不同应力循环路径下砂岩的能量演化特征

Energy evolution characteristics of sandstone under different stress cycle paths

为从能量角度探究砂岩在循环应力作用下的力学响应,开展10 MPa初始峰值应力逐级递增、80 MPa初始峰值应力逐级递增及恒定100 MPa峰值应力3种循环路径下的加卸载试验,结合应力应变曲线和能量计算结果,分析砂岩各项能量随循环次数和峰值应力的演化特征。结果表明:随循环次数和峰值应力增加,初始峰值应力80和100 MPa时滞回环仅在第2次循环时向应变增大方向明显迁移。不同应力循环路径下单位体积耗散能表现出不同的阶段性演化特征,分级循环加卸载时,砂岩各项能量值随峰值应力增加由二次函数向线性函数增长特征转变,而峰值应力恒定为100 MPa等幅度循环加卸载时,随循环次数增加,砂岩单位体积能和单位体积弹性能呈开口向下的二次函数趋势下降,单位体积耗散能呈指数趋势下降。不同应力区间内循环路径对砂岩的能量影响有较大差异,分级循环加卸载与单次加卸载相比,峰值应力为80 MPa时各项能量相差均小于10%,峰值应力为100 MPa时各项能量相差达到22.74%~62.58%。

In order to investigate the mechanical response of sandstone under cyclic stress from the energy perspective,the loading and unloading tests conducted along three cyclic paths which included an incremental step-by-step increase in initial peak stress at 10 MPa,an incremental step-by-step increase in initial peak stress at 80 MPa,and a constant peak stress maintained at 100 MPa.Subsequently,we analyzed the evolution characteristics of each energy component of sandstone based on the stress-strain curve and energy calculation results,taking into account the number of cycles and peak stress.The results show that as the number of cycles and peak stress increase,the hysteresis loop migrated to the direction of strain increase obviously only in the second cycle at the initial peak stress of 80 and 100 MPa.Different stress cycle paths make the unit volume dissipated energy shows different stage evolution characteristics.The energy values of sandstone change from a quadratic to a linear growth law as the peak stress increases in the hierarchical cyclic loading and unloading way,while in the constant amplitude cyclic loading and unloading way with the peak stress constant at 100 MPa,the unit volume energy and unit volume elastic energy of sandstone show an open downward quadratic decrease trend and the unit volume dissipative energy shows an exponential decrease trend.The effect of cyclic paths on the energy of sandstone in different stress ranges is quite different.In a comparison of the single and the hierarchical cyclic loading and unloading way,the energy difference is less than 10%when the peak stress is 80 MPa,while the energy difference is 22.74%-62.58%when the peak stress is 100 MPa.