Simulation of low-velocity impact damage in sandwich composites

Composite materials have been increasingly used in many wind energy and transport applications due to their high strength, stiffness and excellent corrosion resistance. One of the main limitations of composites is their high susceptibility to impact-induced damage, which may result in significant strength reduction or even structural collapse. A detailed understanding of the extent and nature of impact damage is thus greatly needed for damage tolerance based structural design and a reliable estimation of the residual strength of a damaged structure. In this thesis, fracture mechanics based progressive damage models, cohesive interface elements and crushable foam models were used to predict the structural response and internal failure mechanisms of sandwich composites subjected to low-velocity impact; various failure modes typically observed in composites including delaminations, fibre fracture and matrix cracking were simulated and implemented into ABAQUS/Explicit through user-defined subroutines VUMAT. Numerical simulations were assessed and validated by a series of experimental analyses carried out through low-velocity impact tests (using drop-weight testing machine) and damage calibration tests (using X-radiography, Ultrasonics and optical microscopy of polished cross-sections). Good agreements were obtained between experiments and predictions not only in terms of structural responses as well as regarding the shape and size of internal damage under various investigated cases.