循环载荷下煤样能量转化与碎块分布特征

Energy conversion and fragment distribution characteristics of coal sample under uniaxial cyclic loading

煤矿开采中煤体常处于反复加卸载过程,研究煤体在不同加载速率重复载荷作用下的能量转化与破坏机制对认清煤矿动力灾害本质具有指导意义。利用MTS815.03伺服实验系统,通过单轴循环加卸载试验,结合能量和分形理论,获得了不同加载速率下煤样变形破坏各阶段能量积聚、耗散和释放的转化机制及其与煤样碎块块度分布规律的内在关系,为开展重复载荷作用下煤岩破裂响应及破坏机制的研究提供依据。试验结果表明:煤样能量转化具有明显的阶段性特征,可分为能量初始积累阶段、能量加速积累阶段和能量快速耗散阶段。煤样破坏前耗散能所占比例经历了高—低—高的过程,而弹性能则相反,加载后期弹性能比例下降或耗散能比例的升高,预示着煤样进入加速破坏阶段;能量集聚和释放与加载速率密切相关,随着加载速率的增大,峰值前弹性能所占比例逐渐增加,煤样破坏前更多的能量以弹性能形式储存在煤样体内,岩石破坏后,更多的能量被释放出来,煤样破坏越剧烈,其宏观破坏形态由剪切张拉和劈裂破坏向弹射破坏过渡;循环加卸载下煤样碎块分形特征具有明显的分段性,在小于尺寸阈值范围内具有较好的自相似性特征,不同加载速率下碎块分形维数为2~3,且随加载速率的增加成线性增长;加载速率越大碎块分形维数越大,煤样破碎程度越高,大块碎块所占比例越小,煤样碎块越破碎且单块碎块质量越小,煤样发生动力灾害的危险性越大。

Coal mass is often affected by consecutive loading-unloading cycles in coal mining,so the study of energy conversion and failure mechanism of coal mass under repeated loads has a guiding significance for understanding the mine dynamic disasters.By using MTS815.03 servo experimental system,the uniaxial loading cycle tests were performed for the mechanism of energy accumulation,dissipation,and release in the failure process of coal sample under different loading rates,and the fragmentation distribution was also analyzed based on the theoretical analysis of energy and fractal.It provides a basis for the study of burst response and failure mechanism of coal and rock under cyclic loading.The experimental results show that the phasic characteristics of energy conversion are obvious,which can be divided into three stages:initial energy accumulation stage,accelerated energy accumulation stage and fast energy dissipation stage.The proportion of energy dissipation of coal samples encounters the course from high to low again to high.The elastic energy has an opposite trend of change,the proportion of elastic energy drops or the proportion dissipated energy increases at the later stage of loading,the coal samples enter the accelerated failure stage.The energy dissipation and the energy release are closely related with the loading rate.With the increase of loading rate,the proportion of elastic energy increases before the peak stress,sample is damaged severely after the peak stress and its macro failure mode transits from shear tension and splitting failure to ejection failure.Because more energy is stored in the coal sample body in the form of elastic energy before the coal sample is destroyed,and more energy is released after the rock is destroyed,which makes the coal sample destroyed more severely.The phasic characteristics of fractal features of coal samples are obvious under cycling loading and unloading and the self-similarity characteristics are obvious within the range of threshold,the fractal dimension is between 2-3,which increases linearly with the loading rate.The fractal dimension of fragments increases with the increase of loading rate,and the higher the broken degree of coal samples,the smaller the proportion of large fragments.In this case,the higher of the broken level of more fragments and the smaller the mass of single fragments is,the greater the risk of dynamic disasters of coal samples.