Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading
Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading
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DOI：
10.1002/2016jb013646
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发表时间：
2017-06-01
影响因子：
3.9
通讯作者：
Mitchell, T. M.
中科院分区：
文献类型：
--
作者：
Browning, J.;Meredith, P. G.;Mitchell, T. M.
利用在三个垂直方向上独立控制应力路径的立方体试件的测量结果，我们对干燥砂岩在常规应力(sigma(1)>；sigma(2)=sigma(3))和真三轴(sigma(1)>；sigma(2)>；sigma(3))应力条件下的裂纹损伤演化进行了比较研究。为了表征裂纹损伤，我们测量了三个主方向上的超声纵波和横波速度的变化，以及与应力和应变同时输出的体声发射(AE)。我们使用声波速度来模拟相对的裂纹密度和方向。本质上，我们创建了两种端元裂纹分布；一种表现为柱面横向各向同性(常规三轴)，另一种表现为平面横向各向同性(真三轴)。在我们的实验中，在应力条件下，我们观察到传统三轴和真三轴之间的声发射事件的数量大约减少了五倍。当声发射数据、速度数据和裂纹密度数据综合在一起时，表明中间主应力抑制了裂纹的总数，并将它们的扩展限制在低于最小主应力的方向。然而，单个裂纹的大小基本上保持不变，受材料颗粒大小的控制。只有当差应力超过某一阈值时，才会产生裂纹损伤。循环加载实验表明，只有当超过先前的最大差应力时，才会开始进一步破坏，而不管平均应力是通过增加最大主应力还是通过降低最小主应力来实现的。
We present a comparative study of crack damage evolution in dry sandstone under both conventional (sigma(1)>sigma(2)=sigma(3)), and true triaxial (sigma(1)>sigma(2)>sigma(3)) stress conditions using results from measurements made on cubic samples deformed in three orthogonal directions with independently controlled stress paths. To characterize crack damage, we measured the changes in ultrasonic compressional and shear wave velocities in the three principal directions, together with the bulk acoustic emission (AE) output contemporaneously with stress and strain. We use acoustic wave velocities to model comparative crack densities and orientations. In essence, we create two end-member crack distributions; one displaying cylindrical transverse isotropy (conventional triaxial) and the other planar transverse isotropy (true triaxial). Under the stress conditions in our experiments we observed an approximately fivefold decrease in the number of AE events between the conventional and true triaxial cases. When taken together, the AE data, the velocities, and the crack density data indicate that the intermediate principal stress suppresses the total number of cracks and restricts their growth to orientations subnormal to the minimum principal stress. However, the size of individual cracks remains essentially constant, controlled by the material grain size. Crack damage is only generated when the differential stress exceeds some threshold value. Cyclic loading experiments show that further damage commences only when that previous maximum differential stress is exceeded, regardless of the mean stress, whether this is achieved by increasing the maximum principal stress or by decreasing the minimum principal stress.