Size-dependent fracture behavior of Guiting limestone under mixed mode loading

The effect of specimen size on mixed mode fracture resistance of a sedimentary soft rock (Guiting limestone) has been investigated. Two types of specimen have been used: edge cracked semi circular bend (SCB) specimens under three-point bend loading and cracked Brazilian disk (BD) specimens under diametral compression. Each type of specimen was tested under different mode mixities from pure mode I to pure mode II. In addition, tests were performed on different sizes of specimen characterized by four specimen radii from 25 mm to 150 mm. The experimental results revealed that the mixed mode fracture resistance increased with specimen size. To predict the size dependent fracture resistance of Guiting limestone under mixed-mode loading, a modified form of the maximum tangential stress (MMTS) criterion was employed. The MMTS criterion takes into account the effects of the second and third terms in the Williams series solution for the tangential stress around the crack tip and considers the critical distance from the crack tip to be a function of specimen size. It is shown that the MMTS criterion can predict the fracture behavior of Guiting limestone by considering the simultaneous effects of specimen size and geometry.