冻融荷载耦合作用下含开口裂隙砂岩宏细观损伤模型研究

Study on macro-meso damage model of sandstone with open cracks under the coupling action of freeze-thaw and load

开展宏细观尺度的岩石损伤演化规律是评价冻岩力学性质劣化过程的基础。以含开口裂隙砂岩为对象,从开口裂隙体积的角度,基于称重法提出宏观损伤变量的计算方法;结合长期冻融循环下岩样成分及物理力学性质变化特性,对张开孔隙率之差进行了合理修正,由此得到冻融作用下的细观损伤变量;考虑荷载作用下初始孔隙、裂隙压密的特点,采用统计损伤力学理论,建立荷载作用下细观损伤变量计算模型;并通过基于Lemaitre应变等效假设的宏细观总损伤计算方法,计算得到宏细观耦合总损伤;根据推广后的应变等价原理,构建了考虑初始压密段影响的总损伤本构模型。为了验证模型的合理性,采用预制的含0°,30°,45°,60°,90°倾角的裂隙砂岩岩样和完整砂岩岩样,开展多次冻融循环后的物理力学性质试验。研究结果表明:冻融循环次数较少时,冻融损伤变量增长较快,冻融损伤变量随循环次数的增加逐渐趋于稳定并缓慢增加;裂隙倾角对砂岩岩样内部微孔隙、微裂隙的扩展影响表现为损伤变量的增速不同,含45°倾角岩样的冻融损伤变量增速最大,冻胀力引起含45°倾角岩样的微孔隙、微裂隙扩展速率最快;细观总损伤模型和宏细观耦合总损伤模型曲线演化规律趋于一致,冻融荷载耦合作用为该次试验砂岩岩样损伤劣化的主要来源;峰值应力处应变的宏细观总损伤试验值与理论计算值变化趋势相近,提出的理论模型可较为合理地描述含裂隙砂岩的损伤劣化机制。

The evolution law of rock damage at macro and meso scales is the basis for evaluating the degradation process of mechanical properties of frozen rock.In this paper,the calculation method of macroscopic damage variable is proposed based on weighing method from the perspective of the volume of open fractures.Combined with the variation characteristics of rock composition and physical and mechanical properties under long-term freeze-thaw cycles,the difference of open porosity was reasonably corrected,and the meso-damage variables under freeze-thaw action were obtained.Considering the compaction characteristics of initial pores and cracks under load,a calculation model of mesoscopic damage variable under load is established by using statistical damage mechanics.The macro-meso coupling total damage is calculated by the macro-meso total damage calculation method based on Lemaitre strain equivalent hypothesis.According to the extended strain equivalence principle,the total damage constitutive model considering the influence of initial compaction section is constructed.In order to verify the rationality of the model,the prefabricated fractured sandstone samples with dip angles of 0°,30°,45°,60°and 90°and the intact sandstone samples were used to carry out the test of physical and mechanical properties after multiple freeze-thaw cycles.The research results show as follows.When the number of freeze-thaw cycles is small,the freeze-thaw damage variables increase rapidly,and the freeze-thaw damage variables gradually tend to be stable and increase slowly with the increase of the number of cycles.The influence of dip angles of fractures on the expansion of micro-pores and micro-fractures in sandstone rock samples is reflected in the different growth rates of damage variables.The increase rate of freeze-thaw damage variable of rock samples with 45°dip angle is the largest,and the expansion rate of micro-pores and micro-fractures of rock samples with 45°dip angle caused by frost heaving force is the fastest.The damage evolution curve of meso total damage model and macro-meso coupling total damage model tends to be consistent,and the coupling effect of freeze-thaw and load is the main source of damage and deterioration of sandstone samples in this test.The variation trends of macro-meso total damage test values of strains at the peak stresses are similar to those of theoretical calculation values,and the proposed theoretical model can reasonably describe the mechanisms of damage and degradation of fractured sandstone.