热冲击对花岗岩动态断裂行为的影响研究

Study on the Effect of Thermal Shock on Dynamic Fracture Behavior of Granite

为了研究热冲击对加热花岗岩的影响,使用3种冷却方法来提供不同的冷却速率,对花岗岩试样施加不同程度的热冲击作用。通过分离式霍普金森压杆(SHPB)系统对热冲击处理后的花岗岩半圆盘中心直裂纹三点弯曲(NSCB)试样进行冲击断裂试验,并利用高速摄像机记录了试样的断裂模式。试验结果表明:随着试样温度和冷却速率的提升,试样的干密度和纵波波速显著下降,试样的孔隙率加大。试样的断裂韧度变化及破坏模式表明热冲击作用使花岗岩试样受到冲击力时抗裂纹扩展的能力下降,尤其在冲击载荷的加载率高于130 GPa·m^(0.5)/s时下降尤为明显。

In engineering operations such as geothermal development and utilization in high-temperature rock formations,underground coal gasification,multiple oil extractions,underground disposal of high-level radioactive waste,and protection and restoration of important buildings after fires,rocks often experience thermal shock due to drastic temperature changes.Thermal shock refers to the phenomenon where an object undergoes a large amount of heat exchange in a short time due to rapid heating or cooling,resulting in the generation of thermal shock stress within the object.To investigate the impact of thermal shock on the dynamic fracture behavior of high-temperature granite,the granite was heated to three different temperature levels of 100,300,600℃,followed by cooling using three different methods of furnace cooling,air cooling,and water cooling to provide different cooling rates,resulting in varying degrees of thermal shock within the granite samples.The notched semi-circular bend(NSCB)specimens of the granite samples subjected to thermal shock treatment were tested using a split Hopkinson pressure bar(SHPB)system for dynamic fracture behavior.The fracture pattern of the specimens was recorded using a high-speed camera.The results show that with the increase in specimen temperature and cooling rate,the dry density and longitudinal wave velocity of the specimens decrease significantly,and the porosity increase.Under thermal shock,the dynamic fracture toughness value of the specimens decreased significantly.At the same loading rate level,the dynamic fracture toughness value of the water-cooled specimens is lower than that of the air-cooled specimens,indicating that the crack propagation resistance of rock materials decrease when subjected to dynamic impact.Furthermore,at loading rates higher than 130 GPa·m^(0.5)/s,the effect of thermal shock on fracture toughness is more pronounced.By analyzing the relationship between dynamic fracture toughness and loading rate,a power law function with a good fitting degree is obtained,revealing the impact of thermal shock on dynamic fracture toughness.Therefore,this study provides valuable reference for the stability of rock formations in cooling treatment projects involving high-temperature rocks,contributing to the design and management of engineering operations such as geothermal development and utilization,underground coal gasification,multiple oil extractions,underground disposal of high-level radioactive waste,and protection and restoration of important buildings after fires.