ABSTRACT

ABSTRACT: The mechanical behavior of hollow cylinders of Niagara dolomite under cyclic internal pressurization was investigated using a computerized testing system. This loading configuration approximately simulates pressurized boreholes or tunnels, or annular caverns under electromagnetic forces of superconductive magnets, subjected to periodic internal pressure fluctuation. We used loading rates equivalent to 10 to 10,000 sec./cycle, approaching levels representing diurnal cycling. Test results suggest that expanding rock opening size weakens the fatigue resistance. Additionally, fatigue life of openings in Niagara dolomite depends both on the maximum cyclic stress level and the cyclic period. Life is reduced when the opening is subjected to higher cyclic stress or/and to longer cyclic periods. The total hysteresis energy, which represents the amount of energy dissipated during cyclic pressurization, and the total permanent tangential strain, are each linearly related to fatigue life when plotted on a logarithmic scale. A potentially useful method is suggested for estimating the fatigue life of circular openings under cyclic internal loading based on the measurement of the average permanent tangential strain per cycle.

1 INTRODUCTION

An understanding of the mechanical behavior of rock under cyclic loading is essential for the rational design and longevity estimation of underground openings subjected to fluctuating forces. The mechanical properties of rock under static loads have been studied by many investigators. But the behavior of rock under cyclic loading, which often causes failure of rock structures below their static strengths, has been largely neglected and is still not fully understood. Information concerning the mechanical response of rock under cyclic load is scarce due in part to the time consuming nature of such laboratory tests, the narrow scope of cases in which cyclic fatigue is of interest and, perhaps, due to a lack of awareness that the cyclic effect can be decisive. The mechanical behavior of rock under uniaxial and triaxial cyclic loading has been investigated at the University of Wisconsin for the last 15 years or so. These and other studies have established that cyclic loading in rock can bring about fatigue (the phenomenon of pre- mature failure at maximum applied stress level lower than the static strength). For any level of maximum applied stress (defined as 'fatigue strength', S, and expressed as the ratio of this maximum stress to the corresponding static strength) above a threshold characteristic of the rock there is a corresponding number of cycles (defined as 'fatigue life', N) that can bring about fatigue failure. Experimental results show that in uniaxial compression rocks can fatigue at S levels as low as 0.7 if loading is repeated hundreds of thousands times (Haimson and Kim, 1971); in uniaxial tension the same fatigue strength requires ten thousands cycles or so (Tharp, 1973); whereas in cyclic uniaxial tension - compression even a relatively low maximum compressive stress (one third of the compressive strength) accelerates the tensile fatigue mechanism, and an S level of 0.7 requires only several hundreds cycles to bring about failure (K. Kim, 1976). On the other hand, cyclic triaxial compression improves fatigue life by several orders of magnitude as compared with uniaxial loading, depending on the confining pressure (Haimson, 1974).

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