Mechanical response with a snapback (class-II behavior) is one of the basic features of brittle rocks. This technical Note presents analytical studies on the critical condition which separates the class-I and class-II rock behaviors. Formulations under conventional triaxial compression are performed by making direct use of the closed-form solutions of the damage-friction coupling constitutive equations for microcracked solids. It is shown from theoretical viewpoints that class-II behavior must present when rock is brittle enough and that the triggering condition can be characterized via the critical damage at failure. Moreover, in the class-II response, two transition points present in stress-strain curves, which coincides with experimental observations upon brittle rocks. An energetic explanation is given for the mechanism behind nonlinear dissipative phenomena.

具有回弹的机械响应（II 类行为）是脆性岩石的基本特征之一。本技术说明介绍了对区分 I 类和 II 类岩石行为的临界条件的分析研究。通过直接使用微裂纹固体的损伤-摩擦耦合本构方程的闭合解来进行常规三轴压缩下的公式化。从理论角度表明，当岩石足够脆时，必须出现 II 类行为，并且触发条件可以通过破坏时的临界损伤来表征。此外，在 II 类响应中，应力-应变曲线中存在两个过渡点，这与脆性岩石的实验观察结果一致。