高径比对砂岩能量积聚与耗散试验及分析方法

Energy accumulation and dissipation test and analysis method of height-diameter ratio sandstone

为了探究高径比对岩石能量积聚与耗散的影响,利用RMT-150B岩石力学测试系统对同直径不同长度的砂岩试件进行了单轴压缩试验、单轴循环加卸载试验以及单轴分级加卸载试验。对比砂岩单轴压缩与单轴循环加卸载曲线发现,单轴压缩曲线近似为单轴分级加卸载曲线的外包络线,并且由于“硬化”作用单轴分级加卸载强度略高于单轴压缩。通过砂岩单轴循环加卸载下力学特性分析发现随着循环次数增多,残余变形逐渐减小,直至某次循环无残余变形产生砂岩出现非线性伪弹性体特征,继续承受加卸载作用非线性伪弹性体特性消失,残余变形再次出现,因此可以将残余变形演化规律归纳为:存在较大残余变形—残余变形逐渐减小—无残余变形—再次出现残余变形。基于砂岩弹性能的演化分析发现循环次数对弹性能的影响较小,即疲劳损伤对砂岩的弹性能影响较小,则可以假定单轴分级加卸载各卸载点的荷载与单轴压缩试验的荷载相等时,两者的弹性能也相等。利用单轴分级加卸载试验各卸载点的弹性能与单轴循环加卸载首次加卸载弹性能进行比较,验证了假设的正确性。进一步对不同高径比(L/D)下砂岩的单轴压缩试验进行能量分析,发现砂岩的弹性能演化服从线性储能规律。基于砂岩的单轴分级加卸载试验结合线性储能规律对同尺寸的单轴压缩试验进行能量演化分析,为砂岩单轴压缩的能量分析提供了新思路。研究表明:当荷载相等时砂岩储存弹性能与高径比成正比,与储能极限成反比。将提出的试验法与传统能量分析方法(利用公式计算得到弹性能)进行比较,发现提出的方法精确度高。基于耗散能演化规律进一步对不同高径比下砂岩的损伤进行了分析,发现砂岩损伤曲线呈非线性演化,并且荷载相等时高径比越大砂岩损伤程度越高。破裂时砂岩内部积聚的弹性能转化成破裂面表面能和碎块动能,储能极限与高径比大小成反比,因此当L/D≥2.0时主要呈单方向的剪切破坏,L/D<2.0时岩石主要为相对复杂的双剪切或圆锥形破坏,并且进一步从能量角度对钻孔卸压进行了讨论。

To explore the influence of aspect ratio on rock energy accumulation and dissipation,the uniaxial compression test,uniaxial cyclic loading-unloading test,and uniaxial stage loading and unloading test were carried out on the sandstone specimens with the same diameter and different lengths using RMT-150B rock mechanics test system.Comparing the uniaxial compression and the uniaxial cyclic loading-unloading curves of sandstone,it is found that the uniaxial compression curve is approximately the outer section of the uniaxial graded loading and unloading curve,and the strength of uniaxial graded loading and unloading is slightly higher than that of uniaxial compression due to hardening.The analysis on mechanical characteristics of the sandstone under uniaxial cyclic unloading demonstrates that as the cycle number increases,the residual deformation decreases gradually until the cycle generates residual deformation of sandstone showing the nonlinear pseudo elastomer characteristics.If the loading continues,the nonlinear pseudo elastomer features disappears,the residual deformation appears again.Therefore,the residual deformation evolution can be summarized as a large residual deformation-the gradual decrease of residual deformation-no residual deformation-residual deformation again.Based on the evolution analysis of the elastic energy of sandstone,the number of cycles has little effect on the elastic energy of sandstone,meaning that fatigue damage has little effect on the elastic energy of sandstone.Therefore,the elastic energy of sandstone is equal to that of the uniaxial compression test when the load at each unloading point is equal to that of the uniaxial compression test.Comparison between the elastic energy of each unloading point and the first loading-unloading in the uniaxial cyclic loading-unloading tests was made to verify the correctness of the hypothesis.It is found that the elastic energy evolution of sandstone obeys the law of linear energy storage by the energy analysis of the uniaxial compression test of sandstone with different aspect ratios.The energy evolution analysis of the uniaxial compression test with the same size is carried out based on the uniaxial loading and unloading test of sandstone and the linear energy storage law,which provides a new method for the energy analysis of the uniaxial compression.The results show that the elastic energy is directly proportional to the height-diameter ratio and inversely proportional to the storage limit when the load is equal.The new method proposed in this paper is compared with the traditional energy analysis method,which calculates the elastic energy by formulas.It is found that the method proposed in this paper is highly accurate.Based on the evolution law of dissipated energy,the damage of sandstone with different aspect ratios is further analyzed.It is found that the damage curve of sandstone exhibits a nonlinear evolution,and the damage degree of sandstone with a larger aspect ratio is higher when the load is equal.The elastic energy accumulated in sandstone during fracture is converted into the surface energy of the fracture surface and the kinetic energy of fragments.The energy storage limit is inversely proportional to the height-diameter ratio.Therefore,when L/D≥2.0,the shear failure mainly occurs in one direction.When L/D<2.0,the rock is mainly a relatively complex double shear or conical failure,and the pressure relief of borehole is further discussed from the perspective of energy.