Nonlinear mechanical properties of coal rock under quasi-static strain rate

In order to study the nonlinear mechanical characteristics of coal rock under excavation disturbance, the uniaxial compression test of quasi-static strain rate of 10-5-10-2 s-1 was carried out by MTS815 servo rigidity testing machine on the coal rock of Fu Rong, Bai Jiao Coal Mine, China. The variation of mechanical properties such as stress-strain curve, compressive strength, elastic modulus and Poisson's ratio of coal rock under quasi-static strain rate were analyzed. Based on the significant nonlinear mechanical characteristics of coal rock, the viscoelastic damage constitutive model was modified and verified. The research shows that ① Under uniaxial compression loading, the coal rock has four stages, such as compaction, linear elasticity, plastic yield and post-peak damage. The strain rate correlation of coal rock mechanical properties is outstanding. With the increase of strain rate, the compaction phase is shortened, the linear elastic phase is increased, and the stress strengthening characteristics in the yielding stage and the stress softening characteristics in the post-peak failure stage tend to be obvious. ② Due to the closure and expansion of the primary fissures, each stage of the loading coal rock presents remarkable nonlinear mechanical characteristics. With the increase of strain rate, the obvious nonlinear characteristics of the compaction stage and the linear elastic stage show a weakening trend. ③ In the range of quasi-static strain rate 10-5-10-2 s-1, the compressive strength increases with the strain rate from 8.83 MPa to 12.57 MPa, the growth rate is 42.35%. The modulus of elasticity generally increases first and then decreases with the increase of strain rate. In the range of 10-5-10-4 s-1 strain rate, the elastic modulus increases from 1 644 MPa to 1 825 MPa, followed by 10-4-10-2 s-1 is reduced within the strain rate interval, and the trend of change is obviously turning. The Poisson's ratio shows a decreasing trend with the increase of strain rate under the quasi-static strain rate, which decreases from 1.53 to 0.71, and decreases value by 53.59%. The mechanical characteristics parameters (compressive strength, elastic modulus, Poisson's ratio) of coal rock under the whole quasi-static strain rate category all shows significant nonlinear changes, and all of them turn around the critical strain rate of 10-3 s-1. ④ Using a damage body and a Maxwell model in parallel, considering the nonlinear variation characteristics of coal rock under quasi-static strain rate, the material constant a is used to modify the Hooke's law, and the constitutive equation of nonlinear damage evolution of coal rock under quasi-static strain rate is derived. The constitutive equation' rationality is verified with the combination of the uniaxial compression test of coal rock. © 2019, Editorial Office of Journal of China Coal Society. All right reserved.