Study on dynamic mechanical properties of coal under impact loading

In order to study the mechanical properties of coal samples with different hardness under impact loading, two different coal samples of briquette and raw coal samples were prepared, and they were subjected to different strain rate conditions using a separate Hopkinson pressure bar experimental device.. The uniaxial impact failure experiments were carried out, and the dynamic stress-strain curves and related dynamic mechanical parameters of two kinds of coal were obtained. The failure deformation characteristics and energy dissipation laws of the two coals were analyzed. The experimental results show that, the two kinds of coal show non-linear deformation at the initial stage under impact loading, which is different from that under quasi-static loading obviously. In general, the failure and deformation process can be summarized as follows: initial non-linear stage, yield stage, strain strengthening stage and unloading failure stage. In the range of strain rate 70～280 s-1, the dynamic compressive strength and peak strain of both coals have the characteristics of strain rate strengthening, which increases exponentially with strain rate, and the strain rate hardening characteristics of the raw coal are more significant. Since the material is affected by the heat activation mechanism and the inertia effect, the dynamic stress increasing factor (DIF) of two kinds of coal shows two different trends with strain rate, and there is a transition strain rate (130 s-1). At the rate, the briquette exhibits higher strain rate hardening and dynamic toughening effects than the raw coal. The dissipation energy density of both coals increas-es exponentially with strain rate, showing a significant rate correlation. However, under the same load, the dissipation energy density of raw coal is higher than that of briquette coal, which requires more energy to be destroyed. The average energy dissipation rates of raw and briquette coal have no obvious strain rate correlation, with an average of 10.1% and 19.3%, respectively. It shows that briquette coal has stronger energy absorption capacity and more significant rate sensitivity than raw coal. The results show that the two kinds of coals are both rate-sensitive materials. At high strain rates, briquette exhibits higher strain rate hardening, dynamic toughening effect and energy absorption capacity than raw coal. The mechanical properties of the two coals at higher strain rates are subject to further experimental research.