三轴压缩条件下北山花岗岩的力学特性及破裂演化

Study on mechanical characteristics and fracture evolution of Beishan granite under triaxial compression

岩石在荷载作用下会产生破裂损伤,力学特性发生退化。为研究北山花岗岩在三轴压缩状态下的力学特性和破裂演化机制,采用MTS815 Flex Test GT和PCI-2声发射(AE)系统对北山花岗岩试样进行了三轴压缩试验,利用采集的声发射信号分析了试样的破裂演化机制,获得了北山花岗岩变形参数,并拟合得到了MohrCoulomb强度准则参数φ=53.25°,c=32.37 MPa,Hoek-Brown强度准则参数σc=173.23 MPa,mi=32.78,s=1,其中参数mi在低围压区间敏感性较强,在高围压区间拟合效果较好。试验结果表明:北山花岗岩的弹性模量与泊松比均随围压的增大呈非线性增长,当围压超过20 MPa时,泊松比趋于稳定;北山花岗岩三轴压缩应力-应变曲线峰前段主要分为初始压密、弹性变形、裂纹稳定扩展和裂纹非稳定扩展4个阶段;基于体积应变模型和体积应变法求得了各阶段分界点的特征应力,即裂隙闭合应力、起裂应力和裂纹损伤应力,特征应力均随围压增加而增大,起裂应力比为0.56~0.58,闭合应力比为0.32~0.44,损伤应力比0.73~0.87;试样的破坏形态主要为单面剪切破坏,且破裂角随围压增加而逐渐减小,低围压下张性劈裂对岩石破坏具有一定的影响,随着围压升高,劈裂成分逐渐减小破裂面趋于平整。

Rocks produced fractures and damages under loading effects,and the mechanical properties are degenerated.The mechanical properties and fracture evolution characteristics of Beishan granites under triaxial compression were studied in this paper. A triaxial compression tests of Beishan granite samples were carried out with MTS815 Flex Test GT and PCI-2 acoustic emission（ AE） test system. The fracture evolution mechanism of the specimen was analyzed according to the acquisition of acoustic emission signals. The deformation parameters of Beishan granites were obtained.Mohr-Coulomb strength criterion parameters,including φ = 53. 25 and c = 32. 37 MPa,and Hoek-Brown strength criterion parameters,such as σc = 173.23 MPa,mi = 32.78,and s = 1 were obtained by fitting. The parameter mi was sensitive to low confining pressure,and the fitting effect was better in the high confining pressure range. The tested results show that the elastic modulus and Poisson＇s ratio of Beishan granite present a nonlinear increase with the increase in confining pressure. When the confining pressure is higher than 20 MPa,the Poisson＇s ratio tends to be stable.The front segment of the three triaxial compressive stress-strain curve of granite is mainly divided into four stages,including initial compaction,elastic deformation,stable crack extension and unsteady crack propagation. Based on the volumetric strain model and the volume strain method,each characteristic stress of the demarcation point at each stage is obtained and increased with the increase in confining pressure,including fracture stress,crack stress and crack damage stress. The split stress ratio is between 0.56 and 0.58,the closed stress ratio is between 0.32 the 0.44,and the ratio of damage stress is between 0.73 and 0.87. The specimen failure mode is mainly single side shear form. In addition,the rupture angle decreases with the increase in confining pressure. The tensile fracture under a low confining pressure has a certain effect on rock failure. With the increase in confining pressure,the splitting component gradually decreases and the fracture surface tends to be smooth.