粉砂质泥岩峰前、峰后应力松弛特性试验研究

Experimental study of pre-peak and post-peak stress relaxation behavior of silty mudstone

采用分级加载方法，对三峡地区巴东组粉砂质泥岩开展峰前、峰后三轴压缩应力松弛试验。依据试验结果，分析峰前弹性段、屈服段，峰后应变软化段、残余强度段岩石应力松弛量、应力松弛度、松弛稳定时间以及应力松弛速率的变化规律，研究峰前、峰后不同阶段岩石应力松弛特征的差异。引入损伤变量对三参量广义Kelvin模型进行改进，建立岩石的非线性应力松弛损伤模型。应用Levenberg-Marquardt算法，辨识得出岩石的应力松弛损伤模型参数。基于辨识结果，分析峰后破裂对岩石应力松弛模型参数的影响作用，研究峰前、峰后不同阶段岩石应力松弛模型参数的变化规律。在试验研究成果的基础上，总结得出峰前、峰后岩石不同的应力松弛机制。研究结果表明：（1）峰前、峰后各阶段中，峰后应变软化段试样的应力松弛量大，应力损失程度高，松弛稳定时间长，应力松弛速率快，工程实践中应尽可能避免使岩石处于峰后应变软化段。（2）峰后各级应变水平试样的G1，G2，凰参数值为峰前0．20％应变水平相应参数值的1％～53％，峰后岩石的应力松弛模型参数值急剧降低。（3）峰后形成的宏观破裂对试样的黏弹性力学特性影响最大，黏性力学特性其次，而对试样的瞬时弹性力学特性影响最小。（4）峰后应变软化段试样的G1，H1模型参数值大幅降低，峰前屈服段试样的G2模型参数值大幅降低。（5）峰前岩石的应力松弛主要是由于岩石内部微裂隙的产生、扩展等原因而引起的，峰后岩石的应力松弛主要是由于岩石中宏观裂纹的产生、扩展以及贯通等原因而引起的。峰前、峰后岩石的应力松弛机制不同。

The pre-peak and post-peak stress relaxation tests under triaxial compression were conducted on silty mudstone samples from Badong Formation strata in the Three Gorges Reservoir area. The pre-peak stress relaxation of rocks was divided into elastic and yield stages, and the post-peak stress relaxation of rocks was separated into strain softening and residual strength stages according to the test results. The variations of relaxation magnitude, degree, time and rate of rocks with time in four stages were analyzed, and the differences between pre-peak and post-peak stress relaxation characteristics of rocks were investigated in detail. A damage variable was proposed considering the deterioration of relaxation moduli of rocks with time. A new nonlinear damage model was established by introducing the damage variable into three-parameter generalized Kelvin model. TheLevenberg-Marquardt algorithm was used to identify the pre-peak and post-peak model parameters of rocks. The variations of model parameters in four stages were analyzed, and the differences between pre-peak and post-peak model parameters were investigated according to the identification results. Finally, the mechanisms of pre-peak and post-peak stress relaxation behavior of rocks were discussed. The results show that： （1） through comparing four values of relaxation magnitude, degree, time and rate of rocks among different stages, it is found that four values of paramters in the strain softening stage are larger than those in three other stages, which should be paid more attention in rock engineering. （2） The post-peak values of G1, G2 and/-/1 in 5 - 9 strain levels are 1 - 53 percent of corresponding pre-peak values in first strain level. （3） The influence of post-peak rapture on the viscoelasticity of rocks is the largest, followed by the viscosity, and that on the instantaneous elasticity of rocks is the smallest. （4） The values of G1 and H1 decrease sharply in the strain softening stage, and the value of G2 drops dramatically in the yield stage. （5） The pre-peak and post-peak stress relaxation behavior of silty mudstone is mainly derived from the initiation and growth of microfissures and the initiation, growth and coalescence of macrocracks in rocks, respectively. Therefore, the mechanisms of pre-peak and post-peak stress relaxation behavior of rocks are different.