Experimental investigation into the damage evolution of sandstone under decreasing-amplitude stress rates and its implications for coalbed methane exploitation

Coalbed methane (CBM) in the overlying strata of coal mines is recognized as an important alternative clean energy source. Under constant and cyclic loading of overlying strata, the mechanical response of sandstone is vital in determining the best stress-relief CBM exploitation position. In this paper, cyclic uniaxial compression tests were conducted in sandstone subjected to different decreasing-amplitude rates. First, the irreversible strain shows a rapid increase and a steady trend in this specific unloading path. The stress–strain response shows that the cumulative damage in the cyclic-unloading stage is not apparent when compared with constant-amplitude and increasing-amplitude loading and is not likely to lead to failure. Then, the gap between the loading and unloading elastic moduli is narrowed at higher stress levels as the cycle number increases. However, these property gaps are not apparent at lower stress levels. Therefore, obvious boundaries may exist between different stress stages. In addition, with a decrease in input energy, the proportion of elastic energy shows a clear linear-decreasing trend. The plastic energy illustrates the opposite trend. The energy proportion is a good indicator for unloading under the constant-fatigue loading path and can describe the procedure well. This loading path is determined to be nonlinear based on other damage indicators, such as elastic moduli and irreversible strain. These results imply that at a higher position in the fractured zone, the overlying strata damage is more evident than observed elsewhere under the same disturbance.