-160℃超低温冻融循环后花岗岩三点弯曲试验研究

Experimental study on three-point bending of granite after cryogenic freeze-thaw cycles at -160 ℃

为了研究超低温条件下冻融循环作用对花岗岩I型断裂韧度的影响,本文采用半圆弯曲(semicircular bending,SCB)试样,选取不同的冻融次数(1、2和3次),对天然状态下的花岗岩进行-160℃超低温冻融循环处理,并对冻融循环后的花岗岩进行三点弯曲试验,分析了-160℃冻融循环对花岗岩I型裂缝尖端局部破坏特征、断裂韧度以及微观结构的影响。结果表明,随着冻融循环次数的增加,花岗岩I型裂缝尖端局部化损伤不断加剧,断裂韧度不断降低,岩石内部微裂纹和孔隙数目增多,裂隙长度增加,孔径增大。最后,对比分析了低温与超低温条件下岩石冻胀力和断裂韧度的变化。与低温条件相比,超低温冻融产生的冻胀力更大,当断裂韧度下降幅度近似一致时,岩石在低温下需要冻融循环的次数更多。研究结果可为超低温环境下液化天然气(LNG)的地下存储提供相应的理论参考。

In order to study the effect of freeze-thaw cycles on the type Ⅰ fracture toughness of granite under ultralow temperature conditions, semi-circular bending(SCB) specimens were used in this study, and different freeze-thaw times(1, 2 and 3 times) were selected. The granite in the natural state was treated with-160 ℃ ultra-low temperature freeze-thaw cycles, and the three-point bending test was carried out on the granite after the freeze-thaw cycle. and microstructure effects. The results show that with the increase of freeze-thaw cycles, the localized damage of I-type crack tip of granite is intensified, the fracture toughness is decreased, the number of microcracks and pores in the rock is increased, the length of cracks is increased, and the pore size is increased.Finally, the changes of rock frost heaving force and fracture toughness under low temperature and ultra-low temperature conditions are compared and analyzed. Compared with low temperature conditions, the frost heaving force produced by ultra-low temperature freezing and thawing is larger. When fracture toughness decreases by approximately the same amplitude, rocks need more cycles of freezing and thawing at low temperature. The research results can provide theoretical reference for underground storage of liquefied natural gas(LNG) in ultralow temperature environment.