Experimental Study of Growth of a Pre-existing Interfacial Flaw using the Digital Image Correlation Technique

Rocks normally contain defects such as joints, cracks, and pre-existing flaws from small to large scales. The behavior of these defects is highly dependent on their geometrical characteristics and locations in the rock mass. In many cases, these cracks are located at the interface of two materials. Most of the investigations on interfacial cracks have been analytical and there are a few experimental studies available. In this study, crack initiation, growth, and coalescence of a pre-existing flaw in a rock-like material are investigated under uniaxial compressive loading. Specimens are made of two types of rock-like materials with different compressive and tensile strength. The flaw is placed at the center of specimens. Flaw width is equally distributed in both materials. To investigate the effect of geometrical parameters of the flaw on crack growth processes, rock-like specimens with different flaw angles were made. To show the repeatability of results, three identical specimens of each geometry were made. Digital image correlation technique was used for a detailed investigation of crack growth processes from initiation to failure. Nikon D7100 camera was used to take pictures of the specimen during loading. Ncorr software was used to analyze the pictures. The results showed that crack growth, coalescence, and failure pattern depended on pre-existing flaw geometry. By varying the angle of inclination from zero to 45°, the compressive strength decreased and then, increased with increasing the angle of inclination from 45 to 90°. Specimen with 45° flaw had the least compressive strength because theoretically maximum shear happens at this angle. For flaws with angles of 0 to 30 degrees, failures are caused by the growth of wing cracks. However, from 45 to 75 degrees, failures often occur with the propagation of the shear crack along the interface. At the angle of 90 degrees, a tensile crack is formed at the center of the sample along the interface. The results showed that crack growth and failure pattern depended on pre-existing flaw geometry.