Two types of cracks are observed in brittle materials in compression: wing cracks and secondary cracks. Wing cracks are tensile cracks that initiate at the tips of pre-existing cracks (flaws) and propagate in a stable manner towards the direction of maximum compression. Secondary cracks are shear cracks that also initiate from the tips of the flaws and propagate in a stable manner. Numerical simulations with the code FROCK, a Hybridized Indirect Boundary Element Method, show that: (1) the initiation of a kink does not eliminate the stressingularity at the tip of the flaw; (2) the stressingularity is eliminated only by a coplanar shear crack with frictional characteristics identical to those of the flaw. A modification of the initiation model of Bobet & Einstein (1998b) is capable of predicting initiation of the tensile crack and of the shear cracks observed in the laboratory.
In rocks and other brittle materials, two types of cracks are observed in compression: (1) wing cracks; and (2) secondary cracks (Fig. 1).
[Figure 1. available in full paper]
Figure 1. Crack Pattern in Uniaxial Compression Wing cracks are tensile cracks that initiate at the tips of pre-existing cracks (flaws) and propagate in a stable manner towards the direction of maximum compression. Secondary cracks initiate also from the tips of the flaws, propagate in a stable manner, and have been recognized by many researchers as shear cracks. In our experiments (Sagong & Bobet (2000)), we have found two possible directions of shear (secondary) crack initiation (Fig. 1): (1) coplanar or quasi-coplanar to the flaw; and (2) oblique to the flaw, parallel to the wing crack but in the opposite direction. These observations are consistent with results from other investigators such as Lajtai (1971, 1974), Petit & B arquins (1988), Jiefan et al. (1990), Chen et al. (1992), and Bobet & Einstein (1998a) on a variety of brittle materials.
In specimens with multiple flaws, coalescence is produced by the linkage of two flaws through a number of combinations of tensile and shear cracks. A total of eight types of coalescence have been observed (Bobet & Einstein (1998a), Sagong & Bobet (2000)); see Figure 2. Coalescence Type I occurs when the two flaws are coplanar or almost coplanar. In this case the linkage of the flaws is through the connection of the two internal (internal refers to cracks located between the two flaws) coplanar secondary cracks which, although stable after initiation, grow unstably near coalescence until they link. Coalescence Type II is produced by an out of plane propagation and linkage of the two internal coplanar secondary cracks; the secondary cracks first propagate "in plane" and in a stable manner up to some point; afterwards, they become unstable and propagate outside their own plane as a tensile crack producing coalescence.
