Coalescence of two parallel pre-existing surface cracks in granite

• Cracking mode on specimen surface is identified by the digital speckle correlation method.
• Petal crack plays an important role in surface crack development.
• Coalescence between surface cracks occurs both on surface and inside of specimen.
• Four surface crack coalescence patterns are classified.

This paper studies experimentally the coalescence mechanism between two parallel three-dimensional (3-D) pre-existing surface cracks in granite specimen under uniaxial compression. The bridge angles between the two pre-existing cracks vary from 0° to 135°. The digital speckle correlation method (DSCM) is used to analyze the captured images producing strain fields during the cracking process, and thus reveals the mode of cracking (either tensile, shear or their combination). Petal crack that initiates from internal crack fronts of the pre-existing surface cracks plays an important role in crack initiation, propagation and coalescence. Microcracks underneath the specimen surface appear as white patches on the surface, which always appear preceding the appearance of surface macrocracks (either wing or anti-wing cracks). Cracks, which grow in a direction opposite to that of wing cracks, were consistently observed before the appearance of wing cracks; and these cracks are therefore called “anti-wing cracks”. In addition, DSCM results suggest that wing crack and anti-wing crack are tensile during initiation but may subsequently turn into mixed mode. The coalescence between the two 3-D pre-existing surface cracks takes place both on the specimen surface (through wing, anti-wing cracks or secondary cracks) and inside the specimen (through internal petal cracks). Depending on the bridge angle β between the two pre-existing cracks, there can be no crack coalescence, coalescence in forms of tensile cracks (tensile mode), coalescence by mixed mode cracks (mixed mode), and finally coalescence in forms of a secondary crack jointing an initiated wing or anti-wing crack, emanating from one tip, to the other tip (secondary crack mode).