循环加、卸载孔隙水压力对砂岩变形特性影响实验研究

 利用MTS815岩石力学实验系统对饱和砂岩进行三轴等围压情况下的循环加、卸载孔隙水压力实验。结果表明：压密阶段的加、卸载曲线中出现了很多“Z”状的波动，这些小波的出现没有规律性，初始残余变形比较大，还没有形成明显的滞回曲线。在弹性耦合阶段稳定滞回曲线加载时，应变呈上凹“Z”状波动，在到达上限值前出现拐点，应变达最大值；卸载时，应变呈下凸“Z”状波动，在到达下限值前出现拐点，应变达最小值，该阶段形成稳定的滞回曲线，表现形式由疏变密，稳定滞回曲线包含弹性变形向塑性变形演化。在孔隙水压力的不同上限值和不同幅值区间的耗散能构成不对称“X”形。在加载段，随着孔隙水压力增大，耗散能逐渐减小；卸载段，随着孔隙水压力减小，耗散能逐渐减小。“X”形的交点出现的位置和夹角与不同上限值和不同幅值区间有关。在循环加、卸载孔隙水压力作用下，残余应变与循环次数的关系符合乘幂负指数关系。

EXPERIMENTAL STUDY OF EFFECTS OF CYCLIC LOADING AND UNLOADING PORE WATER PRESSURES ON DEFORMATION CHARACTERISTIC OF SANDSTONE

Triaxial compression experiments have been carried out to study the effects of the cyclic loading and unloading pore water pressures on the deformation characteristic of saturated sandstone when the axial stress and confining pressure are invariant using the MTS815 rock mechanics testing system. The experimental results have shown that there are anomalistic Z-shaped nonlinear wave in the stress-strain hysteresis curve at the stage of compaction under loading and unloading；and initial residual deformation is relatively large，therefore，the hysteresis loop is not so obvious. In addition，it has been observed that at the elastic coupled stage，the stress-strain curve which rises with concave Z-shaped wave appears inflection point before coming to a head；at this point，strain is the maximum at loading. While at unloading，it is inflexion at the stress-strain curve which drops with convex Z-shaped wave before touching bottom；at the same time，the strain fetches the minimum. Meanwhile the stress-strain hysteresis loop，showing the stage from sparse to dense，is stable which indicates evolution from elastic deformation to plastic deformation. The dissipation energy of different upper limits and different amplitude areas forms into asymmetrical X-shaped at the pore water pressure. The dissipation energy decreases gradually at loading with the increase of pore water pressure. Otherwise，at unloading，it decreases gradually with reducing of pore water pressure. The position and angle of the X-shaped crossing are related to the different upper limits and amplitude areas. The relationship between the residual strain and cyclic numbers accords with the power of negative index under the cyclic loading and unloading pore water pressures.