Experimental Study of Opening-Mode Crack Growth in Rock. Technical Progress Report

摘要

Measurements of fracture strength have been made on rocks containing a wide range of minerals and microstructures. Elastic moduli determined from data recorded during the fracture experiments were used in the computation of the strain energy release rate. Measured steady-state fracture energies range from about 40 J/m exp 2 (Portland sandstone) to about 280 J/m exp 2 (Barre granite). While the energy of crack initiation and crack propagation depends on the strength of the constituent minerals, each is controlled primarily by the microstructure of the rock. The density of pores and microcracks and the degree to which they are interconnected are most important. The weakest rocks are those that have either a weak structural component that forms nearly continuous, non-branching surfaces within the microstructure (as in the Salem limestone) or a very high porosity (as in the Portland sandstone). The strongest rocks are polymineralic and have connected microcrack networks that permit formation of multiple crack paths through the structure (as in the granites). The fracture energy depends on crack speed and chemical environment. Water weakening ranges from a reduction of more than 50% of the strength in the Portland sandstone to nil in diabase, marble, and the Cardium sandstone. (ERA citation 08:019015)