Identification of Failure Mechanisms in Sandwich Structures with Foam Core Thickness Mismatches

An analytical and test program was designed to gain understanding of the failure mechanisms and strength reductions that occur in sandwich structures containing two sections of foam having different thicknesses, typically joined by a butt joint. A three point bend test configuration was used to induce a state of nearly uniform shear stress and to evaluate the effect of butt joints. Three distinct failure modes were identified using finite element analysis and these modes were confirmed with high speed photography of three point bend test specimens. The investigation demonstrated that the failure mode changed as the mismatch height increased. A distinct failure criterion was proposed for each failure mode and the resulting strength predictions were in reasonable agreement to test data. This study found that sufficiently small thickness mismatches do not cause any strength reduction, while a sufficiently large thickness mismatch can cause significant strength reduction.