Crack coalescence in specimens with open and closed flaws: A comparison

A comparison is presented between experimental observations made from gypsum specimens loaded in uniaxial compression, with open and closed flaws. Three types of cracks are observed: wing cracks, coplanar and oblique secondary cracks. Wing cracks are tensile cracks that initiate at or near the tip of the flaws, are stable, and propagate towards the direction of maximum compression. Secondary cracks are shear cracks; they are initially stable and may become unstable near coalescence. Coalescence is produced by the linkage of two flaws by a combination of wing and secondary cracks. When coalescence occurs through wing cracks or a combination of wing and secondary cracks, the process is stable. When coalescence occurs through secondary cracks only, the phenomenon is unstable. Eight types of coalescence have been identified, which apply to specimens with open and closed flaws. The types of coalescence are classified based on the types of cracks that produce linkage, and are closely related to the orientation, spacing, and continuity of the flaws. Interestingly, coalescence produced through the linkage of shear cracks only occurs at higher stress than coalescence produced by a combination of shear and wing cracks. The smallest coalescence stress occurs when produced only by wing cracks. The main difference between experimental results from open and closed flaws is that initiation stresses and coalescence are higher for closed than for open flaws. This is explained by the friction along the closed flaws, which needs to be overcome before a crack can initiate, and also by the capability of closed flaws to transmit normal stresses. Thus the differences are not fundamental, and so a unified conceptual framework and a common terminology are proposed for fracturing phenomena in brittle materials with open or with frictional pre-existing discontinuities.