Creep characteristics of coal and rock investigated by nanoindentation

In coal mining industry, with the depth growing of coal mines, the creep behaviours of coal and rock can extensively affect the mining safety, coalbed methane recovery and geo-sequestration. To acquire a better insight into their creep characteristics, an efficient and robust researching technique, nanoindentation, was applied to investigate the creep performances of coal and rock samples obtained from two coal mines in the east of China. Creep characteristics were reflected by evaluating the curves of creep depth versus creep time of nanoindentation tests during the load-holding period at the peak load of 30 mN. These curves can be divided into two stages: transient stage and steady stage; and the time of load-holding period of 5 s, which is the dividing point between two stages, can efficiently avoid the influence of creep displacement on the unloading curves. The exponential function can perfectly fit creep curves and Kelvin model can be used to calculate the rheological parameters of coal and rock samples. Calculated results yield values for the creep modulus and viscosity terms of coal and rock. This study also settled a particular emphasis on the selection of the positions of indentations to obtain the rheological properties of mineralogical constituents in heterogonous coal and rock samples and their elastic aftereffect. (C) 2020 Published by Elsevier B.V. on behalf of China University of Mining & Technology.