Experiments and discrete element simulations of crack initiation angle of mixed-mode I/II in PMMA material

•The crack initiation angle increase with the increase of the size ratio.•Exponential formula can describe the crack initiation angle in mixed-mode I/II.•Increase of the symmetry factor results in decrease of the crack initiation angle.•Influence of crack length is more significant than that of symmetry factor. Complex crack configuration causing mixed-mode I/II usually occurs in various situations in engineering, and it is difficult to accurately predict the direction of crack initiation by theory. In fact, there is a great difference between the finite-plate theoretical prediction and the experimental results. To investigate the effects of crack configuration on crack initiation angle, new brittle polymethyl methacrylate (PMMA) specimens were designed. Multiple sets of fracture experiments of mixed-mode I/II were performed systematically to obtain the crack initiation angle of PMMA specimens. The variation of the crack initiation angle with the size ratio was obtained experimentally, which indicated that the crack initiation angle increases with the increase of the size ratio. As a supplement to the physical experiments, corresponding numerical simulations were carried out based on the Discrete Element Method. Firstly, the material parameters of the numerical simulation were calibrated by reproducing the failure process of the sample in the experiment. Then, the numerical modeling and calculation were carried out for the samples that are inconvenient to process in the experiment. Combining the results of both experiment and numerical simulation, a prediction formula for the crack initiation angle was obtained. Furthermore, the effects of symmetry factor and preexisting crack length on the tensile capacity and the crack initiation angle were investigated by simulation systematically. By adjusting the crack configuration parameters, the direction of crack initiation can be artificially controlled, which is of great importance for ensuring engineering safety.