Mode II crack paths under compression in brittle solids - a theory and experimental comparison

A study of crack propagation paths in the situation where the crack is suppressed to open during growth due to high compressive forces has been performed. This problem was analyzed theoretically very recently by the authors and is here extended to involve a limited number of illustrative experimental results reported elsewhere in the literature. By analyzing the experimental crack growth patterns, the conclusion is that the model cannot describe the more realistic microscopic failure in detail. Since shear crack growth on the microscale strongly depend on inhomogenities in the material, like cavities, grains or inclusions; the closed crack growth patterns observed are not smooth or free of kinks. Nevertheless, the model show good agreement with the reported experimental observations of the paths of closed macroscopic mode II cracks on samples in brittle materials, induced under overall compression. Failure patterns experimentally observed supports the theory that the growth of mode II cracks under compression in brittle materials follow a propagation path described by a function y=λxb, where b=3/2. This is strongly supported by the measured values obtained from various experiments. In all the studied experiments, the exponent b was found in the interval [1.43–1.58]. Further, an investigation of the curvature parameter λ has been performed and the conclusion is that λ does also agree with the simplified model, even though not as good as the exponent b. However, λ differs in general <15% from the theoretical value predicted by the model. The process of crack growth is in the simplified model assumed to be controlled by the mode II stress intensity factor KII of the main crack and the difference between the compressive remote normal stress parallel with the crack plane (σ11∞) and the compressive remote normal stress perpendicular to the crack plane (σ22∞).