The Effect of Flaw Geometry and Infilling on Crack Initiation, Propagation, and Coalescence of a Parallel Flaw Pair in Marble

A series of uniaxial compression tests were conducted on unfilled or filled marble specimens with a parallel flaw pair to study the effect of the flaw geometry and infilling on the crack coalescence pattern. Special attention was paid to the crack initiation, propagation, and coalescence processes, which were traced by the CCD camera and digital image correlation (DIC) technique. The experimental results have shown that the appearance of the white patch and strain localization band precedes the initiation of macroscopic crack. The tensile crack evolves from a narrow strain band or white patches with clear boundaries, while the shear crack evolves from a wider strain band or white patch with blurry boundaries. The high-strength fillers suppress the initiation and propagation of the tensile cracks (i.e., T1, T2, and T3) and promote the appearance of the coplanar shear cracks and mixed tensile-shear cracks (i.e., M1, M2, and S1). The variation in the infilling condition leads to the difference in the fundamental crack types, hence resulting in different coalescence patterns for a given flaw geometry. For small rock bridge angles (i.e., β = 30°, 45°), no coalescence and indirect coalescence tends to occur between the open and gypsum-filled flaws, while shear coalescence is prone to occur between the cement- and resin-filled flaws. The coalescence patterns are dominated by the flaw geometry for medium rock bridge angles (i.e., β = 60°, 90°), but the crack coalescence stress and peak strength of specimens are greatly affected by the infilling conditions. For large rock bridge angle (i.e., β = 120°), the increase in the strength of the filler tends to generate straighter wing cracks for the tensile coalescence at the rock bridge and promotes the initiation of mixed tensile-shear cracks for additional coalescence outside the rock bridge.