体积应变率对北山花岗岩疲劳特性的影响

Influence of volumetric strain ratio on the fatigue characteristics of Beishan granite

采用MTS815岩石力学试验系统对北山花岗岩进行动态三轴循环加、卸载试验,研究体积应变率的变化对岩石疲劳特性的影响。通过在静态加载阶段确定岩样损伤应力所对应的体积应变值,并实时监测体积应变的变化确定岩石疲劳初始点和体积应变率,进而获得岩样在动态循环加、卸载过程中的力学参数,以揭示北山花岗岩在不同体积应变率下的动态疲劳特征和损伤演化规律。研究结果表明:(1)岩样在疲劳破坏时的瞬时应变和最大塑性应变对体积应变率的变化不敏感。(2)岩样塑性体积变形随加、卸载循环次数的增加表现出初始变形、等速变形和加速变形3个阶段,在等速变形阶段,塑性体积应变率和弹性模量衰减率强烈依赖于体积应变率的变化,二者均随体积应变率的增加呈非线性减小的趋势。(3)采用塑性体积应变和弹性模量定义损伤变量可合理描述岩样的损伤演化过程,在损伤稳定发展阶段,损伤演化速率随体积应变率的增加呈非线性增大。(4)体积应变率的变化致使岩样在动态疲劳前产生初始损伤的差异,从而导致其疲劳寿命随体积应变率的增加而显著降低。

Dynamic triaxial cyclic loading-unloading tests on Beishan granite were performed using the rock mechanical test system MTS815. The main objective is to investigate the influence of the volumetric strain ratio on the fatigue characteristics of the rock. For each test,the volumetric strain at the threshold of crack damage stress was identified in the static loading stage. The fatigue initiation and the volumetric strain ratio were subsequently determined by measuring the variation of the volumetric strain for each specimen. The mechanical parameters were further obtained during dynamic loading. Then,the development of fatigue and damage behaviors of the specimens were revealed under different volumetric strain ratios. The instantaneous strains and the maximum plastic strains at failure are found not sensitive to the volumetric strain ratio. With the increasing ofloading-unloading cycles,the plastic volumetric deformation presents three characteristic stages. In the stage of uniform deformation rate,the plastic volumetric strain rate and the Young？s modulus decrease nonlinearly with the increase of volumetric strain ratio. The plastic volumetric strain and the Young？s modulus were used to define the damage variables,which describe reasonably the damage process of the tested rock. Moreover,in the steady stage of damage development,the damage velocity of the specimens increases with the increasing of volumetric strain ratio. Different volumetric strain ratios lead to the difference of initial damage for specimens before dynamic fatigue testing. Hence,the fatigue life decreases significantly with the increasing of volumetric strain ratio.