Experimental and numerical investigations of impact splitting of naturally bedded layered slate rock

Alam, Md. Rabiul (2005) Experimental and numerical investigations of impact splitting of naturally bedded layered slate rock. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In this thesis investigation, experimental and numerical studies were carried out to determine the axial impact splitting loads and the splitting process of naturally bedded layered slate rocks. This effort was undertaken to examine an alternate slate mining procedure so that the wastage in slate can be reduced from the present-day 95%. This was the first effort made in the use of this procedure for slate excavation. -- Experimental impact forces were determined by carrying out splitting tests on different sizes of slate blocks from small to intermediate scales, using a hydraulic actuator with a wedge-shaped indenter in the laboratory. Generally the slate blocks split parallel to their natural bedding planes, exhibiting clean and even split faces. Thus the use of this procedure for slate mining will minimize the damage produced by the use of optimum explosives; this reduction in wastage has been approximately estimated to be nearly 20% to 25% less than the present-day 95%. It has been observed in some cases that breaking loads were not consistent with the sizes of blocks (probably due to the presence of preexisting flaws within the block). Some of the larger blocks needed lesser loads to break than those found for relatively smaller blocks. -- Numerical analyses were carried out to predict the impact splitting loads required to split a finite sized layered slate rock based on the observed mechanism of plane crack propagation in plane strain and three-dimensional slate blocks, obtained from impact splitting of slate blocks in the laboratory. In order to characterize the material for numerical analysis, properties of the slate material, such as elastic moduli, Poisson's ratio, compressive and tensile strengths, coefficient of friction (between slate and steel), plane strain fracture toughness, hardness, crack propagation velocity etc., were determined after carrying out extensive experimental investigations. The experimental values obtained in this study were generally within the ranges specified for slates of other origins. It must also be mentioned here that the results presented in this thesis on the physical and mechanical properties of slate are the first study carried out on slates of Newfoundland origin. -- ABAQUS finite element software was used to simulate the entire wedge impact splitting process during numerical analysis. Mode I (plane strain opening mode) dynamic crack propagation was simulated numerically by the sequential releasing of the restraining node on the symmetric plane of the analyzed specimen. Mode I stress intensity factors (KI) were determined for different crack lengths and two types of crack propagation velocities. It was found that the crack propagation velocities had little (less than 10%) or almost no effect on the variation of the mode I stress intensity factors (SIPs). The possibility of crack propagation was determined by comparing KI with the plane strain fracture toughness (KIC). For a chosen slate block, the mode I stress intensity factor was obtained to be almost the same as its fracture toughness, when the experimental impact splitting load was applied on it. Also in a three-dimensional splitting scenario, a parabolic crack front was required to model an equal fracture toughness (or stress intensity factor) value all along the crack front. -- Numerical results on the effect of size of the specimen (width and depth) on the mode I stress intensity factor were also examined and a correlation equation was established between the mode I stress intensity factor, splitting force and plane strain geometry of the slate block. Breaking loads calculated using the correlation equation for plane strain fracture toughness were compared with those obtained from experimental work. The differences between the numerical and experimental impact forces were found to vary between +0.78 to -32.34%. A field problem, considering the separation of a large sized slate slab from its intact state, was also analyzed numerically. The breaking load obtained from this study was compared with those obtained from the developed correlation equation. A good agreement (difference was less than 15%) was obtained between them. A discussion is also given concerning the possible reduction of wastage by the use of this process as a slate mining procedure.

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