层理方向对砂岩断裂模式及韧度的影响规律试验研究

为了探索具有层理面的砂岩断裂力学性质的各向异性,开展了具有不同层理方向的半圆形砂岩试样在不同切缝角度下的三点弯试验研究,揭示了层理方向对砂岩应力强度因子、断裂韧度及破裂模式的影响规律。试验结果表明试样破裂模式受层理面与荷载方向夹角θ控制:θ=0°时,沿层理面张裂破坏;θ=30°时,沿层理面剪切破坏;θ=45°,60°时,切层和沿层理面混合破裂;θ=90°时,切层破坏。不同层理角度的试样测得的断裂韧度差异较大,切缝角α=0°时,θ=90°试样断裂韧度最大,θ=0°试样断裂韧度最小,且KImax/KImin=2.36。运用有限元计算了各试样的无量纲化应力强度因子,结果表明切缝角α=0°时,无量纲化II型应力强度因子YII受层理面与荷载方向夹角θ影响显著:θ=0°,90°试样YII=0,呈现I型断裂;θ=45°,60°试样YII≠0,呈现出I-II复合型断裂;θ=30°试样YII最大,以II型断裂占主导,其余切缝角度下试样无量纲化I型应力强度因子与II型应力强度因子随层理角度θ的变化呈现不同的变化规律。通过扩展有限元XFEM计算出的试样起裂角、断裂韧度及断裂路径与试验结果吻合较好,结果表明各试样的起裂角随层理面与荷载方向夹角θ及切缝角α的变化呈现一定的各向异性。试验所得规律有助于更全面理解具有层理面岩石的断裂特性,并可作为对各向异性岩石断裂力学理论研究和数值计算的有益补充。

Fracture pattern and toughness of layered sandstone influenced by layer orientation

In order to study the anisotropic properties of layered sandstone on fracture mechanics, the three-point bending tests on the semi-circular specimens with different layer orientations and crack angles are conducted. The variations of normalized stress intensity factors, fracture toughness and fracture patterns of these specimens with layer orientation are revealed. The results illustrate that the fracture pattern of the specimens is closely related to the angle θ between the layer orientation and the loading direction, that is, tensile splitting along the layer when θ=0°, shearing along the layer when θ=30°, tensile splitting across the layer when θ=90° and composite shearing failure across and along the layer θ=45° and 60°. The fracture toughness of the specimens with different layer orientations greatly differs with the angle between the layer orientation and the loading direction; when crack angle α=0°, the specimens with θ of 90° has the maximum fracture toughness; and when those with θ of 0° have the minimum one, the ratio equals 2.36. Modes I and II normalized stress intensity factors are calculated by using the finite element code ABAQUS. It is shown that the mode II normalized stress intensity factor of the specimens varies more evidently with the layer orientation when α equals 0°, that is, mode I fracture when θ=0°and 90°, mixed mode when θ=45° and 60°, and mode II dominated fracture when θ=30°. In addition, for the specimens with α=30°~60°, the mode I and mode II normalized stress intensity factors show different variations with the layer orientation. The crack initiation angle, mixed-mode fracture toughness and fracture trajectory of the specimens are calculated by using the extended finite element method, and are in good agreement with the experimental results. The results indicate that crack initiation angles are influenced by the layer orientation and crack angle. The findings prove to be helpful for understanding the fracture characteristics of the layered rock materials and enriching the researches on fracture mechanics and numerical simulation of anisotropic rock materials.