The impact response of graded foam sandwich structures

Low velocity impact tests have been undertaken on sandwich structures based on cores fabricated by bonding foams of different densities together. Here, a range of linear PVC, crosslinked PVC and PEI foams were bonded together to produce a three layer core. Carbon fibre skins were then bonded to the core and the structures were loaded by a drop-weight impact carriage with a hemispherical head. It has been observed that the majority of the panels failed in a through-thickness shearing mode, leaving a clear cylindrical hole in the multi-layered core. A limited number of structures also exhibited cone cracking on the exit surface, due to failure in a mixed tensile/shear mode. The impact response of the graded sandwich structures was modelled by finite element analysis and the predicted load-displacement responses and failure modes compared. Agreement between the FE model and the experimental data was good across the range of structures investigated, with the model accurately predicting the impact responses and failure characteristics observed within the panels. It has also been shown that graded core structures can out-perform their monolithic counterparts. Finally when normalised by their unit cost, significant differences in the perforation resistances of the structures have been observed.