Crack Initiation, Propagation, and Coalescence Experiments in Sandstone Brazilian Disks Containing Pre-Existing Flaws

Table 2

Crack propagation patterns and characteristics in pre-cracked specimens.

Flaw types in tested specimens	Patterns and characteristics
β = 0	1. Wing cracks were initiated from the middle portion of the pre-existing crack in the direction of the maximum compressive stress; 2. Specimens fractured and split away due to the indirect tensile effect like uncracked specimens in conventional Brazilian tests; 3. No secondary cracks
β = 15°, 30°, 45°, 60°, 75°, and 90°	1. Wing cracks were initiated from the tips of the pre-existing crack in the direction parallel to the maximum compressive stress; 2. No secondary cracks
Forward echelon crack system; Inverse echelon crack system; Two parallel cracks	1. Wing cracks were initiated from the tips of the pre-existing cracks in the direction of the maximum compressive stress; 2. Secondary cracks were initiated after wing cracks; 3. Cracks coalescence paths appeared between the two pre-existing cracks
Three parallel cracks	1. Wing cracks were initiated from the tips of the pre-existing crack in the direction parallel to the maximum compressive stress; 2. No secondary cracks
Crack system with barrier cracks	1. Wing cracks were initiated from the tips of the pre-existing crack in the direction of the maximum compressive stress; 2. Crack propagation of the pre-existing crack was not blocked by the barrier cracks; 3. Crack propagation paths have changed after they passed through the barrier cracks