In-situ rock failures can result from stress changes due to pure loading and/or unloading. Understanding of the damage evolution behavior in brittle rocks during loading and unloading is imperative for the designs of rock structures. In this paper, we investigate the damage evolution characteristics of a granitic rock during loading and unloading after a series of triaxial experiments performed at different confining pressures. The axial stress-axial strain variations of the tested specimens revealed that the specimens undergoing unloading fail with a lower axial strain compared to the specimens failed purely by loading. Higher confining pressures were observed to exacerbate the difference. Volumetric strain versus axial strain curves indicated that the curves reverse the trend with the beginning of major damage of specimens. We suggest here a new form of equation to describe the secant modulus variation of brittle rocks against the axial stress for the unloading process. Failure mechanisms of tested specimens showed two distinct patterns, namely, specimens under pure loading failed with a single distinct shear fracture while for the unloading case specimens displayed multiple intersecting fractures. In addition, analysis of the evolution of dissipation and elastic energy during deformation of the specimens under loading and unloading conditions showed differentiable characteristics. Moreover, we evaluated the variations of two damage indices defined based on the energy dissipation and secant modulus evolution during deformation and observed that both of them satisfactorily distinguish key stages of damage evolution.%在不同围压下进行三轴试验,研究花岗岩在加载和卸载过程中的损伤演化特征.通过分析试样的轴向应力、轴向应变变化表明,在轴向应变较低时,试样卸荷破坏失效与加载条件下的不同.围压越高这种不同越明显.本文提出了一种新的描述脆性岩石割线模量随轴向应力变化的方程.试件的破坏机制表现出2种不同的模式:在纯加载荷下,试样没有明显的剪切断裂,而卸载试样则表现出多个相交的断裂.此外,分析了在加载和卸载条件下,试样在变形过程中的耗散和弹性能的演化规律.此外,根据损伤过程中的能量耗散和割线模量演化来评价2种损伤指数的变化,并观察到2种损伤的演化过程都能很好地区分损伤演化的关键阶段.
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