Abstract

Investigating the coupling effects of temperature levels and number of heating-water cooling cycles on thermal damage, cracking failure mechanisms, and mechanical responses of HDR (hot dry rock) is a vital issue during the exploitation of geothermal energy. In this study, a large number of granite samples exposed to high temperature treatment (25, 150, 300, 450, 600 °C) with multiple heating and water cooling cycles (1, 5, 10, 15) were carried out micro observations of X-ray diffraction (XRD) and scanning electron microscopy (SEM), and uniaxial compression tests with acoustic emission (AE) signals monitoring. The results show that, as the temperature level or number of heating and water cooling cycles increases, uniaxial compressive strength, elastic modulus, crack damage stress, and the damage index all present a reduction, while the peak axial strain, dilatancy volumetric strain and Poisson’s ratio increase. A genetic algorithm is established to evaluate the coupling influences of temperature and number of heating-water cooling cycles on the uniaxial compressive strength and dilatancy volumetric strain. The AE activities tend to be active due to crack initiation, development and coalescence. For a large temperature level or number of cycles, the AE signals remain active during the whole deformation process, while increase rate of the accumulated AE counts with time gradually slows down. The granites undergo a typical splitting failure mode, characterized by several tensile cracking planes at a low temperature or number of cycles, but present complicated crack networks of a turtle shape with increasing temperature or number of cycles, resulting in a larger fractal dimension of the induced surface cracks. The failure mechanism of rock gradually transfers from brittleness-dominated to ductility-dominated due to accumulated thermal damage. Then, the micro thermal damage mechanism of uncompressed granites is analyzed through microscopic observations of XRD and SEM. A power function is proposed to describe the relations between longitudinal wave velocity and uniaxial compressive strength. With increasing temperature or number of cycles, the maximum diffraction intensity of quartz decreases by 25.82%–38.89% and 13.42%–31.93%, respectively, and the micro thermal defects gradually develop, resulting in weakened macro bearing capacity.

Article highlights

• Heating and water cooling cycles effects on mechanical properties of granites.

• Heating and water cooling cycles effects on micro thermal damage and failure mechanisms of granites.

• Micro mechanisms for the macro strength responses.

中文翻译：

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多次加热和水冷循环对花岗岩物理力学响应的影响

摘要

研究温度水平和加热水冷却循环次数对 HDR（干热岩）的热损伤、开裂失效机制和机械响应的耦合效应是地热能开发过程中的一个重要问题。本研究对大量暴露于高温处理（25、150、300、450、600°C）和多次加热和水冷循环（1、5、10、15）的花岗岩样品进行了微观观察。 X 射线衍射 (XRD) 和扫描电子显微镜 (SEM)，以及带有声发射 (AE) 信号监测的单轴压缩测试。结果表明，随着温度水平或加热和水冷循环次数的增加，单轴抗压强度、弹性模量、裂纹损伤应力和损伤指数均呈下降趋势，而峰值轴向应变、剪胀体积应变和泊松比增加。建立了一种遗传算法来评估温度和热水冷却循环次数对单轴抗压强度和剪胀体积应变的耦合影响。由于裂纹的萌生、发展和聚结，AE 活动趋于活跃。对于较大的温度水平或循环次数，AE 信号在整个变形过程中保持活跃，而累积 AE 计数随时间的增加速率逐渐减慢。花岗岩经历典型的劈裂破坏模式，其特征是在低温或循环次数下出现多个拉伸开裂面，但随着温度或循环次数的增加，呈现出复杂的龟形裂纹网络，导致诱导表面裂纹的分形维数更大。由于累积热损伤，岩石的破坏机制逐渐从脆性为主转变为延性为主。然后，通过XRD和SEM显微观察分析了未压缩花岗岩的微观热损伤机制。提出了幂函数来描述纵波速度与单轴抗压强度之间的关系。随着温度或循环次数的增加，石英的最大衍射强度分别下降25.82%~38.89%和13.42%~31.93%，微观热缺陷逐渐发展，宏观承载力减弱。由于累积热损伤，岩石的破坏机制逐渐从脆性主导向延性主导转变。然后，通过XRD和SEM显微观察分析了未压缩花岗岩的微观热损伤机制。提出了幂函数来描述纵波速度与单轴抗压强度之间的关系。随着温度或循环次数的增加，石英的最大衍射强度分别下降25.82%~38.89%和13.42%~31.93%，微观热缺陷逐渐发展，宏观承载力减弱。由于累积热损伤，岩石的破坏机制逐渐从脆性为主转变为延性为主。然后，通过XRD和SEM显微观察分析了未压缩花岗岩的微观热损伤机制。提出了幂函数来描述纵波速度与单轴抗压强度之间的关系。随着温度或循环次数的增加，石英的最大衍射强度分别下降25.82%~38.89%和13.42%~31.93%，微观热缺陷逐渐发展，宏观承载力减弱。提出了幂函数来描述纵波速度与单轴抗压强度之间的关系。随着温度或循环次数的增加，石英的最大衍射强度分别下降25.82%~38.89%和13.42%~31.93%，微观热缺陷逐渐发展，宏观承载力减弱。提出了幂函数来描述纵波速度与单轴抗压强度之间的关系。随着温度或循环次数的增加，石英的最大衍射强度分别下降25.82%~38.89%和13.42%~31.93%，微观热缺陷逐渐发展，宏观承载力减弱。

文章亮点

• 加热和水冷循环对花岗岩力学性能的影响。

• 加热和水冷循环对花岗岩微观热损伤和破坏机制的影响。

• 宏观强度响应的微观机制。