the discretisation of partial differential equation models. The controller based on the reduced-order model, when in closed-loop with the large-scale system, ought to endow certain properties, in primis stability, but also satisfaction of state constraints and recursive computability of the control law in the case of constrained control. In this paper we introduce a new approach to the design of model predictive controllers to meet the aforementioned requirements while the on-line complexity is essentially tantamount to the one that corresponds to the low-dimensional approximate model.Pantelis Sopasakispantelis.sopasakis@imtlucca.itDaniele Bernardinidaniele.bernardini@imtlucca.itAlberto Bemporadalberto.bemporad@imtlucca.it2013-10-25T08:30:43Z2014-06-16T10:17:42Zhttp://eprints.imtlucca.it/id/eprint/1841This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/18412013-10-25T08:30:43ZMPC for Sampled-Data Linear Systems: guaranteeing continuous-time positive invarianceModel Predictive Controllers (MPC) designed for sampled-data systems can be shown to violate the constraints in continuous time. A reformulation of the initial problem will guarantee constraint satisfaction throughout the intersample period. Polytopic inclusions of the continuous trajectory are used in this paper to establish additional constraints leading to a linearly constrained quadratic optimization problem. Continuous time asymptotic stability and continuous-time positive invariance are proven for the reformulated problem.Pantelis Sopasakispantelis.sopasakis@imtlucca.itPanagiotis Patrinospanagiotis.patrinos@imtlucca.itHaralambos Sarimveis2013-09-17T13:10:30Z2013-09-17T13:10:30Zhttp://eprints.imtlucca.it/id/eprint/1770This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/17702013-09-17T13:10:30ZLipschitz Continuity of the Solutions to Team Optimization Problems RevisitedSufficient conditions for the existence and Lipschitz
continuity of optimal strategies for static team optimization problems are studied. Revised statements and proofs of some results in “Kim K.H., Roush F.W., Team Theory. Ellis Horwood Limited Publishers, Chichester, UK, 1987” are presented.Giorgio Gneccogiorgio.gnecco@imtlucca.itMarcello Sanguineti2011-07-27T09:42:01Z2011-08-05T12:45:52Zhttp://eprints.imtlucca.it/id/eprint/624This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/6242011-07-27T09:42:01ZFurther switched systemsMixed logical dynamical systems and linear complementarity systems are representations of switched systems, which under the conditions described here are equivalent to the model used in Chapter 4. They are particularly useful for model-predictive control. The equivalences of several hybrid system models show that different models, which are suitable for specific analysis and design problems and have been investigated in detail, cover the same class of hybrid systems. The analysis of the well-posedness of the models leads to conditions on the model equations under which a unique solution exists.Alberto Bemporadalberto.bemporad@imtlucca.itM.Kanat ÇamlıbelW.P.M.H. HeemelsArjan J. Van der SchaftJ.M. SchumacherBart De Schutter2011-07-27T09:02:47Z2011-08-04T07:29:08Zhttp://eprints.imtlucca.it/id/eprint/471This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4712011-07-27T09:02:47ZSuboptimal Explicit Receding Horizon Control via Approximate Multiparametric Quadratic Programming Algorithms for solving multiparametric quadratic programming (MPQP) were recently proposed in Refs. 1–2 for computing explicit receding horizon control (RHC) laws for linear systems subject to linear constraints on input and state variables. The reason for this interest is that the solution to MPQP is a piecewise affine function of the state vector and thus it is easily implementable online. The main drawback of solving MPQP exactly is that, whenever the number of linear constraints involved in the optimization problem increases, the number of polyhedral cells in the piecewise affine partition of the parameter space may increase exponentially. In this paper, we address the problem of finding approximate solutions to MPQP, where the degree of approximation is arbitrary and allows to tradeoff between optimality and a smaller number of cells in the piecewise affine solution. We provide analytic formulas for bounding the errors on the optimal value and the optimizer, and for guaranteeing that the resulting suboptimal RHC law provides closed-loop stability and constraint fulfillment.Alberto Bemporadalberto.bemporad@imtlucca.itCarlo