Crack initiation, propagation and coalescence, from pre-cracked rock specimens, were detected in the laboratory using elastic wave transmission and reflection signals. The experiments were conducted on prismatic Indiana limestone specimens with two parallel pre-existing flaws subjected to uniaxial compression. Digital image correlation was used to monitor the cracking process around the tips of the flaws by imaging surface displacements. Compressional and shear wave pulses were transmitted and reflected continuously through the specimen while the uniaxial compressive load increased. The normalized amplitude of transmitted waves (shear waves with horizontal polarization) was observed to decrease with increasing uniaxial load, which was associated with the elastic deformations of the specimen. However, prior to tensile crack initiation, a large reduction in amplitude occurred. In addition, an additional decrease in amplitude was observed close to crack coalescence. These changes in amplitude occurred at least 1.3 MPa before the detection of damage by DIC imaging. The normalized amplitude of reflected signals (from the flaw tips) also increased significantly before the initiation of new cracks. These experimental results indicate that changes in transmitted and reflected seismic waves provide a potential method to detect crack initiation inside rock, as well as to determine the location of new cracks.
A large number of experimental studies have been conducted on crack initiation, propagation, and coalescence in pre-cracked brittle materials. In most of these studies, the pre-existing crack (flaw) has been subjected to mixed mode loading (mode I: opening or tensile and mode II: in plane shear) [1–11]. Two types of cracks have been observed in these experiments: tensile (wing) and shear (secondary) cracks, which are shown in Fig. 1. Tensile cracks initiate at or near the tip of the flaw. They propagate toward the direction of maximum compression and they are stable (i.e. further propagation of these cracks require application of additional load). These cracks are also characterized by a plumose structure on their surface. Shear cracks initiate from the tips of the flaws, they are initially stable, but they may be unstable close to crack coalescence or specimen failure. These cracks have been classified into two groups: coplanar or quasi-coplanar (making an angle of 45° or less with the flaw plane) and oblique (with an initiation angle larger than 45° with the flaw plane). The shear cracks are formed in areas of high compression and/or shear stress. They are characterized by pulverized material on the surface and by high roughness . Park and Bobet  performed laboratory uniaxial compression tests on gypsum specimens with two, three, and 16 parallel flaws. They identified eight types of coalescence based on different flaw geometries (see Table 1 in ). Observations in the laboratory have relied on visual inspection using optical magnification and high-speed cameras. While these techniques have been instrumental in the understanding of cracking phenomena at the macroscopic scale, what is needed, is a local or microscopic characterization of new cracks forming inside the brittle materials.