The effect of loading rate on the behavior of samples composed of coal and rock

To accurately evaluate the danger from rock bursts during coal mining, uniaxial compression tests of composed coal rock at different loading rates were carried out. The effect of the rate and path of loading and unloading on the mechanical properties of the composed coal rock has been analyzed. The overall elastic modulus, peak strength, and residual strength of the composed coal rock lie among that of roof, coal, and floor. The stress-strain characteristics of the composed coal rock depended on the part of the system with the smallest stiffness. The stress-strain curve during the post-peak failure stage was smoother than that of a single rock specimen on the whole. The stiffness difference markedly influenced the failure form and propensity to rockburst of the composed coal rock. With an increase of loading rate, the strain increment of the composed coal rock in the elastic phase, the plastic phase, and the failure phase gradually increased, and the strain at the peak point rose linearly. Faster strain rates made the curves smoother with smaller slopes. In comparison to monotonic loading, all the strain increments of the composed coal rock in the elastic phase, the plastic phase, and the peak strength point of cyclic loading were smaller; both the elastic modulus and peak strength were greater. Fast loading enhanced the capacity of the composed coal rock to convert outside energy to its own elastic energy. With an increase of loading rate, both the post-peak secant modulus and the strain softening modulus of the composed coal rock gradually decreased as negative logarithmic curves on the whole. The dynamic failure time gradually decreased as a hyperbolic curve, with enhanced brittleness. Consequently, composed coal rock and the rapid loading and unloading effect induced by mining should be considered for evaluating rockburst propensity. (C) 2013 Elsevier Ltd. All rights reserved.