真三轴应力条件下加卸荷速率对砂岩力学特性与能量特征的影响

Effect of loading and unloading rates on mechanical properties and energy characteristics of sandstone under true triaxial stress

为了更准确地认识真三轴应力条件下加卸荷速率对岩石力学特性与能量特征的影响规律,利用自主研发的"多功能真三轴流固耦合试验系统"开展了砂岩真三轴加卸荷力学特性试验,实现了最小主应力方向上的单面卸荷,模拟实际围岩应力演化过程。试验结果表明:随着卸荷速率的增大,砂岩破坏时的最大主应力、最大主应变、最小主应变和体积应变均减小、中间主应变增大,扩容起始点提前,岩样破坏模式逐渐由剪切破坏转为张拉破裂,且张性裂纹多集中于卸荷面附近。加载速率的增大,砂岩破坏时的最大主应力、最大主应变、最小主应变和体积应变增大,扩容起始点滞后,岩样破坏模式逐渐由张剪破坏转向剪切破坏,产生非贯通性裂纹。引入应变偏应力柔量分析不同加卸荷速率下砂岩变形规律,最小主应变和体积应变的偏应力敏感性与卸荷速率呈正相关,最大主应变的偏应力敏感性与加载速率呈正相关。此外,岩石在峰值应力前能量演化有明显的阶段性,峰前吸收的能量大多以可释放弹性应变能的形式存储,耗散能在峰后超过弹性应变能。耗散能比例Ud/U随着最大主应变的增加呈现出先增后降再增的趋势,峰值应力时Ud/U随着卸荷速率的增大而减小,随着加载速率的增大而增大。达到峰值应力时,岩石吸收的总能量U、弹性应变能Ue、耗散能Ud和相应的应变能增量与时间间隔的比值u均随着卸荷速率的增大而减小,随着加荷速率的增大而增大。

In order to understand exactly the effect of loading and unloading rates on the mechanical properties and energy characteristics of rock under true triaxial stress conditions,a self-developed true triaxial fluid-solid coupling experiment system was applied to investigate the mechanical properties of sandstone under different true triaxial loadingunloading stress paths.During the tests one-side platen was unloaded in the direction of the minimum principal stress to simulate the actual stress evolution of surrounding rock.The results indicate that with the increase of the unloading rates,the maximum principal stress,the maximum principal strain,the minimum principal strain and the volume strain of the sandstone at peak strength are all decreased,the intermediate principal strain is increased,and the volume expansion point is advanced. The failure mode of rock sample gradually changes from shear to tensile,and the tensile cracks are mostly concentrated near the unloading surface.As the loading rate increases,the maximum principal stress,the maximum principal strain,the minimum principal strain and the volume strain increase when the sandstone fails.The volume expansion point gradually lags,and the fracture pattern of rock sample changes from tension-shear to shear,and the non-penetrating crack appears.By introducing a description variable presented as strain-deviatoric stress compliance,the deformation laws of sandstone specimens under the different loading and unloading rates were analyzed.The deviatoric stress sensitivity of the minimum principal strain and the volume strain is positively correlated with the unloading rates.The deviatoric stress sensitivity of the maximum principal strain is positively correlated with the loading rates.Additionally,the energy evolution of rock before peak stress can be divided into obvious different stages,and the energy absorbed before the peak is mostly stored in the form of the elastic strain energy,and the dissipated energy exceeds the elastic strain energy after the peak. The dissipated energy ratio Ud/U tends to increase first,then decrease,and finally increase again with the increase of the maximum principal strain. The rates of Ud/U at peak strength decrease with the increase of unloading rates and increase with the increase of loading rates. When approaching the peak stress,the absorbed total energy U,the elastic strain energy Ue,the dissipated energy Udand the corresponding ratios of strain energy increments to time intervals decrease with the increase of unloading rates and increase with the increase of loading rates.