Filippi2011-07-27T08:53:49Z2011-08-04T07:29:08Zhttp://eprints.imtlucca.it/id/eprint/566This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/5662011-07-27T08:53:49ZData classification and parameter estimation for the identification of piecewise affine modelsThis paper proposes a three-stage procedure for parametric identification of piece wise affine auto regressive exogenous (PWARX) models. The first stage simultaneously classifies the data points and estimates the number of submodels and the corresponding parameters by solving the MIN PFS problem (partition into a minimum number of feasible subsystems) for a set of linear complementary inequalities derived from input-output data. Then, a refinement procedure reduces misclassifications and improves parameter estimates. The last stage determines a polyhedral partition of the regressor set via two-class or multi-class linear separation techniques. As a main feature, the algorithm imposes that the identification error is bounded by a fixed quantity δ. Such a bound is a useful tuning parameter to trade off between quality of fit and model complexity. Ideas for efficiently addressing the MIN PFS problem, and for improving data classification are also discussed in the paper. The performance of the proposed identification procedure is demonstrated on experimental data from an electronic component placement process in a pick-and-place machine.Alberto Bemporadalberto.bemporad@imtlucca.itAndrea GarulliSimone PaolettiAntonio Vicino2011-07-27T08:47:44Z2012-06-26T11:21:20Zhttp://eprints.imtlucca.it/id/eprint/451This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4512011-07-27T08:47:44ZHybrid modelling and optimal control of a multi product batch plantThis paper addresses the problem of optimally selecting the production plan for a Multiproduct Batch Plant. The proposed approach can also be applied to a broader class of optimal control problems for systems with discrete inputs. The plant is modelled as a Discrete Hybrid Automaton (DHA) using the high level modelling language, HYbrid System DEscription Language (HYSDEL), which allows conversion of the DHA model into an Mixed Logical Dynamical (MLD) model. The solution algorithm, which takes into account a model of a hybrid system described as an MLD system, is based on reachability analysis ideas. The algorithm abstracts the behaviour of the hybrid system into a “tree of evolution”, where nodes of the tree represent reachable states of the system, and branches connect two nodes if a transition exists between the corresponding states. To each node a cost function value is associated and, based on this value, the tree exploration is driven, searching for the optimal control profile.Bostjan PotocnikAlberto Bemporadalberto.bemporad@imtlucca.itFabio Danilo TorrisiGasper MusicBorut Zupancic2011-07-27T08:45:11Z2011-08-04T07:29:07Zhttp://eprints.imtlucca.it/id/eprint/453This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4532011-07-27T08:45:11ZA bounded-error approach to piecewise affine system identificationThis paper proposes a three-stage procedure for parametric identification of piecewise affine autoregressive exogenous (PWARX) models. The first stage simultaneously classifies the data points and estimates the number of submodels and the corresponding parameters by solving the partition into a minimum number of feasible subsystems (MIN PFS) problem for a suitable set of linear complementary inequalities derived from data. Second, a refinement procedure reduces misclassifications and improves parameter estimates. The third stage determines a polyhedral partition of the regressor set via two-class or multiclass linear separation techniques. As a main feature, the algorithm imposes that the identification error is bounded by a quantity δ. Such a bound is a useful tuning parameter to trade off between quality of fit and model complexity. The performance of the proposed PWA system identification procedure is demonstrated via numerical examples and on experimental data from an electronic component placement process in a pick-and-place machine.Alberto Bemporadalberto.bemporad@imtlucca.itAndrea GarulliSimone PaolettiAntonio Vicino2011-07-27T08:45:06Z2011-08-05T13:45:04Zhttp://eprints.imtlucca.it/id/eprint/532This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/5322011-07-27T08:45:06ZBounded model checking of hybrid dynamical systemsBounded model checking (BMC) has recently emerged as a very powerful methodology for the verification of purely discrete systems. Given a horizon of interest, bounded model checking verifies whether all finite-horizon trajectories satisfy a temporal logic