Experimental study on dynamic mechanical characteristics of coal specimens considering initial damage effect of cyclic loading

Abstract

In the mining of nearly horizontal open-pit coal mines, transportation flat plates are usually arranged on the end-slope. Reciprocating transport operations of large heavy-duty trucks cause significant cyclic loading and unloading effects on the overburden rock, leading to initial damage to the coal rock body. At the same time, the strong impact load generated by large-scale blasting during open-pit mining further deteriorates the mechanical properties of the coal rock body, thereby inducing large area instability of the end-slope. Therefore, it is crucial to conduct experimental research on the dynamic mechanical properties of coal rock under cyclic loading and unloading damage conditions for the stability of the end-slopes and safe mining in open-pit coal mines. The dynamic mechanical properties of coal samples under the initial damage condition of cyclic loading and unloading and their damage and fracture mechanism were systematically analyzed by using the split Hopkinson pressure bar test system and scanning electron microscope test system. The results show that: ① With the increase of the loading strain rate, the dynamic compressive strength and elastic modulus of the coal sample increase rapidly, and the strengthening effect of the strain rate on the mechanical properties of the sample is significant. The dynamic compressive strength and elastic modulus decrease linearly and rapidly as the initial damage increases under the condition of fixed strain rate, indicating that the initial damage weakens the mechanical properties of the coal sample; ② There exist small-scale primary cracks and pore defects in the natural state of the coal samples, and the occurrence of damage leads to the emergence of new cracks and pores inside the coal sample. With the increase of the damage intensity, the defect scale and number gradually increase and expand rapidly, which is the fundamental reason for the weakening of its mechanical properties; ③ The failure mode of the coal sample changes from tension failure to shear failure with the increase of the initial damage and strain rate, and at the same time, its degree of damage, dissipation energy during the failure process, and energy dissipation capacity show a rapid increasing trend; ④The dynamic failure of the damaged coal sample is mainly brittle failure, but under the condition of high initial damage, significant sliding separation and toughness fracture morphology can be observed locally, which is also the main reason for the high energy dissipation capacity of the coal sample under the condition of high initial damage.

Keywords

• open-pit coal mine
• cyclic loading and unloading
• initial damage
• dynamic mechanical characteristics
• energy dissipation