A creep model for lean coal based on hardening-damage mechanism

Rock creep is the evolution result of the interaction of hardening effect and damage effect inside it. A series of uniaxial compression creep experiments of raw lean coals are conducted by stepwise loading in this work, and the increasing instantaneous elasticity modulus and decreasing viscosity coefficient are taken to analyze the hardening-damage mechanism of lean coal. The results show that at low stress load, coal strain is just instantaneous, and only hardening effect exists. When the stress reaches or exceeds the creep start stress threshold, damage effect emerges, and interacts with hardening effect, resulting in transient creep and steady creep. In long term creep, coals are first hardened and then weakened and finally fail due to large accumulative damage. On basis of the strain characteristics of lean colas at different stress loads, a combined viscous-elastic plastic model is established; and then the hardening function and damage function are both introduced into the model; thus the creep constitutive equation is obtained. This model is used to depict lean coal's creep curves and the results show the model curves fit the experimental data very well. This model is simple with clear physical meaning, and it can reflect the hardening-damage creep characteristics of lean coal in uniaxial compression very well.