Experimental uniaxial compression loading tests and scanning electron microscope (SEM) tests are carried out on rock-like specimens containing single pre-existing cracks to study the mechanical properties and microscopic damage evolution. The present study has distinguished tensile or shear cracks based on different SEM observations on a micro scale. Specifically, six typical micro patterns are defined according to their geometry shapes, namely, flocculent, flaw, circle, flow, layered, and broken circle pattern. These micro patterns display distinct characteristics on structure surfaces, boundary lines, and the distribution of grain debris. Moreover, the microscopic damage of both tensile and shear cracks is quantitatively studied using the image post-processing technique. The damage evolution, which associates the macroscopic cracking processes, has been investigated. It is indicated that the microcracks develop from the pre-existing cracks prior to the initiation of any macroscopic observable cracks, and the damage is not rapidly accumulated after the initiation of both tensile and shear cracks.
Natural rock contains discontinuities, including fractures, pores, and other defects, which govern the fracturing behaviors of the rock masses under loading. Numerous theoretical, experimental, and numerical studies have been carried out to study mechanical properties of jointed rocks or other rock-like materials (Griffith, 1921; Brace and Bombolakis, 1963; Horii and Nematnasser, 1985; Bobet and Einstein, 1998; Wong and Einstein, 2009a; 2009b; 2009c; Park and Bobet, 2010; Zhang and Wong, 2012; 2013; Gonçalves da Silva and Einstein, 2013; Haeri et al., 2014; Yang et al., 2017; Zhao et al., 2018). In these researches, tensile and shear cracks are always be regarded as two basic crack types and fundamental of the rock mechanic. (Cheng and Wong, 2018). Bombolakis (1963) firstly observed the propagation of tensile wing cracks from straight cracks under uniaxial compression, which consists well with the Griffith theory. Lajtai (1974) carried out uniaxial compression loading tests on plaster of Paris, the results consist of five crack types, including both tensile and shear cracks. Petit and Barquins (1988) observed that shear zones develop extensively in addition to the occurrence of tensile wing cracks. Using scanning electron microscope (SEM), Sagong and Bobet (2003) investigated tensile and shear cracks in gypsum specimens on a micro scale. Li et al. (2005) conducted experimental tests on marble specimens, and they discovered two cracking phenomena: wing cracks and secondary quasi-coplanar cracks. Although the mechanical properties of these two cracks were not clearly identified by the authors, it is accepted that wing cracks are tensile cracks and secondary cracks are shear cracks. Wong and Einstein (2009a) systematically characterized the tensile/shear cracks which emanate from a single pre-existing crack. Seven different crack types (including three tensile types, three shear types, and one mixed type) were identified based on geometry and propagation mechanism. Subsequently, they studied the orientation of microcracking zones of the wing cracks (Wong and Einstein, 2009c). As a summary, the previous studies focused on the differences between tensile cracks and shear cracks in three main aspects (Cheng and Wong, 2018): First, the tensile/compressive stress concentration phenomenon around the pre-crack tips; Second, the initiation direction and propagation trajectories of observable cracks; Third, the microscopic observation of the crack surfaces.
