Degradation and improvement of mechanical properties of rock under triaxial compressive cyclic loading

Abstract

An extensive experimental study on mechanical properties of a sandstone subjected to cyclic loadings are presented and discussed in this study. Cyclic loading test were performed in triaxial condition at confining pressure of 4 MPa. The mechanical properties of the rock were altered by cyclic loading; dependent upon the applied stress level during cyclic loading the rock may experience damage or hardening as a result of cyclic loading. It was identified that a critical maximum normalized deviator stress (i.e. deviator stress at the beginning of unloading normalized by rock peak strength) exists between 92-95% which defines the limit between hardening and damage behavior during cyclic loading. If the cyclic loading deviator stress is lower than this critical boundary, the peak strength my increase in postcyclic monotonic loading. This increase in peak strength is directly proportional to the normalized deviator stress in unloading; the amount of increase in peak strength decreases with a decrease in cyclic loading deviator stress. In cyclic damage tests, in general, the increase of Poisson’s ratio, v, and the degradation of tangent Young’s modulus, Etan, indicate the progressive accumulation of damage in the rock sample over the loading cycles. In cyclic hardening tests, however, v values for the cyclic hardening test remained fairly constant and Etan values slightly increased. Moreover, it was found that in cyclic damage tests the amount of axial and lateral strains cumulated during cyclic testing in much greater than those values for cyclic hardening tests.