Opening and mixed mode fracture processes in a quasi-brittle material via digital imaging

Fracture tests were performed on sandstone specimens under three-point bending with a variety of notch positions to achieve KII/KI = 0–12%. At peak load, the maximum crack opening displacement measured by digital image correlation was 45 μm under mixed mode loading and 30 μm under mode I, and these values were used to determine the length of the cohesive zone: 10–12 mm for mixed mode and 5–7 mm for mode I. For mixed mode fracture, the displacement in the cohesive zone was identified to be opening only, while sliding and opening were detected along the remaining length.