Failure Of Geologic Materials Under Low-Cycle Fatigue In Tension ? Introduction

The term "fatigue" in mechanics, is used to refer to reduction in the strength of a material subjected to fluctuating or alternating stresses. Stresses that vary with time are called fluctuating stresses and if during fluctuation the stresses also change sign, they are called alternating stresses. In the field of rock mechanics, the term fatigue (static fatigue) has often been misused to refer to failure of geologic materials in time domain under constant load. To avoid confusion, we use the ten cyclic fatigue here to refer to failure of rock under alternating or fluctuating stresses. Fatigue in metals has been studied for over 100 years. Fatigue properties of geologic materials have, however, received little attention in the past. This is probably because until recently very little was known in regard to the behavior of geologic materials even under static loading. Secondly, suitable equipment for conducting cyclic fatigue experiments was not available until quite recently. A few cyclic fatigue studies have been undertaken on geologic materials to relate it to percussive or rotary drilling mechanisms (e. g. Grover et a11*; Burdine2). The results of these studies were inconclusive due to limited effort. Cyclic fatigue in concrete has been studied more extensively than geologic materials (e.g. Nordby3, Gray et al 4 and Bennett et a15) and mechanisms of fatigue in concrete are better understood than in geologic materials.