Determination of delamination damage in composites under impact loads

This contribution describes recent progress on materials modelling and numerical simulation of fibre reinforced composite shell structures subjected to high velocity impact loads. A Continuum Damage Mechanics (CDM) model for fibre reinforced composites is applied to model both in-ply damage and delamination failure during impact loading. The CDM model has been implemented in a commercial explicit finite element (FE) code in which a laminate is modelled by stacked shell elements with cohesive interfaces which fracture when a delamination failure energy criterion is reached. A comparison of structural response and failure modes from numerical simulations is then conducted. In particular, emphasis is put on the influence of delamination damage on the impact behaviour of composite plates and stiffened panels. Delamination damage is measured in low velocity drop tower impact tests and also in high velocity gas gun tests on composite plates with steel projectiles carried out at the DLR. In both types of test it is seen that at lower impact energies delamination damage was often the main damage mechanism, whereas at higher impact energies there was fibre fracture and plate penetration by the hard projectiles. In this case, the presence of some delamination provides an additional energy absorption mechanism in the composite plate, which can reduce complete penetration. It follows that modelling and analysis of impact damage require composites failure models which include both in-plane ply damage and delamination damage