基于微裂纹演化的岩石蠕变损伤跨层次分析

Creep is one of the most important causes of deformation, instability and failure of engineering surrounding rocks. The existence of a lot of micro-cracks as well as their evolution has crucial effects on the creep, damage and failure of rock materials. In order to find the primary causes and mechanisms related to the damage and failure of the rocks, systematic uniaxial/triaxial creep tests are to be performed for typical rock materials at different stress levels, temperatures and confining pressures, and the constitutive behavior, the failure mode, and the evolution of micro-cracks are to be investigated. The initiation, the growth and the coalescence of the microcracks during creep are to be described with fracture mechanics, and the effects of microcracks, including the interaction between mediums, on the mechanical properties of the rocks are to be evaluated with a micro-macro approach. A fourth-order damage tensor is to be derived with a proper statistical mechanics or homogenization scheme, with which, the description for the constitutive behavior and damage of the rocks subjected to creep deformation will be obtained in the framework of continuum damage mechanics and irreversible thermodynamics with internal variables. The corresponding numerical algorithm and the USER Material subroutine are to be developed and embedded in a commercially available FE code, with which the constitutive response, damage and failure of rocks during creep are to be simulated and verified with experimental results. The model, the approaches, and the results obtained in this project can provide a theoretical basis for structure design, support optimization and stability evaluation in geotechnical engineering.

蠕变是岩土工程围岩变形、失稳破坏的重要原因之一，岩石材料中存在的大量微裂纹及其演化对其蠕变损伤破坏有重要影响。选择典型岩石进行计及应力水平、温度和围压效应的单轴和三轴蠕变试验，进行细观力学实验，观察不同条件下材料中微裂纹的演化规律，分析岩石蠕变损伤与失效的影响因素及作用机理。用细观力学方法研究微裂纹的开裂、扩展规律，分析岩石蠕变损伤行为与微裂纹演化行为的相互作用机制，研究微裂纹演化与材料宏观破坏之间的关系，引入统计力学和均匀化理论，分析微裂纹群的演化行为对岩石变形的影响，建立岩石的四阶损伤张量；基于连续介质力学基本理论和含内变量的不可逆热力学，结合岩石材料蠕变损伤行为的分析，建立能反映微裂纹演化的岩石蠕变损伤细-宏观跨层次本构模型，发展相应的数值分析方法，模拟实际岩土工程中材料的蠕变损伤特性，为岩土工程结构设计、支护优化以及稳定性评估提供依据。

蠕变是岩石材料的重要力学特征，也是岩土工程围岩变形、失稳破坏的重要原因之一，岩石材料中存在的大量微裂纹及其演化对其蠕变损伤破坏有重要影响。利用自行研制的三轴蠕变仪，对泥岩进行了系统的单轴和三轴蠕变试验，试验中考虑了轴压和围压对蠕变的影响。结果表明，当围压一定时，轴向应力增加，蠕变加快，在稳态蠕变阶段的应变率增大，试件的寿命缩短。而当应力差保持不变时，围压增加，蠕变减慢，稳态蠕变阶段的应变率也减小，试件的寿命增加。. 通过对盐岩进行的不同温度和应力水平下的三轴蠕变试验表明，在围压和温度不变时, 轴压对蠕变和蠕变率的影响与泥岩相同。随着温度的增加，蠕变应变增大，稳态蠕变应变率也相应增大。. 在理论方面，基于含内变量的不可逆热力学和连续介质损伤力学，通过在不可逆应变和牛顿时间所构成的空间中合理的定义广义时间、引入四阶各向异性损伤张量，建立了岩石的蠕变损伤本构模型。该模型能够考虑复杂应力状态下材料的响应特性，各向异性损伤及其损伤的方向特征，静水压力和温度的影响等。发展了相应的数值分析方法，并根据泥岩三轴蠕变试验结果进行了验证。. 研究了含瓦斯煤岩中椭圆形微裂纹的变形及演化行为对煤岩变形及破坏的影响，针对煤岩同时受三轴应力和空隙压力作用，分析了椭圆形微裂纹的张开和闭合对煤岩变形的影响，利用能量平衡原理建立了椭圆形微裂纹的扩展准则，发展了一种椭圆形微裂纹偏折扩展的计算方法，建立了椭圆形微裂纹的汇合准则，分析了微裂纹偏折扩展、汇合与煤岩破坏的关系，建立了含瓦斯煤岩的细观损伤模型。引入概率密度函数，将基于Taylor方法建立的三维宏细观损伤模型，应用于分析含瓦斯煤岩的三轴压缩实验，理论结果与实验结果基本符合。