Influence factors of pop-in in the nanoindentation micromechanical property measurement of gas-bearing shale

Studying the micromechanical properties of gas-bearing shale is very important for understanding it. In the nanoindentation measurement of gas-bearing shale, the load-displacement curve often shows pop-in at the loading stage, namely the change of load is small but the change of penetration depth is large. Pop-in can introduce a large error in the calculation of the mechanical parameter. To investigate the influence factors of pop-in, the maximum load, load rate and sample characteristics have been addressed. The results show that pop-in is related to cracks and pores in the shale. Due to the heterogeneity of shale, pop-in cannot be avoided, but can be reduced by choosing reliable load modes and test parameters. Pop-in can increase the maximum indentation depth and decrease the elastic modulus and hardness. The influence of pop-in on hardness is larger than on the elastic modulus. At a high load rate it is easy to induce pop-in. The influence of the load rate on pop-in is larger than that of the maximum load.