Experimental and numerical studies on dynamic crack growth in layered slate rock under wedge impact loads: part II – non‐plane strain problem

Abstract

A B S T R A C T Dynamic crack propagation in non-plane strain (or 3D) slate blocks under wedge impact loads was investigated numerically in this part of the paper. A parabolic-shaped crack trajectory was taken into consideration to model the crack propagation in slate blocks for analyzing the impact splitting of layered slate rock. Major and minor axes of the parabola were determined from the condition of equal mode I stress intensity factors (SIFs) along the crack front. Mode I SIFs were determined for experimental breaking loads for each increment of crack growth in a manner similar to that mentioned in part I of this paper. These values were compared with the plane strain material fracture toughness value obtained from experimental studies and very good agreement was obtained between them, for the case of actual load applied on the specimen. Numerical analysis of a field problem, i.e., separation of a large-sized slate slab from the rock strata in a slate quarry using wedge impacting, was also carried out in this paper. It can be observed that a large magnitude of load is required to break large-sized slate blocks; but this load is applied through a number of smaller load-capacity actuators-in-parallel, requiring large power capacity for the hydraulic pumps. However, this required power could be reduced considerably if the load applied on the line of hydraulic actuators is cascaded across the (line of) actuators (starting from centrally placed actuators) with a small time delay (equal to the initial crushing time in slate rock).These studies were motivated by the need to estimate theoretically the required in-plane impact loads for splitting layered slate rocks; in addition, the design of the necessary machinery/equipment to apply that load was also required in this study. The experimental testing of large-scale specimens will generally be beyond the reach of a researcher working in a medium-sized research laboratory. The other way to do the same is to verify and establish a numerical methodology from the results of small-scale experimental investigations and then to apply the same numerical procedure to compute the approximate breaking loads for large-sized slate blocks. From these computed breaking loads, the field equipment can be designed and fabricated.Two-dimensional and their equivalent plane strain (3D) finite-element analyses of the dynamic crack propagation of slate blocks, broken experimentally under plane strain conditions (transverse length of the block was equal to the transverse length of the wedge), were reported in part I of this paper. However, the prevalence of plane strain conditions for a rock breaking process is rather a rare event. Generally the loading area, or the extent of line load, would be much smaller than the length of the slate rock over which the failure of slate block is to be initiated. This leads to a 3D loading condition. Therefore, it is necessary to analyze large-sized slate blocks numerically as well. In part II of this paper, laboratory test results, and numerical analy...

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