Protection of sandwich plates from low-velocity impact

Sandwich plates used in marine structures are made of laminated face sheets bonded to a structural foam core. Low-velocity impact by a stiff indenter may cause local deflection of the face sheet and permanent deformation or crushing of an underlying volume of the foam core. Upon unloading, the face sheet tends to spring back, while the permanent strain in the core gives rise to residual stresses that may nucleate local interfacial cracks. Extension of such interfacial cracks by applied loads can compromise structural integrity. This article examines the protective effect of thin, ductile interlayers, inserted between the face sheet and the foam core, that absorb the face sheet deflection and thus prevent or reduce the extent of damage to the foam core. Finite element solutions are obtained for a stiff, incompressible, and very ductile polyurethane (PUR) interlayer, and also for a compliant and compressible elastomeric foam (EF) layer, in a sandwich plate made of quasi-isotropic AS4/3501-6 laminated face sheets and H100 PVC core. Loading is applied by a concentrated force acting on a rigid cylindrical indenter. Results show that the PUR interlayer better supports the impacted face sheet than does the foam core or the EF interlayer. The PUR layer also reduces the extent of permanent deformation of the foam core. At a given magnitude of applied load, the energy release rate for cracks extended by residual stresses at the interface between the foam core and either interlayer is much lower than that at the interface between the face sheet and the foam core in the conventional design.