Mixed mode notch fracture toughness assessment of quasi-brittle polymeric specimens at different scales

Abstract

In this study, the mixed mode I/II notch fracture toughness (NFT) of polymeric (PMMA and GPPS) specimens weakened by U- and key-hole notches is assessed experimentally and theoretically. For this aim, first, by performing experiments, the apparent fracture toughness values of cracked polymeric semi-circular bending (SCB) and Brazilian disk (BD) specimens with four different radii are specified. Next, multiple fracture experiments are conducted on the notched polymeric SCB and BD specimens at four different scales, from which the critical loads are measured and converted to the NFT values. Then, the critical radius of the fracture process zone (FPZ) for the notched polymeric specimens is acquired using two different equations already suggested in literature for cracks. A well-established stress-based failure model, namely the point stress (PS) criterion, is employed for anticipating the NFT values of the specimens tested at various scales, by taking advantages of two different critical distance formulas. The results show that at all of the four scales, the fracture toughness values of U- and key-hole notches achieved from PS criterion are in good agreement with the experimental results. Also, shown in this research is that the mixed mode NFT is considerably dependent on the nominal radius of the specimen, such that, as the specimen radius increases, the mixed mode NFT increases significantly. Moreover, it is exhibited that for the present specimens tested, both the critical distance formulas give almost the same predictions, proposing that each of them can be arbitrarily chosen for the NFT prediction.