Abstract: In recent years, scholars have made plenty of researches on the mechanical characteristics and constitutive models of unloading rock through the unloading tests of rock specimen. Also, substantial achievements have been obtained. Compared with loading condition, rock performs differently in many aspects under unloading condition, such as strength characteristics, deformation regularities and failure modes. Unloading rock has significant tensile cracks and volume dilatation, and always performs as tensile-shear failure. Moreover, deformation and failure mechanism are impacted greatly by unloading speed, confining pressure and stress path. The constitutive models of unloading rock can be divided into phenomenological constitutive model and micro-mechanical constitutive model. The former does not consider the failure evolution mechanism of unloading rock, while the latter does not consider the micro crack groups and the error caused by mathematical simplification. These scientific problems make the future of unloading rock mechanics research work full of opportunities and challenges.

Key words: rock mechanics, unloading test, deformation feature, fracture characteristics, constitutive model