On the role of fracture process zone size in specifying fracturing mechanism under dominant mode II loading

The double-edge notched Brazilian disk (DNBD) test is used to investigate the failure mechanism of the polymeric material of polymethyl-methacrylate (PMMA) under dominant shear loading. It has previously been shown that DNBD is a simple and efficient test for measuring shear fracture toughness of rock materials. However, the results in this study show that a tension-based fracturing type prevails in the DNBD samples of PMMA. To predict the type of failure, the previously proposed fracture type (FT) criterion was employed. It is shown that the FT criterion is able to accurately predict the fracturing type, the fracture initiation load and the kink angle of the DNBD specimens. In addition, a comparison made between the results of the DNBD tests of PMMA and rock samples demonstrates the main reason for the different observed fracturing types. Thanks to a large fracture process zone in rocks, DNBD samples of rock materials fail with self-planar extension of cracks due to shear stresses. In contrast, due to small fracture process zone in PMMA, the DNBD samples of PMMA show tension-based failure mechanism with the formations of kinks. As the final discussion, an incremental finite element simulation is used to justify the reason behind the stable tension-based crack growth in the DNBD specimens of PMMA, and to predict the crack propagation trajectory.