Abstract:

The stability of rock structures plays a crucial role in various mining processes, including the study of open-pit mine slopes, predictive analyses of mine pressure in roadways, and the stability of underground quarries. Consequently, research into the strength and stability of rocks has remained a prominent area of focus in mining engineering. Numerous studies in rock mechanics have shown that the tensile strength of rock is significantly lower than its compressive strength, with most rock failures resulting from cleavage damage. As such, tensile strength is a key indicator of rock integrity. Current research on the tensile strength of laminated rocks primarily investigates the relationship between tensile strength and lamina inclination, with little attention paid to the influence of lamina spacing. Additionally, studies often describe the relationship between tensile strength and lamina inclination without exploring the underlying microstructural mechanisms responsible for rock failure. This study aims to investigate the relationship between tensile deformation characteristics, lamina spacing, and loading direction. Experimental samples were prepared with lamina spacing of 2d, 23/3d, d on the cuts of S₁, S₂, S₃ surface and level S, respectively. Brazilian splitting tests were performed to determine the tensile strength of siltstone under different loading directions. Full-field strain measurements using Digital Image Correlation(DIC)were employed to observe the deformation characteristics, and microstructural analysis was conducted based on crystal space distribution theory.

The results of the experiments are as follows:

(1)When the loading direction is parallel to the lamination direction, the tensile strengths of the S₁(2d), S₂(23/3d), and S₃(d) surfaces are 29.99MPa, 26.56MPa, and 18.92MPa, respectively. When the loading direction is perpendicular to the lamination direction, the tensile strengths of the same surfaces are 32.76MPa, 30.44MPa, and 27.77MPa. These findings indicate that tensile strength decreases with decreasing lamina spacing. Thus, as the laminae become more closely spaced, the tensile strength weakens, highlighting the contact surfaces between laminae as weak planes.

(2)The tensile strength of the sample with the S surface as the layer is not primarily influenced by weak interfaces, but rather by the mineral arrangement within the layer. When the loading direction aligns with denser mineral arrangements, the sample is more prone to cracking along the direction of looser particle arrangements, resulting in lower tensile strength. This demonstrates that preferential mineral orientation significantly impacts tensile strength.

(3)For samples with the same lamina spacing, when the loading direction is parallel to the laminae, the tensile region is concentrated along the centerline of the sample. As loading progresses, the tensile region propagates, eventually causing the sample to split into two. When the loading direction is perpendicular to the laminae, the tensile region is more dispersed initially, with several strain concentration points. These points are typically located along the transverse axis, and the sample splits in a manner similar to the parallel loading case. When the loading direction is diagonal to the laminae, the distribution of tensile regions is irregular, with the weak contact positions between laminae contributing to intermittent and tilted strain distributions.

(4)As lamina spacing decreases, the concentration of strain zones becomes denser under the same loading direction, further supporting the hypothesis that laminae contact surfaces act as weak planes.

The above findings provide valuable insights into the tensile behavior of laminated rocks and the influence of lamina spacing and mineral arrangement on rock strength.

Key words: Brazil split, tensile strength, mesostructure, layering

摘要：

为探究拉张变形特征与层理间距及加载方向的关系, 通过特定方式切割得到层理间距为2d、2 \sqrt{3}/3d、d的切面以及层面。通过巴西劈裂实验测定粉砂岩的抗拉强度, 利用DIC全场应变实验观察岩样不同方向的变形特征, 根据晶体空间分布理论分析粉砂岩岩样的微观结构特征。实验结果表明: 1)当加载方向与层理平行时, 随层理间距的减小, 其抗拉强度值为29.99MPa、26.56MPa、18.92MPa; 当加载方向垂直于层理时, 随层理间距的减小, 其抗拉强度值为32.76MPa、30.44MPa、27.77MPa, 说明抗拉强度值随层理间距减小而减小。当层理间距相同时, 垂直于层理方向的抗拉强度大于平行于层理方向的抗拉强度, 以上说明层与层之间的接触面为薄弱面。2)层面的抗拉强度值不受薄弱面控制, 而是与层面矿物排布有关, 当加载方向与矿物排布较密的方向相同时, 其抗拉强度较低, 说明当矿物排布存在明显的优选取向时, 也会影响其抗拉强度值。3)在层理间距相同的条件下, 当加载方向平行于层理方向时, 受拉区域集中在岩样中心线上, 随着应变增加受拉区域逐渐贯通形成连续的破裂面; 当加载方向垂直于层理方向时, 变形初期受拉区域较为分散, 但应变大致集中于层理方向, 随着应变增加, 破裂相互贯通; 当加载方向与层理方向斜交时, 横向受拉区域应变分布不规则, 呈现倾斜断续分布。4)当加载方向相同时, 应变集中带随着层理间距的减小而变密。

关键词: 巴西劈裂, 抗拉强度, 细观结构, 层理