On the Indentation Resistance of a PC Layer on PVC Foam Substrate

Abstract

The indentation response of a polycarbonate face sheet bonded to a PVC foam substrate is investigated experimentally and numerically. The deformation mode involves foam crushing, and membrane stretching of the PC layer at large indenter displacements; this is quantified using optical strain measurement techniques. The bottom corners of the foam substrate lifts off its underlying support when the foam layer is sufficiently thin. Peak load is dictated by tensile failure of the foam on the bottom face. Finite element simulations suggest that a deep foam core prevents this lift-off and results in a greater load carrying and energy absorption capacity.Polymer foams are widely used as lightweight structural materials in sandwich panels, packaging, and many other applications. Sandwich panels are prevalent throughout the automotive and aerospace industries; they provide a high strength to weight solution for structural components, and are used in applications where flexural loading is of primary concern. Typically, sandwich panels comprise a core made from polymer foam or metal foam and two face sheets made from a material of high modulus and strength, such as an aluminum alloy or glass fiber reinforced polymers. In addition to increasing the structural load-bearing capacity, the face sheets afford protection against indentation and impact. Traditional sandwich panels have face sheets of low ductility, such that an indentation load results in face sheet failure by the formation of elastic-brittle or plastic-brittle hinges at indentation displacements less than the face sheet thickness. The present study probes the indentation of a new combination of sandwich panel materials: a polyvinyl chloride (PVC) foam core and a polycarbonate (PC) face sheet. PC has a sufficiently high tensile ductility that it fails at indentation depths that are much larger than the face sheet thickness, such that the face sheet is taken into the membrane range. This gives the possibility of enhanced energy absorption.

Indentation of a layer on foam substrate

“…Note that the above analyses, assuming rigid, perfectly-plastic behaviour, ignore elastic stretching of the top face sheet and elastic compliance of the foam core. It is anticipated that the role of elastic deformation is significant in bi-layers where each phase has a high value of yield strain, such as PC face sheets on a polymer foam core, as noted by Boyce et al [28]. In order for membrane stresses to exist within the face sheet, there must be load transfer between the face sheet and the foam substrate.…”

“…14b. Equating the horizontal contraction of the core and top face sheet gives tan cos (28) The indentation load of the sandwich beam is determined by a work calculation along the same lines as that of Ashby et al [8]. The external plastic work done by load during an incremental punch displacement is (29) This is balanced by the internal plastic work which includes the contribution from two modes of deformation:…”

“…The indentation load (33) for small can be rewritten in terms of ̅ and ̅ using (24), (27) and (28) along with (34) as…”