Evaluation of the plate twist test to characterize mode III fracture of sandwich panels with a face/core interface crack

Sandwich panels with a face/core interface edge crack loaded in torsion have been analyzed using finite element analysis (FEA) and experimental testing to characterize mode III delamination propagation. Symmetric sandwich panels with steel face sheets bonded to a high density (H250) PVC foam core were considered. The test specimens were square plates of side length of 90 mm with 3 mm thick face sheets and core thicknesses of 3, 12 and 25.4 mm supported at two corners and loaded in torsion by application of transverse loads at two diagonally opposite corners. The energy release rate (G) was determined from stress intensity factors calculated from the crack flank displacements. The results show dominant mode III crack loading. The mode III component of the energy release rate (GIII) was nearly uniform along the central region of the crack front and mode II contribution was only significant near the load introduction and support pins. The measured and calculated compliance for the sandwich panels examined agreed reasonable. The fracture toughness (Gc) determined from measured critical loads and the compliance calibration method was 119 ± 27 J/m2.