三类岩石热损伤力学特性的试验研究与细观力学分析

运用偏光显微技术,比较不同温度处理后砂岩、花岗岩和大理岩微观结构的不同变化特征。分析对比常温~800℃高温处理后三类岩石纵波波速、孔隙率、弹性模量、峰值应力及应变的变化规律,并讨论其与微观结构变化的内在联系。结合岩石热损伤后初始损伤程度增大、微裂纹刚度弱化及张开度增大等特征,采用细观损伤力学模型研究热损伤岩石应力–应变曲线显著的非线性特征。研究结果表明:(1)热处理砂岩细观结构的变化主要表现为胶结物变化及矿物相变,矿物内无明显热裂纹发育;热处理花岗岩内热裂纹发育明显,800℃处理后最大裂纹宽度可达100μm,较400℃时增加约1个数量级;大理岩热裂纹以晶界裂纹为主,600℃处理后最大裂纹宽度达20μm,约为400℃时的2倍。(2)花岗岩和大理岩的弹性模量随热处理温度的增大持续降低,但砂岩的弹性模量在500℃热处理温度阈值之后才显著下降。(3)三类热损伤岩石的宏观物理力学性质与其形成条件、矿物组分、微裂纹发育密切相关。(4)基于均匀化理论的细观损伤力学模型的计算值与试验值吻合良好,热损伤岩石应力–应变曲线初始压密阶段显著延长的力学行为与微裂纹密度和刚度直接相关。

Laboratory study and micromechanical analysis of mechanical behaviors of three thermally damaged rocks

In this study，the alterations in the microstructures of three types of rocks (i.e. sandstone，granite and marble) were observed with the polarized light microscopy after thermal treatment at different temperatures. The variations in the physical-mechanical properties(including the longitudinal wave velocity，porosity，Young?s modulus，peak stress and the corresponding strain) of three types of rocks with the thermal treatment from room temperature to 800 ℃ were analyzed and compared and were correlated to the microstructural variations. The strong ductile behavior of the thermally cracked rocks was interpreted with a micromechanical model by taking into account the normal stiffness reduction of micro-cracks. A notable change in the carbonate cementation was observed in thermally cracked sandstone，in which the thermally induced cracking was not developed across the mineral particles. Both transgranular and intergranular cracking developed in thermally cracked granite with a maximum opening of 100 μm at 800 ℃，which was one order of magnitude larger than that at 400 ℃. The cracks in thermally treated marble were mainly intergranular，with the maximum width of about 20 μm at 600 ℃. Different from granite and marble，a dramatic decrease of elastic modulus occurred in thermally cracked sandstone when the treatment temperature was over 500 ℃，which was mainly ascribed to the phase transition of quartz. It was found that the physical-mechanical properties of the thermally cracked rocks depended much on the diagenetic processes，mineral compositions and cracking patterns. Moreover，the numerical simulation results are in good agreement with the experimental data，showing that the mechanical behaviors of the thermally cracked rocks are closely related to the density and stiffness of the thermally induced cracks.