Abstract: Rock is characterised as the elastoplastic coupling, which means its elastic parameters change with the variation of plastic deformation beyond the threshold of yield. Based on current classification, definition and constitutive function of strain, the following work was conducted under the framework of elastoplastic coupling. Firstly, through compressive tests, the evolution of elastic parameters was analysed from the aspects of the damage and plastic deformation. The strain was mainly studied under the condition of loading incremental step. It was suggested that the strain increment corresponding to the loading increment could be decomposed into elastic strain increment , elastic strain increment and plastic strain increment respectively. Furthermore, was following the generalised Hooke’s law, while was against Hooke’s law. The elastic relationship was constituted with elastic parameters after loading incremental step. Constitutive functions were then formulated in strain and stress spaces under loading, neutral loading and unloading conditions, respectively. The discussion was made on the internal variable increment reflecting the microstructure change. In general, the classification and definition of strain are explicit in concept and suitable for strain hardening and softening sections. Moreover, the elastoplastic coupling constitutive function considers the influences of different loading conditions on the changing rates of elastic parameters. Overall, the strain definition and constitutive function are in accordance with rock elastoplastic coupling characters.

Key words: elastoplastic coupling, constitutive function, elastic parameter, strain