physical and numerical evaluation of mode ii fracture of quasi-brittle materials

Mode ii fracture testing of quasi-brittle materials is a challenging task as mode i fracture and tensile microcracking are normally involved in this situation. in this study, a double edge notched compression (denc) graphite specimen was tested for the mode ii fracture toughness (kiic) measurement. digital image correlation (dic) was used to identify the surface displacement field and the crack initiation and propagation. the ca2 hybrid discrete-finite element computer program was utilized for the numerical simulation. the graphite was modeled as a bonded particle system to allow initiation and evolution of microcracking. the dic measurement detected the tensile kinked crack initiation and propagation at the notch tip, however failed to determine the shear dislocation. this was due to the influenced displacement field by the kinked crack. the bonded particle simulation captured the initiation and propagation of the kinked tensile crack and shear crack around the notch tip. given this observation, this study alludes to the inaccuracy of dic measurements in situations with complicated displacement fields. the numerical tests suggest that the critical stress for initiation of shear crack (at the notch tip) directly depends on the particle size of the material. this study also implies that the critical stress is not a fixed percentage of the rock strength. the following ratios for shear crack initiation stress to the peak stress were determined: 0.85, 0.76, and 0.5 with the average particle radius of 0.2, 0.1, and 0.05 mm, respectively. the numerical kiic is consistent with the physical measurement, which insinuates that the bonded particle models are useful techniques for studying mode ii fracture of quasi-brittle materials.