Experimental Study on Degradation of Mechanical Properties of Sandstone Under Different Cyclic Loadings

An experimental investigation was carried out on the Hawkesbury sandstone to study changes in mechanical rock behavior (i.e., strength and deformability) during various cyclic loading conditions under uniaxial and triaxial testing situation. Axial load, confining pressures and axial and lateral deformations were measured continuously from start of loading until the post-peak state. Cyclic tests were carried out at different confining pressures, stress levels and unloading amplitudes. In cyclic loading tests, a relatively uniform accumulation in axial, lateral and volumetric strain is observed followed by a rapid strain increase as it heads toward failure. This rapid accumulation in strain occurred on average at approximately 65 % of the cumulative axial strain. During systematic cyclic loading a continuous degradation in the tangent Young's modulus, Etan, until beginning of large plastic deformation was observed. The rate of stiffness decrease increased rapidly during few cycles before the failure. Poisson's ratio showed a similar trend of behavior and increased continuously with cyclic loading. Moreover, Etan values measured during interval cyclic loadings were affected by two factors: (1) stiffening by cyclic loading because of elasto-viscoplastic properties; and (2) damage effects that have taken place during cyclic loading, in particular, if cyclic loading begins close to the monotonic peak strength. Depending to the stress level at initiation of cyclic loading with respect to the monotonic peak strength, either of these two factors could become significant and affect stiffness of rock during or after a cyclic loading. Finally, it was found that damage increases with an increase in unloading stress level and unloading amplitude, and fatigue life of a confined specimen is longer than a sample without confinement.