Effects of loading rate and bedding on the dynamic fracture toughness of coal: Laboratory experiments

In order to study the influence of bedding angle and loading rate on the initiation fracture toughness (IFT) of coal, notched semi-circular bending (NSCB) specimens were made by using a split Hopkinson pressure bar (SHPB) system. The dynamic IFT of NSCB coal specimens with various bedding angles and crack lengths under different loading rates were analyzed and discussed. A high-speed high-resolution digital camera was used to record the processes of cracking initiation and propagation in coal specimens. The experimental results indicate that the effects of bedding plane on the cracking pattern decreases with the increase of both loading rate and crack length in the specimens. The impact velocity dominates the evolution of the dynamic IFT of coal, and the anisotropy caused by bedding planes in the specimens plays a more critical role in the evolution of dynamic IFT than the crack length. The effects of bedding angles on the dynamic IFT decrease with the increase of impact velocity. The growth behavior of dynamic IFT of coal distinctly changes after the loading rate exceeds a critical value. In addition, a model of rate dependency was established to describe the dynamic IFT of coal.