Study on variations of peak strength of a sandstone during cyclic loading

An experimental investigation was carried out on Hawkesbury sandstone samples to study the peak strength variations during triaxial monotonic and cyclic compressive testing. Axial load, confining pressures and axial and lateral deformations were measured precisely from start of loading until post-peak state. Cyclic tests were carried out at different stress levels and unloading amplitudes. All the tests were performed at 4 MPa confining pressure. A methodology to conduct cyclic loading testing was devised including the development of an innovative peak strength prediction method. The results confirmed that beginning of cyclic loading at higher stress will result in failure after fewer cycles. Moreover, it was found that if the specimen experience more cyclic loading the failure mode will be more brittle. The mechanical properties of the rock were altered by systematic cyclic loading; dependent upon the applied stress level during cyclic loading the rock may become weaker or stronger 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 93.7 and 94.1 % which defines the limit between peak strength hardening and weakening during cyclic loading. If the cyclic loading deviator stress is lower than this critical boundary, the peak strength increases up to 11 %. 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.