formula by first translating the problem to a large satisfiability SAT-problem and then relying on extremely powerful state-of-the art SAT-solvers for a counterexample or a ertification of safety. In this paper we consider the problem of bounded model checking for a general class of discrete-time hybrid systems. Critical to our approach is the abstraction of continuous trajectories under discrete observations with a purely discrete system that captures the same discrete sequences. Bounded model checking can then be applied to the purely discrete, abstracted system. The performance of our approach is illustrated by verifying temporal properties of a hybrid model of an electronic height controller.Nicolò GiorgettiGeorge J. PappasAlberto Bemporadalberto.bemporad@imtlucca.it2011-07-27T08:43:46Z2011-08-05T13:40:55Zhttp://eprints.imtlucca.it/id/eprint/454This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4542011-07-27T08:43:46ZAn MPC/hybrid system approach to traction controlThis paper describes a hybrid model and a model predictive control (MPC) strategy for solving a traction control problem. The problem is tackled in a systematic way from modeling to control synthesis and implementation. The model is described first in the Hybrid Systems Description Language to obtain a mixed-logical dynamical (MLD) hybrid model of the open-loop system. For the resulting MLD model, we design a receding horizon finite-time optimal controller. The resulting optimal controller is converted to its equivalent piecewise affine form by employing multiparametric programming techniques, and finally experimentally tested on a car prototype. Experiments show that good and robust performance is achieved in a limited development time by avoiding the design of ad hoc supervisory and logical constructs usually required by controllers developed according to standard techniques.Francesco BorrelliAlberto Bemporadalberto.bemporad@imtlucca.itMichael FodorDavor Hrovat2011-07-27T08:40:46Z2011-08-05T13:23:15Zhttp://eprints.imtlucca.it/id/eprint/455This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4552011-07-27T08:40:46ZOptimal control of continuous-time switched affine systemsThis paper deals with optimal control of switched piecewise affine autonomous systems, where the objective is to minimize a performance index over an infinite time horizon. We assume that the switching sequence has a finite length, and that the decision variables are the switching instants and the sequence of operating modes. We present two different approaches for solving such an optimal control problem. The first approach iterates between a procedure that finds an optimal switching sequence of modes, and a procedure that finds the optimal switching instants. The second approach is inspired by dynamic programming and identifies the regions of the state space where an optimal mode switch should occur, therefore providing a state feedback control law.Carla SeatzuDaniele CoronaAlessandro GiuaAlberto Bemporadalberto.bemporad@imtlucca.it2011-07-27T08:40:35Z2011-08-05T13:20:08Zhttp://eprints.imtlucca.it/id/eprint/490This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4902011-07-27T08:40:35ZA decomposition algorithm for feedback min-max model predictive controlAn algorithm for solving feedback min-max model predictive control for discrete-time uncertain linear systems with constraints is presented in this note. The algorithm is based on applying recursively a decomposition technique to solve the min-max problem via a sequence of low complexity linear programs. It is proved that the algorithm converges to the optimal solution in finite time. Simulation results are provided to compare the proposed algorithm with other approachesDavid Muñoz de la PeñaTeodoro AlamoAlberto Bemporadalberto.bemporad@imtlucca.itEduardo F. Camacho2011-07-27T08:39:04Z2011-08-04T07:29:06Zhttp://eprints.imtlucca.it/id/eprint/509This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/5092011-07-27T08:39:04ZHybrid modeling and control of a multibody magnetic actuator for automotive applicationsThis paper investigates model predictive control (MPC) techniques based on hybrid models for a multi-mass magnetic actuator. The actuator has four operating modes depending on the mutual interaction of two moving masses and is modeled as a hybrid dynamical system. The control law optimizes a performance index and enforces several types of constraints: soft-landing during collisions to reduce mechanical wear, current limits and consequent position-dependent nonlinear bounds on the available magnetic force, and restrictions