砂岩真三轴循环加卸载的能量演化与损伤分析

Energy evolution and damage analysis of true triaxial cyclic loading and unloading of sandstone at different rates

为了探究不同速率对砂岩真三轴循环加卸载破坏过程中的能量和损伤演化的影响,利用自主研发的真三轴扰动卸荷岩石测试系统,借助立方体砂岩试件,分别进行了真三轴压缩试验、真三轴一次加卸载试验以及真三轴循环加卸载试验。结果表明:真三轴条件下砂岩的峰值强度随循环加卸载速率的增大而增大;σ^(1)方向弹性能密度演化基本不随循环加卸载速率的改变而不同,而耗散能密度随循环加卸载速率的增大而减小,2者都随循环数呈现非线性增长;随着循环加卸载速率的增大,σ^(1)方向上耗散能密度占总输入能密度比例减小,弹性能密度占总输入能密度比例增加;随着循环数的增加,σ^(2)、σ^(3)方向上弹性能密度的增长率逐渐增大,而耗散能密度增长率在前5次循环时变化较小,在循环末尾阶段大幅提升;随着循环加卸载速率的增大,σ^(2)、σ^(3)方向上弹性能密度也随之提高,耗散能密度则与之相反;σ^(2)方向上弹性能密度占输入能密度的比例随循环数递增,在第6次循环出现拐点,随后突降;σ^(3)方向上弹性能密度占输入能密度的比例随着循环数的增加先减小后增大,呈现“V”型变化趋势;研究发现,弹性能密度和耗散能密度均与输入能密度存在线性函数关系,并且弹性能密度-输入能密度和耗散能密度-输入能密度均存在线性函数关系。真三轴循环加卸载作用下砂岩损伤分为初始损伤、损伤缓慢发育、损伤快速发展阶段3个阶段,损伤变量随着循环数的增加呈现非线性增长。

To investigate the effects of different rates on the energy and damage evolution of sandstone during true triaxial cyclic loading and unloading damage,the true triaxial compression test,true triaxial primary loading and unloading test,and true triaxial cyclic loading and unloading test were conducted with the aid of cubic sandstone specimens by using the self-developed true triaxial disturbance unloading rock test system.The tests show that the peak strength of sandstone under true triaxial conditions increases with the cyclic loading and unloading rate.The evolution of the elastic energy density in theσ^(1)direction does not vary with the cyclic loading and unloading rate,while the dissipative energy density decreases with the cyclic loading and unloading rate,both of which show a nonlinear increase with the number of cycles.With the increase of the cyclic loading and unloading rate,the ratio of the dissipative energy density to the total input energy density in theσ^(1)direction decreases and the ratio of the elastic energy density to the total input energy increases.With the increase of the number of cycles,the growth rate of elastic energy density in theσ^(2)andσ^(3)directions gradually increases with the increase of cycle number,while the growth rate of dissipative energy density changes less in the first five cycles and increases significantly at the end of the cycle.The elastic energy density in theσ^(2)andσ^(3)directions increases with the increase of cyclic loading and unloading rate,while the dissipative energy density increases in the opposite direction.The elastic energy density in theσ^(2)direction increases with the cycle number as a percentage of the input energy density,and an inflection point appears in the sixth cycle.The elastic energy density to input energy density in theσ^(3)direction decreases and then increases with the increase of cycle number,showing a“V”-shaped trend.It is found that the elastic energy density and dissipative energy density are linearly related to the input energy density,and the elastic energy density-input energy density and dissipative energy density-input energy density are linearly correlated.The damage of sandstone under the action of true triaxial cyclic loading and unloading is divided into three stages:initial damage,slow development of damage,and rapid development of damage,and the damage variables show nonlinear growth with the increase of cycle number.