Mechanical heterogeneity characterization of coal materials based on nano-indentation experiments

基于纳米压痕实验的煤体材料力学非均匀性表征

To investigate the complex macro-mechanical properties of coal from a micro-mechanical perspective,we have conducted a series of micro-mechanical experiments on coal using a nano-indentation instrument.These experiments were conducted under both dynamic and static loading conditions,allowing us to gather the micro-mechanical parameters of coal for further analysis of its micro-mechanical heterogeneity using the box counting statistical method and the Weibull model.The research findings indicate that the load–displacement curves of the coal mass under the two different loading modes exhibit noticeable discreteness.This can be attributed to the stress concentration phenomenon caused by variations in the mechanical properties of the micro-units during the loading process of the coal mass.Consequently,there are significant fluctuations in the micro-mechanical parameters of the coal mass.Moreover,the mechanical heterogeneity of the coal at the nanoscale was confirmed based on the calculation results of the standard deviation coefficient and Weibull modulus of the coal body’s micromechanical parameters.These results reveal the influence of microstructural defects and minerals on the uniformity of the stress field distribution within the loaded coal body,as well as on the ductility characteristics of the micro-defect structure.Furthermore,there is a pronounced heterogeneity in the micromechanical parameters.Furthermore,we have established a relationship between the macro and micro elastic modulus of coal by applying the Mori-Tanaka homogenization method.This relationship holds great significance for revealing the micro-mechanical failure mechanism of coal.

为了研究纳米尺度下煤体的力学非均质性,通过纳米压痕动静态力学实验获得了余吾煤样的硬度、弹性模量和断裂韧度等微观力学参数,并利用盒计数统计方法和Weibull模型对煤体的力学非均质性进行了分析。研究结果表明,在两种加载模式下,各测点的荷载−位移曲线具有重复性差和离散性高的特点,并经历荷载增加、荷载保持不变,弹性变形逐渐恢复的3个阶段;在加载过程中,煤体内部由于单个微单元力学属性的差别更加容易产生应力集中现象,导致煤体微观力学参数具有明显的波动性。两种加载模式煤体的力学参数标准差系数和Weibull模数表明纳米尺度下煤样的力学参数离散性比较大,煤样的力学非均质性比较明显。另外,SEM电镜扫描结果显示煤样内部存在的微观结构缺陷,影响了受载煤体的应力场分布的均匀性和微缺陷结构的延展特征,同时煤样内部存在的石英以及高岭石等黏土类矿物质组分影响了各测点的力学性质,使其表现出明显的力学非均质性。此外,通过Mori-Tanaka均匀化方法建立了煤体宏微观弹性模量之间的关系,对于从微观角度研究煤体的宏观力学性质具有借鉴意义。