中文摘要：

试验研究表明，岩土材料在加卸荷时表现出不同的变形破坏特性，卸荷时脆性增加。建议了一个可以描述材料脆性变化的脆性指数B，并验证了B指标能较好地描述岩土材料在加卸荷时的脆性变化。应用二元介质模型，对岩样在卸载周围应力过程中的应力-应变关系进行了模拟，表明这一模型不但可以模拟岩样在低围压下的应变软化到高围压下的应变硬化现象，而且可以模拟岩样在卸载周围应力过程中的变形破坏过程，特别是对于表现为应变软化的岩样，可以反映出应力峰值的后卸围压时的变形破坏过程的差别。

英文摘要：

Experimental study reveals that geomaterials under loading or unloading conditions exhibit different deformation and failure properties, and the brittleness increases on unloading. In the paper a brittleness index B is proposed and used to describe the brittleness variety of geomaterials under loading or unloading conditions. And the binary medium model proposed recently by the authors is used to simulate the process of deformation and failure of the rock specimens under different loading paths. It is found that the model can simulate the transformation of deformation mode from the strain softening under the low confining pressures to the strain hardening under the high confining pressures and the process of deformation and failure when reducing the confining pressures situations; especially it can reflect the difference of failure behavior between samples with different stress states, i.e. before or after peak stress when the confining pressure is unloaded.