An experimental study of the transition from tensile failure to shear failure in Carrara marble and Solnhofen limestone: Does “hybrid failure” exist?

Failure of brittle rocks under axisymmetric extensional loading, when the minimum principal stress is tensile, results in the formation of one or more opening-mode cracks that are oriented normal to the extension axis. When all the principal stresses are compressive, failure occurs through the formation of numerous grain-scale tensile cracks, which coalesce into a shear-mode fault that is inclined at >20° to the maximum principal stress. There have been few attempts to study the transition between these failure modes, particularly those that incorporate microstructural analyses. A transitional mode of failure, termed “hybrid failure”, is often proposed to describe the orientation of faults between these end-member loading conditions, but this concept has traditionally been based on a parabolic, “Griffith-type” failure envelope that describes the growth of the single most critically-oriented crack. By integrating axisymmetric rock deformation experiments with microstructural analyses, it can be shown that failure under these transitional conditions involves the formation of several transgranular tensile cracks, which coalesce into a shear-mode fault that is inclined at <10° to the maximum principal stress. The stress intensity at the tip of each crack interacts with adjacent cracks to produce a stair-step geometry along the failure surface. These are what may be appropriately considered “hybrid faults”, the angle of which systematically increase as the maximum principal stress, and thus the differential stress increases. There is, however, no evidence of a distinct “hybrid failure” mode, which involves the in-plane propagation of a single crack that is subject to a combined shear stress and tensile normal stress. The results of this experimental study provide novel insights into the mechanical behaviour of carbonate rocks and into the interpretation of meso-scale natural examples of failure under mixed stress-states.