Crack initiation and path selection in brittle specimens: A novel experimental method and computations

We present a novel experimental method aiming at investigating aspects of dynamic crack propagation in brittle materials under in-plane, quasi-static, mixed mode loading. The method consists in gluing a precracked specimen into a rectangular hole in an aluminum frame using thin layers of epoxy resin. The driving force for crack initiation and propagation lies in the mismatch between the coefficients of thermal expansion (CTE) of the aluminum frame and the specimen, following modest heating of the assembly on an electrical heating stage. The main advantages of this method are in its avoidance of gripping problems and of the need to employ a complicated loading device. An important benefit of this method is the ability to analyze, numerically, the assembly containing the specimen as a boundary value problem by means of finite element analysis without any prior assumptions regarding the boundary conditions. The method enables investigation of various aspects of dynamic crack propagation in brittle materials, including crack initiation, crack path selection criteria, and surface instabilities under a relatively low energy-speed regime. To validate the method's applicability, we first evaluated the fracture toughness, K _IC, of soda lime glass specimens. We then performed fracture experiments of slow and fast crack propagation in these specimens under combined tensile and shear stresses, which revealed the paths selected by the cracks. These paths were calculated using quasi-static finite element analysis (FEA), code Franc2D, and the dynamic eXtended FEA Method, using the criteria for crack path selection. It was found that the crack paths obeyed the law of local symmetry (K _II=0) for both the quasi-static and dynamic crack propagation.