Non-linear creep damage model of sandstone under freeze-thaw cycle

Abstract

To study rock damage characteristics under long-term freeze-thaw cycles and loads, rock freeze-thaw and creep damage factors were defined based on nuclear magnetic resonance porosity and volume strain, respectively. The damage factor is introduced into the basic rheological element, and the non-linear creep damage constitutive model and freeze-thaw rock equation are established to describe non-linear creep characteristics under a constant load. Simultaneously, the creep test of freeze-thaw rock under step loading is performed. Based on the test data, the applicability and accuracy of the creep damage freeze-thaw rock model are analyzed and verified. The results show that freeze-thaw cycles result in continuous rock pore structure damage and deterioration, and nuclear magnetic resonance porosity enhancement. The constant load induces increasing rock plastic deformation, volume, and creep aging damage. As the loading stress increases, the instantaneous rock elastic parameters increase, and the rheological elastic and viscosity parameters decrease. Furthermore, the damage degradation of freeze-thaw cycles weakens the rock viscoplasticity, resulting in a rapid decrease in the viscosity parameter with an increase in freeze-thaw cycles. Generally, the continuous damage of the rock is degraded, and the long-term strength decreases continuously.

摘要

为了研究冻融循环与载荷的长期作用下岩石内部的损伤特性, 以流变理论、损伤力学为基础, 定义了基于核磁共振孔隙度的岩石冻融损伤因子与基于体应变的蠕变损伤因子, 并将损伤因子引入基本流变元件中, 建立了冻融岩石的非线性蠕变损伤本构模型与方程, 用以描述冻融岩石在恒定载荷下的非线性蠕变特征。同时, 开展了冻融岩石的分级加载蠕变试验, 获得了冻融循环前后核磁共振孔隙度与岩石横向应变、纵向应变的变化情况。以试验数据为基础, 分析并验证了冻融岩石蠕变损伤模型的适用性与准确性。结果表明, 在冻融循环的作用下, 岩石内部孔隙结构不断损伤劣化, 核磁共振孔隙度持续增加; 在恒定载荷的作用下, 岩石的塑性变形增加, 体积增长明显, 蠕变时效损伤增强。随着加载应力的增加, 岩石的瞬时弹性参数增大, 流变弹性参数、黏性参数在快速降低; 其中高应力水平对岩石具有硬化作用从而减弱部分损伤效应, 使得后期流变弹性参数在小范围波动, 而冻融循环的损伤劣化作用, 使得岩石的黏塑性减弱, 导致黏性系数随着冻融次数的增加而快速降低, 但总体上岩石不断的损伤劣化, 长期强度持续降低。本文建立的基于损伤因子的冻融岩石蠕变模型, 能够准确描述冻融岩石的非线性特征。