Testing of Mode-I Fracture Toughness of Sandstone Based on the Fracturing Mechanism of an Explosion Stress Wave

The dynamic fracture toughness of rock materials under blast load is the basis for studying the stability of rock blasting engineering. To study the dynamic fracture characteristics of sandstone with an explosive load, the initiation and propagation times of pre-crack samples with different lengths were obtained through physical experiments. The experimental-numerical method was used to obtain the stress intensity factor (SIF) curve and the crack initiation toughness of the specimens with different pre-crack lengths. By comparing and analyzing the dynamic SIF and explosion load curves at different pre-crack lengths, the following results and conclusions were obtained: (1) a polyvinylidene fluoride pressure gauge effectively measured the blast pressure curves at the blast hole wall and the blast hole with a diameter of 40 mm in the presence of a coupling water medium that could effectively avoid the formation of crushed zones and reduce the attenuation of the blast energy. (2) The crack-tip SIF curves were well correlated to the blast pressure curves with a minimal SIF lagging of 0.3-2.9 mu s after the peak of the blast pressure curve and a maximum SIF lagging of 0.1-22.9 mu s after the time when the peak blast pressure decayed to zero, and the lagging time increased with the pre-crack length. (3) The crack was affected by the reflected wave during the propagation process, and the reflected wave did not affect the crack propagation within 0-109.8 mu s. The longer the pre-crack length was, the longer the reflected wave influence time was. (4) The SIF at the pre-crack tip was controlled to a degree such that the first wave was superposed on the surface wave of the crack surface. Both the peak value of the stress intensity factor curve and the crack initiation toughness value decreased with the increase of the pre-crack length, and an obvious separation inflection point appeared on the stress intensity factor curve with the increase of the pre-crack length. (5) The implications of these findings particularly regarding the test method for dynamic fracture of materials tested by explosion method are discussed.