Mixed-mode tensile and in-plane shear fracture analysis for PUR foams using two sub-sized short bend beam configurations

Two edge cracked beam-shaped specimens called Inclined and Asymmetric Short Bend Beams (ISBB and ASBB) were designed in this work to characterize the in-plane fracture properties of the polyurethane (PU) foams. To calculate the fracture resistance using these two test samples, a broad variety of fracture toughness tests were conducted on PU foams with densities of 100, 145, and 300 kg/m3 and with varying mode mixities. For each foam density, the mixed mode I/II fracture envelope obtained from the ISBB specimen was noticeability lower than the fracture curve of the ASBB method. Indeed, the fracture envelopes obtained from the ISBB and ASBB specimens can provide lower estimate and upper estimate predictions for the PU foam materials under mixed mode I/II. The intrinsic fracture toughness values (as material properties for modes I and II cases; i.e., KIcand IIc) for the tested PU foams were predicted by plotting the fracture resistance versus the critical T-stress for the tested ASBB and ISBB foam samples. Ultimately, a master design diagram was developed by introducing a fracture threshold zone to address an accurate mechanical design against in-plane mixed-mode fracturing of polymeric foams.