on the positions of the moving masses. Two different approaches are considered: (i) a hybrid MPC design based on the full two- mass model, and (ii) a switched MPC control design, which switches between two simpler hybrid MPC controllers, one for the case in which the masses are moving in contact, and the other case where the masses are decoupled, commanded by a simple switching logic. Simulation results and performance comparisons of the two control schemes are discussed.Alberto Bemporadalberto.bemporad@imtlucca.itStefano Di CairanoIlya KolmanovskyDavor Hrovat2011-07-27T08:39:02Z2011-08-04T07:29:06Zhttp://eprints.imtlucca.it/id/eprint/520This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/5202011-07-27T08:39:02ZHybrid model predictive control based on wireless sensor feedback: an experimental studyThis paper presents the design and the experimental validation of model predictive control (MPC) of a hybrid dynamical process based on measurements collected by a wireless sensor network. The proposed setup is the prototype of an industrial application in which a remote station controls the process via wireless network links. The experimental platform is a laboratory process consisting of four infrared lamps, controlled in pairs by two on/off switches, and of a transport belt, where moving parts equipped with wireless sensors are heated by the lamps. By approximating the stationary heat spatial distribution as a piecewise affine function of the position along the belt, the resulting plant model is a hybrid dynamical system. The control architecture is based on the reference governor approach: the process is actuated by a local controller, while a hybrid MPC algorithm running on a remote base station sends optimal belt velocity set-points and lamp on/off commands over a network link exploiting the information received through the wireless network. A discrete-time hybrid model of the process is used for the hybrid MPC algorithm and for the state estimator.Alberto Bemporadalberto.bemporad@imtlucca.itStefano Di CairanoErik HenrikssonKarl Henrik Johansson2011-07-27T08:36:41Z2014-01-20T15:22:08Zhttp://eprints.imtlucca.it/id/eprint/507This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/5072011-07-27T08:36:41ZDecentralized model predictive control of constrained linear systemsAlessandro AlessioAlberto Bemporadalberto.bemporad@imtlucca.it2011-07-27T08:36:36Z2011-08-05T12:59:53Zhttp://eprints.imtlucca.it/id/eprint/485This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4852011-07-27T08:36:36ZCombined design of disturbance model and observer for offset-free model predictive controlThis note presents a method for the combined design of an integrating disturbance model and of the observer (for the augmented system) to be used in offset-free model predictive controllers. A dynamic observer is designed for the original (nonaugmented) system by solving an Hprop control problem aimed at minimizing the effect of unmeasured disturbances and plant/model mismatch on the output prediction error. It is shown that, when offset-free control is sought, the dynamic observer is equivalent to choosing an integrating disturbance model and an observer for the augmented system. An example of a chemical reactor shows the main features and benefits of the proposed method.Gabriele PannocchiaAlberto Bemporadalberto.bemporad@imtlucca.it2011-07-27T08:36:28Z2011-08-05T12:58:24Zhttp://eprints.imtlucca.it/id/eprint/619This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/6192011-07-27T08:36:28ZSuboptimal hybrid model predictive control: Application to sewer networksThis paper presents an application of the suboptimal hybrid model predictive control (HMPC) algorithm previously proposed by the authors to large scale sewer networks. HMPC relies on the on-line solution of mixed integer programs (MIP) that are known to be NP-complete and whose worst case complexity scales exponentially with problem size. Modern MIP solvers are on the other hand highly efficient at taking advantage of problem structure and usually achieve average optimization times that are much better than the worst case predicts. But as the MIP constraints depend on the current state of the plant, complexity can vary considerably and unpredictable behavior can occur. To circumvent unpredictability and to be able to enforce hard real-time computation constraints, the number of feasible nodes in the MIP problem is limited online by adding constraints to the number of possible mode sequences over the prediction horizon. It is shown that in realistic scenarios concerning control of