Opening and mixed-mode fracture initiation in a quasi-brittle material

An electronic speckle pattern interferometry (ESPI) system, which delivers high-resolution displacement data from fringes that are formed by the subtraction of laser speckle patterns, was constructed to study fracture initiation in a quasi-brittle material. Mode I opening and Mixed-Mode I and II fracture experiments were performed with a homogeneous, fine-grained (0.1-0.8 mm grain size) sandstone using the three-point bending test. Specimens were notched at various lengths and positions of the beam edge to produce the desired loading condition, with K_II=K_I 50-13%. The experimental results indicate that the length of the localized damage zone at peak load for Mode I fracture is 6-7 mm, which is about 10 times the (largest) grain size. From the mixed-mode loading tests, the zone length at peak load increased to 10-12 mm, and the length was more or less constant for K_II=K_I 55-13%. ESPI also provided detailed information on the horizontal displacement profiles along the damage zone. For center notch specimens at peak load, the horizontal (opening) displacement at the notch tip was 40 μm, which can be interpreted as the critical opening displacement if the damage zone is fully formed at peak. For mixed-mode specimens, the critical horizontal displacement at peak load is 60-80 μm, but the vertical displacement is needed to resolve the critical opening and sliding components.