The b-value analysis of bedded shale under cyclic loading tests

Abstract

Research on the damage evolution of bedded shale which can be described by different scales of cracks from micro- to macro-damage is significant. The b-value representing the relative proportion of micro-cracking to macro-cracking could be a good index for estimating the damage process of granite, sandstone, and marble as most of the previous studies have reported. The variation of the b-value of shale specimens with different bedding plane orientations subjected to cyclic load was investigated here. To ensure the reliability of the determination of the b-values, both the least squares method and maximum likelihood estimate were applied. And the sample range of acoustic emission events in data processing was redefined so that the previously generated cracks as well as the current formed cracks in this time window could all be used to calculate the b-value which reflects the accumulation of damage in one loading cycle. Cyclic load tests on stress levels with the first peak load of 30%-50% and above 80% of the estimated uniaxial failure strength, respectively, were conducted for shale specimens with bedding planes parallel and perpendicular to the loading direction. A change trend of the b-value in each test is observed as decreasing for one load cycle around the failure strength as well as the cyclic loading process prior to the final failure. Thus, this phenomenon of shale, only appearing as the stress exceeds 80% of the failure strength, has neither a relationship with the bedding plane nor the loading paths.