large scale sewer networks, depending on the value of parameters related to the mode sequence constraints (MSC), drastic reductions can be achieved in optimization time. Practical issues of the approach are also addressed.Carlos Ocampo-MartinezAri IngimundarsonAlberto Bemporadalberto.bemporad@imtlucca.itVicenç Puig2011-07-27T08:36:17Z2011-10-12T14:23:48Zhttp://eprints.imtlucca.it/id/eprint/614This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/6142011-07-27T08:36:17ZEnergy-aware robust model predictive control based on wireless sensor feedbackFlexibility, ease of deployment and of spatial reconfiguration, and low cost make wireless sensor networks (WSNs) fundamental component of modern networked control systems. However, due to the energy-constrained nature of WSNs, the transmission rate of the sensor nodes is a critical aspect to take into account in control design. Two are the main contributions of this paper. First, a general transmission strategy for communication between controller and sensors is proposed. Then, a scenario with a controller and a wireless node providing measures is investigated, and two energy-aware control schemes based on explicit model predictive control (MPC) are presented. We consider both nominal and robust control in the presence of disturbances, and convergence properties are given for the latter. The proposed control schemes are tested and compared to traditional MPC techniques. The results show the effectiveness of the proposed energy-aware approach, which achieves a profitable trade-off between energy consumption of wireless sensors and loss in system performance.Daniele Bernardinidaniele.bernardini@imtlucca.itAlberto Bemporadalberto.bemporad@imtlucca.it2011-07-27T08:34:14Z2014-07-02T14:30:10Zhttp://eprints.imtlucca.it/id/eprint/433This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4332011-07-27T08:34:14ZModel predictive control for systems with time delay: an application to air-fuel ratio control in automotive enginesTo meet increasingly stringent emission regulations modern internal combustion engines require highly accurate control of the air-to-fuel ratio. The performance of the conventional air-to-fuel ratio feedback loop is limited by the combustion delay between fuel injection and engine exhaust, and by the transport delay for the exhaust gas to propagate to the air-to-fuel ratio sensor location. The combined delay is variable, since it depends on engine speed and airflow. Drivability, fuel economy and emission requirements result in constraints on the deviations of the air-to-fuel ratio, stored oxygen in the three-way catalyst, and fuel injection. This paper proposes an approach for air-to-fuel ratio control based on Model Predictive Control (MPC). The approach systematically handles both variable time delays and pointwise-in-time constraints. A delay-free model is considered first, which takes into account the dynamic relations between the injected fuel and the air-to-fuel ratio and the dynamics of the oxygen stored in the catalyst. For the delay-free model, the explicit MPC law is computed. Delay compensation is obtained by estimating the delay online from engine operating conditions, and feeding the MPC law with the state predicted ahead over the time interval of the estimated delay. The predicted state is computed by combining measurement filtering with forward iterations of the nonlinear dynamic equations of the model. The achieved performance in tracking the air-to-fuel ratio and the oxygen storage setpoints while enforcing the constraints is demonstrated in simulation using real data profiles.Sergio TrimboliStefano Di CairanoAlberto Bemporadalberto.bemporad@imtlucca.itIlya Kolmanovsky2011-07-27T08:34:12Z2011-08-05T12:43:59Zhttp://eprints.imtlucca.it/id/eprint/513This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/5132011-07-27T08:34:12ZHybrid modeling, identification, and predictive control: an application to hybrid electric vehicle energy managementRising fuel prices and tightening emission regulations have resulted in an increasing need for advanced powertrain systems and systematic model-based control approaches. Along these lines, this paper illustrates the use of hybrid modeling and model predictive control for a vehicle equipped with an advanced hybrid powertrain. Starting from an existing high fidelity nonlinear simulation model based on experimental data, the hybrid dynamical model is developed through the use of linear and piecewise affine identification methods. Based on the resulting hybrid dynamical model, a hybrid MPC controller is tuned