In geothermal engineering, dynamic loads (e.g., drilling, hydraulic fracturing) and thermal shock between working fluids and high-temperature reservoirs significantly affect well-wall stability and exploitation efficiency. This study performed a series of dynamic tensile experiments on granite flattened Brazilian disc (FBD) specimens treated by cyclic thermal shock (CTS) using a split Hopkinson press bar (SHPB) system. The influence of temperature and thermal cycles on the physical and tensile mechanical properties, failure process, and morphological characteristics was analyzed, and the coupled failure mechanism was discussed in combination with numerical simulation. The results show that the P-wave velocity and dynamic tensile strength of granite decrease with increasing CTS temperature and cycles, while porosity increases, and a response surface model for the degradation of physical properties is developed. The central cracking of the FBD specimens is controlled by pure tensile, and the cracking time lags with increasing CTS temperature and cycles. The specimens exhibit tensile-shear composite failure, and the failure forms change from Y-shape to X-shape with increasing temperature, but transition from Y-shape to mono-diagonal shape with increasing thermal cycles. The fractal dimension and joint roughness coefficient (JRC) of the fracture surface are positively correlated with CTS temperature and cycles. Moreover, the CTS-induced thermal cracks increase with temperature and thermal cycles in a polynomial and negative exponential function law, respectively, making the granite transgranular fracture more prominent. The transition from parallel cleavage to exfoliation cleavage of mica may be an important reason for the increase of granite ductility at high temperatures.

中文翻译：

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干热花岗岩在循环热冲击和动态拉伸载荷作用下的热机械性能和断裂行为

在地热工程中，工作流体和高温储层之间的动态载荷（如钻井、水力压裂）和热冲击对井壁稳定性和开采效率有显著影响。本研究使用分体式霍普金森压杆 （SHPB） 系统对经循环热冲击 （CTS） 处理的花岗岩扁平巴西盘 （FBD） 试样进行了一系列动态拉伸实验。分析了温度和热循环对物理和拉伸力学性能、破坏过程和形态特性的影响，并结合数值模拟讨论了耦合破坏机制。结果表明，花岗岩的 P 波速度和动态抗拉强度随着 CTS 温度和循环次数的增加而降低，而孔隙率增加，建立了物理性能退化的响应面模型。FBD 试件的中心开裂由纯拉伸控制，开裂时间随 CTS 温度和循环的增加而滞后。试件表现出拉伸-剪切复合材料破坏，破坏形式随温度的升高从 Y 形变为 X 形，但随着热循环次数的增加，破坏形式从 Y 形转变为单对角线形状。裂缝表面的分形维数和节理粗糙度系数 （JRC） 与 CTS 温度和周期呈正相关。此外，CTS 诱导的热裂纹分别以多项式和负指数函数律随温度和热循环的增加而增加，使花岗岩穿晶裂缝更加突出。云母从平行解理转变为剥落解理可能是花岗岩在高温下延展性增加的重要原因。