含弧形预制裂隙砂岩力学特征试验研究
Mechanical characteristics of sandstone containing arc prefa- bricated fissures
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摘要: 天然岩体中富含近似弧形裂隙缺陷,在外荷载作用下弧形裂隙容易萌生新生裂纹,新生裂 纹的扩展容易导致岩石工程的失稳,但针对含弧形预制裂隙岩石力学相关特征还不够全面,基于此 在板状黄砂岩试样内预制了不同 γ 值(弧形高度和直径比值)的弧形裂隙,研究含弧形预制缺陷砂 岩的力学特征。 利用高压水射流切割机在完整板状黄砂岩试样中切割出不同 γ 值弧形裂隙缺陷, 采用 YNS-2000 型电液伺服控制试验系统、DS2 声发射和数字照相采集系统研究了不同弧形裂隙 γ 在单轴压缩作用下对黄砂岩峰值强度、平均模量和割线模量、声发射、破坏过程、起裂形式、起裂 应力和破坏形式的影响规律。 利用复变函数求解在弧形裂隙应力场中所构建的黎曼-希尔伯特问 题,获得尖端应力强度因子表达式。 试验结果表明:① 弧形裂隙缺陷砂岩试样随着 γ 的增大,峰后 试样的承载能力逐渐降低,试样峰值强度、平均模量和割线模量出现总体减小趋势,当 γ = 0.2时峰 值强度最大为 27.09 MPa;当 γ = 1 时峰值强度最小为 18.99 MPa。 ② 预制弧形裂隙 γ 对试样起裂 应力、起裂位置、破裂演化过程、声发射特征和破坏模式均具有重要影响,随着 γ 值的增大,试样起 裂应力呈总体减小趋势,当 γ = 0.8时,试样的平均起裂应力最小,其值为 4.99 MPa;当 γ = 0 时,试 样的平均起裂应力值为 11.08 MPa,平均起裂应力值最大。 ③ 随着 γ 的增大,试样的破坏模式由 拉剪混合破坏向拉伸破坏转变,当 γ = 0 时,试样为拉剪破坏;当 γ 在 0.2~1.0 时,试样为拉伸破 坏。 初始起裂裂纹扩展并不是导致试样最终破坏的原因,次生裂纹的扩展与自由面贯通才是导致 试样整体破坏原因,试样破坏瞬间释放大量弹性能导致岩块折断和表面剥落现象。 通过对含弧形 裂隙裂纹扩展机制探讨推导出应力强度因子表达式,并根据该表达式求出相应应力强度因子值,曲 线拟合后发现其变化趋势与试验值变化趋势基本吻合。 研究仅通过单轴压缩作用研究发现弧形裂 隙 γ 对砂岩力学参数、破裂演化过程和破坏形式影响明显,为了充分研究外界荷载对含弧形裂隙砂 岩的破坏特征,将通过改变加载形式和加载路径深入研究含该类缺陷岩石的力学特征。Abstract: The natural rock mass is rich in similar arc-shaped crack defects,and the arc-shaped crack is prone to gen-erate new cracks under external load,and the growth of new cracks will easily lead to the instability of rock engineering structure. However,the mechanical characteristics of rock containing arc-shaped prefabricated frac-tures are not com-prehensively understood. Based on this,the arc-shaped fractures with different γ values (arc-shaped height and diame-ter ratio) were prefabricated in the sheet yellow-sandstone sample to study the me-chanical characteristics of sandstone containing arc-shaped prefabricated fractures. Using high pressure water jet cutting machine to cut arc-shaped fractures with different γ values in complete sheet yellow-sandstone samples,the effects of different arc-shaped fractures γ under uniaxial compression on peak strength,mean modulus and secession modulus,acoustic emission,failure process,initia-tion mode,initiation stress and failure mode of yellow-sandstone were studied by the YNS-2000 electro-hydraulic servo control test system,DS2 acoustic emission and digital photographic acquisition system. The complex function was used to solve the Riemann-Hilbert problem in the curved fracture stress field. The experimental results show that:① The bearing capacity of the sample after peak decreases gradually and the peak strength,average modulus and secant mod-ulus of the sample generally decrease with the increase of γ. The maximum peak intensity was 27. 09 MPa when γ = 0. 2 and the peak intensity was at least 18. 99 MPa when γ = 1. ② The inclination γ of arc crack has an important in-fluence on the initiation stress, initiation position, fracture evolution, acoustic emission characteristics and failure modes. The crack initiation stress of the test specimen decreases in general with the increase of the antithesis value. The average crack initiation stress of the test specimen was at least 4. 99 MPa when γ = 0. 8,and the average crack in-itiation stress of the sample was 11. 08 MPa with the maximum average crack initiation stress when γ = 0. ③ With the increase of γ,the failure mode of the sample changes from the mixed failure of tension and shear to the tensile failure. The sample is subjected to tension and shear failure when γ = 0. The specimen is tensile failure when γ is within the range of 0. 2-1. 0. The initial crack growth is not the cause of the ultimate failure of the sample,but the secondary crack growth and free surface penetration are the causes of the overall failure of the sample. The expression of the stress intensity factor was derived from the discussion of the crack propagation mechanism of the curved crack,and the corresponding value was obtained according to the expression. In this paper,only through uniaxial compression study, it is found that arc-shaped fracture has obvious influence on the mechanical parameters,fracture evolution process and failure mode of sandstone. In order to fully study the failure characteristics of sandstone containing arc-shaped fracture under external load,the mechanical characteristics of rocks containing such defects will be further studied by changing the loading form and loading path.