循环荷载下硬质层理砂岩疲劳损伤机制试验研究

Experimental study on fatigue damage evolution mechanism of hard layered sandstone under cyclic loading

列车动载扰动作用下隧道基底围岩的变形开裂蕴含着复杂的力学损伤问题。为探究循环荷载下硬质层理砂岩的疲劳损伤演化机制,对层理砂岩进行了疲劳力学试验及电镜扫描试验。结果表明:层理砂岩的宏观裂隙与动态应力-应变曲线存在时效对应关系,层理效应越明显,滞回环跃迁越显著。层理砂岩力学参数具有明显的层理劣化效应。随层理倾角增大,峰值强度呈缓慢衰减—快速降低—急剧增大的趋势,疲劳寿命与峰值强度呈正相关。弹性模量呈急剧增大—缓慢增大—趋于平缓或降低的趋势,周期比分界点为28.57%和81.81%。砂岩破裂模式与层理效应密切相关,包括张拉破裂(Ⅰ型和Ⅱ型)、斜剪破裂及复合破裂;张拉破裂Ⅱ型具有压杆效应,破裂面粗糙度略小;斜剪破裂面粗糙度显著降低,压—剪作用导致复合破裂断面出现破碎带和光滑断口。循环加卸载效应导致层理砂岩的临界损伤呈非线性快速增大—近似线性增长—非线性急剧增大演变,Logistic反函数损伤模型可很好描述临界损伤规律,疲劳敏感性顺序依次为斜剪破裂型>复合破裂型>张拉破裂型。

The deformation and cracking of tunnel basement surrounding rock under the disturbance of train dynamic load contain complex mechanical damage problems.The fatigue mechanics tests and electron microscope scanning tests are carried out to explore the evolution mechanism of fatigue damage of hard layered sandstone under cyclic loading.The results show that there is an aging correspondence between macroscopic cracks and dynamic stress-strain curves of layered sandstone.The more obvious the layer effects,the more frequent the stress drop and the more significant the hysteresis loop transition.The peak strength presents a trend of slow decay-fast decrease-sharp increase and the fatigue life is positively correlated with the peak strength.The elastic modulus presents a trend of sharp increase-slow increase-flattening or decrease,and the dividing points of the period ratio are 28.57%and 81.81%.The fracture mode is closely related to the layer effects,and it presents tensile fracture(ⅠandⅡtypes),oblique shear fracture type and compound fracture type.The tensile fracture type II has a compression rod effect and the roughness of fracture surface is slightly smaller.The roughness of the fracture surface of oblique shear fracture type decreases significantly,and compression-shear action leads to fracture zone and smooth fracture of the composite fracture section.The cyclic loading and unloading effects result in the evolution of critical damage of layer sandstone with an evolution trend of nonlinear rapid increase-approximate linear increase-nonlinear sharp increase.The logistic inverse function damage model can well describe the critical damage laws,and the order of fatigue sensitivity is oblique shear fracture type>compound fracture type>tensile fracture type(ⅠandⅡ).