Determination of crack closure stress under constant-fatigue loading based on damage variable evolution

Abstract

Crack closure stress is a key stress threshold of rocks under various loading. The determination of which plays essential roles in deformation behavior and gas seepage characteristics of overlying strata. However, considering damage indicators evolution of rock under the constant-fatigue loading has rarely been studied under constant-fatigue unloading conditions. In this study, a series of uniaxial decreasing amplitude stress constant-fatigue tests were conducted on sandstone specimens to determine crack closure stress. The evolution of damage indicators depended strongly on the applied stress level. In addition, the influence of cycle numbers on the deformation and crack damage parameters was investigated. The results show that irreversible strain, elastic moduli, and load-unload response ratio show two-phase evolution characteristics. The energy evolution illustrates linear evolution when the stress level exceeds crack closure stress. However, when the stress level approaches crack closure stress, damage indicators show a three-phase evolution stage, and the energy evolution fluctuates greatly. Apart from those, the crack closure stress is a range rather than a specific value. It may be due to the progressive pre-existing or stress invasion cracks reopen. The proposed method can effectively complement existing methods to determine the crack closure stress and evaluate the stress relief extent, which is vital to improve gas drainage efficiency in overlying strata.