Enhancement of low velocity impact damage resistance of sandwich plates

Conventionally designed sandwich plates used in marine and ship structures consist of thin, laminated composite face sheets bonded to a relatively thick, compressible structural foam core layer. In service, the external face sheet often makes low velocity contact with floating objects or moorings that may cause reversible local deflection of the face sheet and permanent compression or crushing of the underlying foam, resulting in interface delaminations. Extension of such interfacial cracks under service loads may impair structural integrity. Modified designs that protect the foam core by inserting a ductile interlayer under the external face sheet, first proposed in our recent paper [Dvorak, G.J., Suvorov, A.P., in press. Protection of sandwich plates from low velocity impact. Journal of Composite Materials 38], are analyzed here in much greater detail and compared with their conventional counterparts. Two different interlayer materials are used, a stiff and incompressible polyurethane (PUR), and a compliant and compressible elastomeric foam (EF), together with the same face sheet and core materials. Response of the three different sandwich plate configurations of constant overall thickness is analyzed under uniformly applied load to a span of a continuous plate, and under forces imparted by two shapes of rigid indenters in a low velocity impact. The results show that the PUR interlayer reduces both overall and local deflections of the face sheet, and also the local compression of the foam core and the residual stresses left after unloading. The EF interlayer offers a much better protection against compression of the foam core, while magnifying both overall and local deflections. Both interlayers substantially reduce the strain energy release rates of interfacial cracks driven by residual stresses generated by foam core compression.