ON LOADING-UNLOADING CRITERION OF ELASTOPLASTIC THEORY OF ROCK AND SOIL
岩土弹塑性理论的加卸载准则探讨

The existent loading-unloading criteria of elastoplastic theory are classified as three classes: the criterion based on the concept of yield surface, the criterion of stress type and the criterion of strain type. The shortcomings of the three classes of criteria are discussed respectively. In the light of analysis on loading-unloading process. it is discovered that strain increment and total strain are suitable to be used as judgement parameters of loading-unloading because they are easy to be calculated and correlate with plastic strain. A new loading-unloading criterion of strain type is presented for the strain increment analysis of harden stage. This criterion is of clear physical meaning~ suitable to the harden stage of geomaterials and can be easily used for practical application. The criterion can be generalized to multi-dimensions and ideal-plastic case. The loading-unloading criteria used for soften. the variation of stress Lode's angle and principal stress axes rotation are discussed also.