Influence of Bedding Planes on Mode I and Mixed-Mode (I–II) Dynamic Fracture Toughness of Coal: Analysis of Experiments

Abstract

To determine the influence of bedding planes on pure mode I and mixed-mode I–II dynamic fracture toughness (DFT) and crack propagation characteristics of coal, a modified split Hopkinson pressure bar (SHPB) system is used to test notched semi-circular bend (NSCB) specimens. The DFT is calculated by the finite element code Abaqus. Two strain gauges are used to measure the crack propagation velocity and the crack propagation path is recorded by a high-speed digital camera. The results show that bedding planes have significant influences on the effective DFT and crack propagation characteristics of coal. As the bedding-plane angles increase (meaning the geometric positional relationship of the loading direction and the bedding-plane direction transforms from perpendicular to parallel), the effective DFT and peak force decrease. The bedding planes affect the crack initiation direction, and the crack propagation path is jointly determined by the direction of maximum principal stress and the bedding planes. For pure mode I, the crack propagation velocities rise with the increase of bedding-plane angles. However, for mixed-mode I–II, the largest velocity is at the bedding-plane angle of 45°. Moreover, the largest loading velocities at the linear stage of the loading history all occur at 45° and decrease rapidly thereafter.