Mixed mode crack propagation in quasi-brittle materials

Crack propagation in rectangular blocks of mortar containing a central notch and subjected to uniaxial compression were studied. Four different notch orientation angles (18°, 36°, 54° and 72°) with respect to the loading direction were used. Holographic Interferometry was used to observe crack initiation and propagation. It was possible to detect crack extensions during the experiments, hence the load vs crack extension curve for each notch orientation angles could be obtained. A separate Holographic Interferometry method was used to measure crack opening and sliding displacements by making four independent observations of holographic fringes from the same holographic plate. Crack initiation theories were employed to study their relative merits for predicting crack initiation angles and loads. A Finite Element Method (FEM) using quarter point singular crack tip element was used to calculate Stress Intensity Factors (SIF) and crack surface displacements for different inclinations and extensions of the propagating cracks. It was found that while crack initiation was predicted well by some of the theories, it was necessary to account for the traction forces on the crack surface before any propagation criterion could be identified. Opening and sliding of the crack faces determined by Holographic Interferometry (HI) and clip gage measurements were used to find the normal and shear traction applied to the propagating crack faces. The SIFs for the traction free cracks were corrected by taking into account normal and shear tractions along the propagating crack. It was concluded that the Maximum Hoop Stress Criterion was reasonable and K_I stress intensity factor at the tip of the propagating crack was dominant in the failure mechanism.