Fracture toughness of closed-cell PVC foam: Effects of loading configuration and cell size

Fracture toughness testing of a large range of PVC foams, used as sandwich core materials, is presented. The influence of loading configuration was examined by testing single edge-notched beam specimens loaded in three- and four-point flexure. Moreover, specially made foams with constant density but a range of cell sizes were tested to examine the influence of cell size and crosshead rate on the fracture toughnesses (KIc and GIc). The results show that the fracture toughness measured using four-point bending significantly exceeds that measured in three-point bending. Indentation tests reveal significant indentation deformation in the load introduction and support regions. For the three-point configuration, the applied load acts on the cracked cross section which causes a reduction in the ligament length and leads to the low apparent fracture toughness. Increased cell size was found to reduce KIc, especially at the lowest crosshead rate. At higher crosshead rates, there was less influence of cell size. Microscopic examination of the fracture surfaces of the foam with large cells tested at low crosshead rates revealed craze-like deformation of the solid polymer in cell edges. At higher rates, brittle fracture surfaces were observed.