Effect of geometric constraint on fracture toughness of PVC foam core sandwich beams

Purpose of this study was to understand the effect of core material thickness (t(C)) on the core deformation constraint and the associated mode I fracture toughness in Double Cantilever Sandwich Beam (DCSB) specimens. Specimens were made from woven roving glass fiber/vinyl ester composite face sheet with PVC core, whose thickness ranged from 3.18 mm to 40.6 mm. The specimens were tested in mode I loading and measured fracture initiation (G(IC)) and resistance (G I R) toughnesses. The G I C was found to be practically same for core thicknesses from 3.18 to 40.6 mm. The G(IR) was found to be 1.02, 0.88 and 0.91 kJ/m(2) for t(c)'s 3.18, 6.25, and 12.7 mm. For t(C) >= 25.4 mm, the crack grew by only few mm's before it deflected to face sheet. Larger G(IR) for t(C) = 3.18 mm is probably due to resin densification of foam cells in the co-cure processing of panels. Three dimensional, material nonlinear finite element (FE) analysis very well simulated the test data. The J(IC) integrals from FE analysis agreed well with GIC from the test. The analysis revealed that the deformation constraint was nearly the same for all core thicknesses considered and thus resulting in nearly identical fracture toughness.