ACKNOWLEDGEMENT

ABSTRACT

Each variable known to affect laboratory measurement of fracture-energy and fracture-toughness is reviewed. Specific examples are cited where each of the variables have been isolated.

I thank Lindamae Peck and Robert B. Gordon for helpful discussion. The research for this paper was done while the author was in the Department of Geology and Geophysics at Yale University. The research was supported by the Division of Engineering, Mathematical and Geoscience Office of Basic Energy Science, Department of Energy, under Contract No. GDE AC-O2-79ER10 445.

INTRODUCTION

Fracture-energy is the energy consumed in producing a unit of crack surface. Fracture-toughness is the value of the stressintensity factor (also referred to as K) at which crack growth commences. Compilations by Ouchterlony (1980) and Barton (1982) reveal considerable variation in measured values of fracture-energy and toughness, even for the same rock. For example, the fracture energy of Indiana limestone measured in eight independent studies under STP conditions ranges from 16 to 230 J/m² (Barton, 1982). Yet within each one of the eight studies the variation is less than 20%. Some variation is expected, for rock is a heterogeneous aggregate material, but the variation between studies is more likely due to the diversity of test conditions. The few studies where each of the variables has been isolated are cited below. Effect of Specimen Configuration and Loading Geometry Specimen configurations and loading geometries that have been used to measure fracture-energy or toughness of rock are shown in figure 1. For a linear-elastic, homogeneous, isotropic material, specimen configuration and loading geometry should not affect the measured fracture-energy and toughness. For rock, which is not such a material, fracture-energy and toughness may not be independent of specimen and loading geometry. For example, in theory the angle of the wedge used to drive the crack in the double-cantilever beam specimen (see figure 1), should not influence these properties. However, the fracture energy of Sioux quartzite is found to depend on the angle of the wedge used to split a double cantilever beam test specimen (Peck & Gordon, 1982). The fracture energy increases 20 percent with decrease in wedge angle from 100º to 10º for cracks propagated at the same velocity. They suggest that the greater component of compressive load accompanying a high wedge angle may open microcracks and thereby reduce the energy needed to propagate the primary crack. The fracture-energy and fracture-toughness of rock increase by as much as 50% during the initial increments of crack growth and then become independent of crack length, see Hoagland et al. (1973a & b), Schmidt (1976), Schmidt and Lutz (1979), Ingraffea and Schmidt (1978), Barton (1981), and Peck (1982). The increasing energy probably is related to the formation of the process zone of microcracks observed alongside of, and ahead of, the advancing crack, whether the crack grows from a notch or from a preexisting fatigue crack. Examples of the length of crack propagation beyond which fracture-energy or toughness is independent of length are cited in Table I.

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