and its effectiveness is demonstrated through closed-loop simulations with the high-fidelity nonlinear model. Giulio RipaccioliAlberto Bemporadalberto.bemporad@imtlucca.itF. AssadianC. DextreitStefano Di CairanoIlya Kolmanovsky2011-07-27T08:32:36Z2014-07-02T14:16:49Zhttp://eprints.imtlucca.it/id/eprint/443This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4432011-07-27T08:32:36ZModel predictive powertrain control: an application to idle speed regulationModel Predictive Control (MPC) can enable powertrain systems to satisfy more stringent vehicle requirements. To illustrate this, we consider an application of MPC to idle speed regulation in spark ignition engines. Improved idle speed regulation can translate into improved fuel economy, while improper control can lead to engine stalls. From a control point of view, idle speed regulation is challenging, since the plant is subject to time delay and constraints. In this chapter, we first obtain a control-oriented model where ancillary states are added to account for delay and performance specifications. Then the MPC optimization problem is defined. The MPC feedback law is synthesized as a piecewise affine function, suitable for implementation in automotive microcontrollers. The obtained design has been extensively tested in a vehicle under different operating conditions. Finally, we show how competing requirements can be met by a switched MPC controller. Stefano Di CairanoDiana YanakievAlberto Bemporadalberto.bemporad@imtlucca.itIlya KolmanovskyDavor Hrovat2011-07-27T08:32:34Z2014-03-05T13:43:45Zhttp://eprints.imtlucca.it/id/eprint/475This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4752011-07-27T08:32:34ZEvent-driven optimization-based control of hybrid systems with integral continuous-time dynamicsIn this paper we introduce a class of continuous-time hybrid dynamical systems called integral continuous-time hybrid automata (icHA) for which we propose an event-driven optimization-based control strategy. Events include both external actions applied to the system and changes of continuous dynamics (mode switches). The icHA formalism subsumes a number of hybrid dynamical systems with practical interest, e.g., linear hybrid automata. Different cost functions, including minimum-time and minimum-effort criteria, and constraints are examined in the event-driven optimal control formulation. This is translated into a finite-dimensional mixed-integer optimization problem, in which the event instants and the corresponding values of the control input are the optimization variables. As a consequence, the proposed approach has the advantage of automatically adjusting the attention of the controller to the frequency of event occurrence in the hybrid process. A receding horizon control scheme exploiting the event-based optimal control formulation is proposed as a feedback control strategy and proved to ensure either finite-time or asymptotic convergence of the closed-loop.Stefano Di CairanoAlberto Bemporadalberto.bemporad@imtlucca.itJorge Júlvez2011-07-27T08:32:07Z2011-08-05T12:39:05Zhttp://eprints.imtlucca.it/id/eprint/434This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4342011-07-27T08:32:07ZModel predictive controller matching: Can MPC enjoy small signal properties of my favorite linear controller?Model predictive control (MPC) strategies can efficiently deal with constraints on system states, inputs, and outputs. However, in contrast with linear control techniques, closed-loop frequency-domain properties of MPC such as sensitivity and robustness to small perturbations are difficult to enforce a priori. This paper considers the problem of transforming a given linear feedback control design, referred to as “favorite controller”, into a model predictive control one. In this way, the MPC controller inherits all the stability, robustness and frequency properties of the given favorite controller in the region around the equilibrium where the constraints are not active. The added value is that the constructed MPC controller is able to properly handle constraints that may be activated during the transient, and that global stability in the set of feasible initial conditions can be guaranteed.Stefano Di CairanoAlberto Bemporadalberto.bemporad@imtlucca.it2011-07-27T08:32:04Z2011-10-12T14:23:42Zhttp://eprints.imtlucca.it/id/eprint/512This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/5122011-07-27T08:32:04ZSimultaneous optimal control and discrete stochastic sensor selectionIn this paper we present the problem of combining optimal control with efficient information