%B Atti del 17? Convegno Nazionale del Gruppo Italiano Frattura (IGF) %T Theoretical and numerical investigation on internal instability phenomena in composite materials %I Gruppo Italiano Frattura %D 2004 %O 17? Convegno Nazionale del Gruppo Italiano Frattura IGF17, Bologna, Italy, June 16-18, 2004 %P 136-141 %A Alberto Carpinteri %A Marco Paggi %L eprints1922 %X Instability phenomena occurring in the microstructure of composite materials are investigated. To this aim, a complete description of the mechanical behavior of bi-material interfaces in composite materials requires the definition of both a cohesive law involving damage for the debonding stage, and a contact model during the closure of the interface. Both formulations are herein presented and implemented in the FE code FEAP. Numerical examples showing the transition from a snap-back instability to a stable mechanical response are presented.