单轴压缩分级松弛作用下煤样变形与强度特征分析

 利用RMT–150B岩石力学系统对煤样进行单轴压缩分级松弛试验，分析煤样常规单轴压缩与单轴分级松弛条件下煤样的变形、强度和破坏的时效特征。结果表明：2种加载方式下煤样峰值前的应力–应变曲线宏观没有明显区别，仍然表现出压密、弹性、屈服和破坏阶段；单轴压缩分级松弛试验的应力–应变曲线，在松弛期间出现局部应力跌落特征；常规单轴压缩得到的力学参数明显高于分级松弛试验值，表现出煤样的力学参数具有时效特征；当松弛应力比低于70%时，应力松弛特征不明显，当松弛应力比高于70%时，煤样内部强度较高材料积蓄的能量能够使低强度材料逐步损伤破坏，高强度材料将承载更高应力也逐渐趋于破坏，松弛应力比越高，持续破坏时间越短，反之则持续破坏时间越长；常规单轴压缩破坏具有明显张剪性破裂面，分级松弛试验时煤样破坏裂纹沿加载方向扩展后碎裂成许多相对较薄片状和片状碎屑，表现出明显的侧向膨胀特征。

ANALYSIS OF DEFORMATION AND STRENGTH CHARACTERISTICS OF COAL SAMPLES UNDER UNIAXIAL COMPRESSION OF STEPPED RELAXATION

The uniaxial compression with stepped relaxation was conducted on coal samples with the testing system of rock mechanics RMT–150B. The aging characteristics of deformation，strength and failure of coal samples under uniaxial compression and uniaxial compression with stepped relaxation were analyzed. The results show that the stress-strain curves ahead of the peak values of coal samples at two loading conditions have four conventional stages，i.e. densification，elastic，yielding and failure. The stress-strain curves of uniaxial compression with stepped relaxations have a local drop of stress during the relaxation. The values of mechanical parameters of uniaxial compression are higher than ones of that with stepped relaxation，showing that the mechanical parameters of coal samples have aging characteristics. When the ratio of stress relaxation is lower than 70%，the characteristics of stress relaxation is not obvious. When the ratio of stress relaxation is higher than 70%，materials of lower strengths were gradually damaged by the energy stored in the materials of higher strength inside coal samples and the materials of higher strength carried higher stresses and were also be damaged gradually. The higher the ratio of the stress relaxation was，the shorter the length of time of failure was. There were noticeable surfaces of tension-shear fracturing for failure samples under uniaxial compression；but the failure cracks of coal samples in the stepped relaxation tests were in loading directions and many chippings and fragments were formed，showing clearly a characteristic of lateral expansion.