Strain rate effects on the crack propagation speed under different loading modes (I, II and I/II): Experimental investigations

Studying rock behavior under the effect of dynamic loads is one of the practical issues in engineering projects that has been considered by many researchers in last decades. Experimental measure of crack propagation speed under three loading modes (mode I, mixed mode I/II and mode II) and under a quasi-static and dynamic strain rate is still a big challenge due to the difficulties associated with measuring this parameter. Taking into account the high strength of hornfels rock and its extensive applications in the scientific and practical life, series tests were conducted on rectangular specimens of this rock type under a quasi-static and dynamic strain rate by using a servo hydraulic machine and drop weight machine, respectively. A special electric circuit was designed in this study to investigate the effect of the loading mode and strain rate on the crack propagation speed. Crack propagation speed was measured under six strain rates (6 x 10(-4), 6 x 10(-3), 6 x 10(-2), 1.9 x 10(+1), 3.8 x 10}1 and 5.6 x 10(+1) S-1) and under three loading modes. The results showed that the crack propagation speed depends on the loading mode and strain rate. By increasing the strain rate from 6 x 10-4 to 5.6 x 10(+1) S-1, the average crack propagation speed under pure mode I, mixed mode loading I/II and pure mode II increases by 12, 4.6 and 4.1 times, respectively. The crack propagation speed under pure mode I and mixed mode loading I/II almost has the same increment rate. But the increment rate of the crack propagation speed under pure mode II is slightly larger. For pure mode I, no significant changes were observed on the crack propagation path by increasing the strain rate. For mixed mode I/II and pure mode II, the crack propagation path under a quasi-static strain rate was a traditional concave-up path. While under a dynamic strain rate, the crack tended to grow through a straight or slightly concave-down path.