Establishing a fatigue life equation linking microplastic strain and strain hardening of freeze-thawed rocks

Abstract

Based on the characteristics of microplastic strain and strain hardening during single loading process of rock materials, high-cycle and low-cycle fatigue damage models describing the characteristics of micro-plastic strain and strain hardening of rocks under freeze-thaw cycles were obtained; a fatigue life equation linking microplastic strain and strain hardening is established. According to the high cycle and low cycle fatigue damage models and freeze-thaw cycle experimental data, the fatigue life and other model parameters in the high cycle and low cycle fatigue damage models are determined under double logarithmic coordinate system. In view of the micro pore expansion and the stress increase caused in rocks due to freeze-thaw cycles, the coupled damage variables and its damage range under the combined action of freeze-thaw and stress were derived. The research results indicate that with the increase of freeze-thaw cycles, the coupled damage shows a high cycle fatigue damage followed by low cycle fatigue damage; when determining the damage value of freeze-thaw rock, low cycle fatigue damage can be equivalent to the increase of high cycle fatigue cycle number; the equivalent damage model based on the Lemaitre strain equivalence hypothesis and high-cycle fatigue damage model only describe the damage evolution within the microplastic strain range; whereas the low-cycle fatigue damage model can reveal the damage evolution from microplastic strain to strain hardening range; in the process of microplastic strain to strain hardening development, the damage evolution rate of freeze-thaw rock gradually decreases, showing brittle reduction and plastic increase.