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Hard rocks are significantly weakened or fatigued when subjected to cyclic loading. In uniaxial tension and in uniaxial compression the fatigue strength for 105 cycles is 60 to 80 percent of the monotonic strength. In triaxial compression the fatigue strength rises as the confining pressure is increased. By far the most damaging cyclic load is the tension-compression type, its fatigue strength reaching 25 percent of the monotonic tensile strength. The strain-time behavior is not unlike that observed in static creep, with primary, steady-state, and tertiary stages. The accumulated cyclic creep in uniaxial compression for different upper peak cyclic stresses appears to be bounded by the complete stress-strain curve. Acoustic emission and specimen photomicrography suggest microfracturing as the principal mechanism of fatigue failure, with distinct differences between cyclic compression and cyclic tension. Acoustic emission is an excellent precursor of imminent cyclic failure. ASTM standardization of cyclic testing is recommended.
rocks, rock mechanical testing, rock fatigue, acoustic emission, cyclic compression, cyclic tension, cyclic tension-compression
Professor of rock mechanics, University of Wisconsin, Madison, Wis.