gathering in an uncertain environment. We assume that the decision maker has the ability to choose among a discrete set of sources of information, where the outcome of each source is stochastic. Different sources and outcomes determine a reduction of uncertainty, expressed in terms of constraints on system variables and set-points, in different directions. This paper proposes an optimization-based decision making algorithm that simultaneously determines the best source to query and the optimal sequence of control moves, according to the minimization of the expected value of an index that weights both dynamic performance and the cost of querying. The problem is formulated using stochastic programming ideas with decision-dependent scenario trees, and a solution based on mixed-integer linear programming is presented. The results are demonstrated on a simple supply-chain management example with uncertain market demand. Daniele Bernardinidaniele.bernardini@imtlucca.itDavid Muñoz de la PeñaAlberto Bemporadalberto.bemporad@imtlucca.itEmilio Frazzoli2011-07-27T08:32:01Z2016-04-06T10:26:11Zhttp://eprints.imtlucca.it/id/eprint/430This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4302011-07-27T08:32:01ZDrive-by-wire vehicle stabilization and yaw regulation: A hybrid model predictive control designElectronic Stability Control (ESC) and Active Front Steering (AFS) have been introduced in production vehicles in recent years, due to improved vehicle maneuverability and the effects in reducing single vehicle accident. We propose a hybrid Model Predictive Control (MPC) design for coordinated control of AFS and ESC. By formulating the vehicle dynamics with respect to the front and rear tire slip angles and by approximating the tire-force characteristics by piecewise affine functions, the vehicle dynamics are formulated as a linear hybrid dynamical system. This model is used to design a hybrid model predictive controller. The proposed model formulation allows one to visually analyze the stability region of the closed-loop system and to assess the stabilizing capability of the hybrid MPC controller. Simulations of the controller in closed-loop with an accurate nonlinear model are presented.Daniele Bernardinidaniele.bernardini@imtlucca.itStefano Di CairanoAlberto Bemporadalberto.bemporad@imtlucca.itH. E. Tseng2011-07-27T08:31:59Z2011-11-17T11:28:12Zhttp://eprints.imtlucca.it/id/eprint/429This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/4292011-07-27T08:31:59ZScenario-based model predictive control of stochastic constrained linear systemsIn this paper we propose a stochastic model predictive control (MPC) formulation based on scenario generation for linear systems affected by discrete multiplicative disturbances. By separating the problems of (1) stochastic performance, and (2) stochastic stabilization and robust constraints fulfillment of the closed-loop system, we aim at obtaining a less conservative control action with respect to classical robust MPC schemes, still enforcing convergence and feasibility properties for the controlled system. Stochastic performance is addressed for very general classes of stochastic disturbance processes, although discretized in the probability space, by adopting ideas from multi-stage stochastic optimization. Stochastic stability and recursive feasibility are enforced through linear matrix inequality (LMI) problems, which are solved off-line; stochastic performance is optimized by an on-line MPC problem which is formulated as a convex quadratically constrained quadratic program (QCQP) and solved in a receding horizon fashion. The performance achieved by the proposed approach is shown in simulation and compared to the one obtained by standard robust and deterministic MPC schemes.Daniele Bernardinidaniele.bernardini@imtlucca.itAlberto Bemporadalberto.bemporad@imtlucca.it2011-07-27T08:31:54Z2011-08-05T12:39:58Zhttp://eprints.imtlucca.it/id/eprint/607This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/6072011-07-27T08:31:54ZAutomotive controlLuca BenvenutiAndrea BalluchiAlberto Bemporadalberto.bemporad@imtlucca.itStefano Di CairanoBengt JohanssonRolf JohanssonAlberto Sangiovanni VincentelliPer Tunest2011-06-21T14:19:10Z2011-08-04T07:29:06Zhttp://eprints.imtlucca.it/id/eprint/623This item is in the repository with the URL: http://eprints.imtlucca.it/id/eprint/6232011-06-21T14:19:10ZTools for modeling, simulation, control, and verification of piecewise affine systemsAlberto Bemporadalberto.bemporad@imtlucca.itStefano Di CairanoGiancarlo Ferrari-TrecateMichal KvasnicaManfred MorariSimone Paoletti