The study of pressure stimulated current (PSC) changes of rocks is significant to monitor dynamic disasters in mines and rock masses. The existing studies focus on change laws and mechanism of currents generated under the loading of rocks. An electrical and mechanics test system was established in this paper to explore the impacts of loading rates on PSCs. The results indicated that PSC curves of different rocks had different change laws under low/high loading rates. When the loading rate was relatively low, PSC curves firstly changed gently and then increased exponentially. Under high loading rates, PSC curves experienced the rapid increase stage, gentle increase stage and sudden change stage. The compressive strength could greatly affect the peak PSC in case of rock failure. The loading rate was a key factor in average PSC. Under low loading rates, the variations of PSCs conformed to the damage charge model of fracture mechanics, while they did not at the fracture moment. Under high loading rates, the PSCs at low stress didn’t fit the model due to the stress impact effects. The experimental results could provide theoretical basis for the influence of loading rates on PSCs.
Coal mining has gradually entered the stage of deep mining in China. Meanwhile, the coal and rock dynamic disasters such as rock burst and coal and gas outburst become increasingly serious and complex, which has strong impacts on efficient and safe production of coal mines [1,2]. Besides the direct hazards, they may easily trigger other tremendous accidents. Gas explosion occurs occasionally during the coal mining, due to a large amount of electric charge released along with the redistribution and fracture of roof rock stress [3–5]. The essential reason and monitoring mechanism of these dynamic disasters are that the roof rock produces freely moving charges when the stress changes [6,7]. Therefore, it is of great significance to study the current changes generated by stress stimulation of the rock to prevent coal mine roof dynamic disasters and the induced secondary disasters.
A large number of studies have proved that currents are formed by freely moving charges produced during the deformation process of rocks under loads. This kind of current is called the pressure stimulated current (PSC). The macroscopic laws and generating mechanism of PSCs have been studied in different types of rocks, such as marble, granite, sandstone and coal rock . Stavrakas, Anastasiadis and etc. studied the change laws of currents generated in marble under the action of uniaxial compression. It is generally believed that these currents are generated during the initiation and propagation of cracks in the rock [9–11]. The most representative theory is moving charged dislocations (MCD) model, proposed by Slifkin and further improved by Vallianatos and Tzanis based on change laws of currents in different rocks under uniaxial compression [12–14]. In some experiments, PSCs will only be detected when the stress applied to the rock specimen exceeds a certain level [15,16]. While in other experiments, PSCs will be generated immediately when any significant stress is applied to the rock specimen [9,17,18]. When the stress is at a constant level, PSCs will attenuate and remain constant when it attenuates to a certain value. The reason for this difference is the different lithology and mechanical properties of rocks [19–21]. A large number of experimental results have indicated that the change of PSC values is closely related to mechanical properties of the tested rock samples [22–26].
In this paper, the uniaxial compression PSC test system was established to explore the influences of the loading rates on PSCs of rocks. The conclusions were given as follows:
(1) PSC curves varied for different rock specimens under low/ high loading rates, which were highly consistent with the stresses. When the loading rate was relatively low, the PSC was firstly small and gentle, and then increased exponentially. In case of high loading rates, PSC experienced the rapid increase stage, gentle stage and sudden change stage.
(2) Pearson correlation coefficients of the granite, marble and sandstone were 0.88, 0.967 and 0.954 in a 95% confidence interval of compressive strength and peak current. The granite’s P value was greater than 0.1, while marble and sandstone’s P values were less than 0.05, indicating that the compressive strength played a key role in generating peak currents in case of rock failure.
(3) The loading rate could determine the average PSC of the rock. Under the continuous loading, the greater the loading rate, the faster the growth rate of the cracks, the higher the electronic level on the separation surface, and the easier the generation of free charges. The loading rate could determine the generation rate of the micro-cracks of the rock specimen, so the faster the generation of the free charges, the larger the currents.
(4) Under low loading rates, when dS/dt=a(a constant), no significant transient PSCs could be observed. When the stress exceeded the elastic limit, micro-cracks began to occur. As the cracks propagated and expanded, the transient and enhanced PSCs could be detected. While under high loading rates, due to the impact effects of the loading rates, even if stresses were applied to the rock specimen in the linear range, the increased PSCs could also be detected.