Experimental investigation of pre-flawed rocks under combined static-dynamic loading: Mechanical responses and fracturing characteristics

Widely existing in many fields of geotechnical engineering practices, rocks inherently containing pre-existing flaws frequently suffer from static pre-compression and dynamic disturbance as the same time. Understanding the mechanical properties and fracturing behaviors of flawed rocks under combined static-dynamic loading is of great importance for rock engineering applications. In this work, pre-flawed rocks are compressed under uniaxial combined static-dynamic loading using the modified split Hopkinson pressure bar (SHPB) system. The influences of flaw intensity and pre-stress on dynamic mechanical responses and progressive fracturing behaviors of pre-flawed rocks are systematically investigated. Experimental results indicate that the dynamic deformation modulus of rocks is not rate-dependent, while the dynamic strength and total strength perform evident rate-dependence. With increasing flaw intensity, the dynamic strength, total strength and deformation modulus show a decease tendency. With increasing pre-stress ratio, the dynamic strength deceases while the deformation modulus increases, and the total strength increases first before pre-stress ratio of 0.6 and then slightly deceases. Progressive fracturing behaviors are comprehensively analyzed by virtue of high-speed camera and digital image correlation (DIC) analysis. With increasing flaw intensity, more mixed cracks are initiated on specimen surface and the crack networks become more complex. With increasing pre-stress ratio, tensile-shear mixed cracks relatively increase while compression-shear mixed cracks relatively decrease. Generally, pre-flawed rock speci-mens failed into compression-shear mixed failure patterns. Fragmentation analysis shows that rock specimens are more fragmented under higher strain rate. The mean fragment size and fractal dimension generally increases with increasing flaw intensity. With increasing pre-stress ratio, the mean fragment size values perform "decrease-increase" tendency while the fractal dimension values perform "increase-decrease" tendency, with the turning point around 0.6.