高应力高水压下砂岩三轴蠕变特性试验研究

对砂岩进行高围压高水压条件下的三轴压缩蠕变试验。试验表明,在整个加载过程中,孔隙水压力主要起到增强轴向变形和横向变形的作用;但在加载的初始阶段,孔隙水压力在一定程度上抑制了轴向变形。当应力水平高于屈服应力时,横向蠕变速率明显大于轴向蠕变速率,且横向蠕变率先进入加速蠕变阶段。本文提出一个新的非线性黏性元件,并引入一个能判断是否进入加速蠕变阶段的计时器元件,组建一个非线性黏塑性加速蠕变启动模型,将该黏塑性模型与Burgers模型串联,构建一个新的非线性黏弹塑性蠕变模型,推导了该模型在常规三轴应力状态下的本构方程。基于试验结果,通过对非线性优化算法(BFGS)的Matlab编程,实现对本文提出蠕变模型的参数识别,识别效果比较理想。对比试验曲线与拟合曲线,二者相当吻合,验证了新提出的非线性黏弹塑性蠕变模型的正确性。

Experimental Study of Sandstone Triaxial Creep Behavior under High Stress and High Water Pressure

Triaxial compression creep experiment of sandstone under high confining pressure and high water pressure was conducted. Results show that during whole loading process, pore water pressure mainly promotes the growth of axial and lateral deformation. However, on the initial stage of loading, pore water pressure inhibits the axial deformation to some extent. When stress level exceeds yield stress, the lateral creep rate is obviously faster than that of axial creep; moreover, the lateral creep enters the accelerated creep stage earlier than axial creep. A new nonlinear viscous component was developed; meanwhile, a timer element was introduced which judges whether creep enters into the accelerated stage. Based on above preparation, a nonlinear viscoplastic accelerated creep startup model was established. Connecting this model with Burgers model in series, a new nonlinear visco-elastic-plastic creep model was structured, and the constitutive equation in conventional triaxial stress condition was derived. According to experimental results, through the nonlinear optimization algorithm (BFGS) Matlab programming, the creep model parameter identification is achieved. The identification effect is ideal. Comparison between experimental curves and fitting curves shows that both are in good agreement, which verifies the correctness of proposed new nonlinear visco-elastic-plastic creep model.