Use of double edge-cracked Brazilian disk geometry for compression-shear fracture investigation of rock

Abstract

A new specimen geometry—the double edge-cracked Brazilian disk and a relevant fracture analysis by weight function method are proposed for the investigation of rock fracture caused by compression-shear loading. Not only can the mixed mode fracture with any ratio of K_I/K_II be achieved, but also the pure mode II crack extension can be obtained. The combined mode fracture analysis for this geometry shows that diametral compression in the farfield can induce a compression-shear stress state in the singular stress field ahead of crack tips. Experimental investigations conducted on marble specimens show that the pure mode II crack extension can be obtained when the dimensionless crack length a>0.7 and the inclined crack angle 5°⩽ϕ⩽40°. Normalized mode I and mode II stress intensity factors decrease from −0.45 and 2.47 at ϕ=5° to −1.65 and 1.52 at ϕ=40°, respectively. The strains at three points of specimen are also measured in order to investigate the influence of stress singularity on initial crack extension. The results show that the principal orientations of strain at three points are very stable in the loading process. The derived formulae are quite explicit, and the specimen geometry is easy to fabricate and convenient to achieve the pure mode II crack extension. Therefore, it can hopefully be used to obtain mode II fracture toughness of rock.

Keywords

rock fracture / stress intensity factor / weight function / cracked Brazilian disk