砂岩裂纹起裂损伤强度及脆性参数演化试验研究

岩石起裂强度σci及损伤强度σcd作为岩石重要的强度特征值,其研究对于分析岩石的渐进破坏过程及预测隧洞脆性破坏有着重要意义。首先采用应变分析及声发射监测方法,研究了两组硬质砂岩试样在单轴及三轴压缩过程中的裂纹演化特征。试验结果表明,试样的侧向膨胀变形及声发射计数值可以较好地反映其内部的裂纹演化情况,二者随着裂纹的萌生积累都表现出一致的阶段性变化规律。同时通过进一步分析应变及声发射曲线中的阶段性变化拐点,确定了砂岩试样的起裂强度及损伤强度值。其中青砂岩平均起裂强度约为其峰值强度的0.42倍,红砂岩平均起裂强度则约为其峰值强度的0.48倍。最后通过对比不同围压下的试验结果,发现两组砂岩试样起裂阶段内摩擦角均小于其峰值阶段,初始φ0约为终值的1/2。由此定义了可反映岩石脆性程度的起裂摩擦水平φ_0/φ,并建立考虑摩擦作用的线性起裂准则。

Experimental study on damage strength of crack initiation and evaluation of brittle parameters of sandstone

The strengths of crack initiation and damage are the important stress thresholds for their significance in analyzing the progressive failure process of rock, which can be applied in the prediction of brittle failure in tunnels. Firstly, the uniaxial and triaxial compression tests are conducted on two sets of hard sandstones, while their strain and acoustic emission (AE) characteristics are analyzed to study the evolution process of crack. The measured lateral dilation strains and the AE counts of rock samples can characterize their crack accumulation accurately according to the test results. And also, there is a consistent segmented transformation law in these two figures as the inner micro-cracks are accumulated. Thus the strengths of crack initiation and damage of the sandstone samples can be determined based on a detailed analysis on stage boundaries of the above curves. The average strength of crack initiation of green sandstone samples is 0.42 of its peak strength, while that of red sandstone samples is 0.48 of its peak strength. Furthermore the confinement influence on evolution process of crack is discussed by comparing different test results under varying confinements. It is found that the initial internal friction angle (φ₀) of sandstone samples at the crack initiation stage is less than its ultimate value (φ) when the peak load is applied. And the calculated initial φ₀ is about 1/2 of the ultimate φ. As a result, the initial internal friction level φ₀/φ of rock is defined, which has an inverse relation with the ratio of uniaxial compressive strength to tensile strength σ_c /σ_t , and it can also reflect the brittleness of rock. And the linear strength threshold of crack initiation is established, while the slope of curve represents the influence of friction effects.