Spatially distributed damage in sandstone under stress-freeze-thaw coupling conditions

In the present study,we tried to understand the spatially distributed damage in sandstone samples under the coupled stress-freeze-thaw(SFT)conditions.Firstly,uniaxial compressive stresses(i.e.0 MPa,10 MPa,20 MPa,and 25 MPa)were applied to the samples,and then freeze-thaw(FT)cycles(0,8,16,and 24)were performed on the uniaxially stressed samples to realize the SFT coupling.Next,real-time CT scanning was conducted to observe the induced damage.The total porosity was introduced to quantitatively evaluate the damage degree.The local porosity variation,with the distance from the center of the sandstone sample,was analyzed to understand the spatial distribution of damage.Finally,the coupling effects of SFT on the damage gradient were discussed.The results indicate that the porosity rises with FT cycles,and the applied stresses can accelerate the increase in porosity.The damage increases exponentially with the distance from the center of the sample.The damage presents a spatial gradient distribution,not the commonly used uniform distribution in various studies.The damage gradient increases with FT cycles,and the increasing rate in damage gradient decreases at uniaxial stress of 0 MPa and 10 MPa first,but the increasing rate in damage gradient increases with FT cycles then at stress increasing to 20 MPa.

考虑应力水平和化学腐蚀效应的冻融砂岩细观损伤试验

寒区工程岩体在经受冻融作用的同时受到外部荷载和化学溶液的影响,直接体现在孔隙结构的变化上,决定着冻融损伤劣化机制。为研究应力、溶液和冻融共同作用下砂岩孔隙结构的演化特征,开展了不同应力水平和化学溶液的砂岩冻融循环试验,通过核磁共振获得的T2谱和扫描电镜技术获取的微观图像,分析了砂岩孔隙孔径分布、孔隙度、不均匀系数、渗透率和弯曲率以及微观结构随冻融循环的变化情况,探讨了应力水平和化学溶液对冻融过程中砂岩孔隙结构演化过程的影响规律和控制机制。研究结果表明:随冻融循环次数增加,不同尺寸孔隙增加,中孔扩展速率加快,且易扩展为更大尺寸的大孔,引起孔隙度、不均匀系数、渗透率增加,而弯曲率降低;稀硫酸溶液促进小孔发育为中孔和大孔。当轴向应力较低(0.3σ_(f))时,在冻融初期垂直于应力方向的孔隙发生闭合,抑制了化学腐蚀和冻胀力的发挥,不同尺寸孔隙数量较无应力下减少,孔隙度、不均匀系数、渗透率减小,弯曲率增加;之后随化学腐蚀和冻胀力交替累加,再加上轴向应力的附加作用,引起不同尺寸孔隙数量增加。当轴向应力较高(0.7σ_(f))时,受应力主控,中孔和大孔增加明显,沿晶裂纹和穿晶裂纹快速萌生和扩展,引起孔隙度、不均匀系数、渗透率快速增大,弯曲率降低,最终导致砂岩破裂。