Shear fracture (Mode II) toughness measurement of anisotropic rock

Fracture toughness of anisotropic shale is an important parameter in shale-gas exploitation technology. Currently available literature is mainly focused on Mode I fracture toughness (KIC) of anisotropic rock under tensile loading. Although there are the cracked straight through Brazilian disk and crack ring disk used to determine Mode II fracture toughness (KIIC) of anisotropic rock under pure shear stress, their fracture trajectories are deviated from the original crack plane and cannot be regarded as the true Mode II fracture. In this paper, shearbox test was firstly adopted to measure KIIC of anisotropic shale. New physical factors YI0 and YII0 were proposed to describe effects of both the geometry and the material parameters on stress intensity factors (SIFs) of the original crack plane (KI0 and KII0) and to derive the calculation formulae of SIFs on arbitrary (KI theta and KII theta). Calculated results show that KII theta reaches its maximum absolute value at theta = 0-10 degrees where KI theta is negative, which promotes occurrence of Mode II fracture. The predicted planes of Mode II fracture agree well with the tested fracture trajectories (amostly along the original crack plane). The KIIC is increased with increase of beta (beta = 0-90 degrees). KIIC is 3-4 times as large as KIC and can be regarded as the true Mode II fracture toughness of anisotropic shale. The shear-box test is an effective method for measuring KIIC of anisotropic rock.

Automated summary

This study measured the Mode II fracture toughness of anisotropic shale using a shearbox test, proposing new physical factors to describe stress intensity factors on the original crack plane. Results showed that the predicted Mode II fracture planes aligned well with tested fracture trajectories and the toughness increased with a certain angle, providing valuable insights for shale-gas exploitation technology.