Experimental research on pre-cracked marble under compression

An experimental approach on propagation and coalescence of pre-existing cracks (fractures) in marble samples under compression is carried out. Two types of newborn cracks are observed: wing (tensile) cracks and secondary (shear) cracks. Both types of cracks initiate from the tips of the fractures and propagate in a stable manner. Wing cracks initiate at an angle with the fractures and tend to propagate towards the direction of the most compressive stress. Secondary cracks, however, initiate in a direction quasi-coplanar to the fractures and also parallel to the wing cracks but in the opposite direction. The orientations and geometries of fractures can decide which kind of wing cracks and secondary cracks will be produced. The interactions between fractures strongly affect the initiation and propagation of wing cracks and secondary cracks, and make examples’ failure patterns thoroughly different. The initial fracture angles of wing cracks range among 52°–68°, which are lower than the results (about 77°) of previous experiments on molding materials. In addition, the characters of acoustic emission (AE) of pre-cracked marble samples are also studied. For some rock samples, a secondary peak value phenomenon of AE counts is observed. For the samples with zigzag fractures, the accumulated AE counts at the moment of failure occupy almost half of the summation of whole load processes. This reveals that the failure of materials with zigzag fractures is more unexpected.