Abstract:Based on mechanical behavior of the different parts of testing machine-specimen system and using the energy conservation principle,a parametric equation of the complete stress-strain curve in direct shear is deduced. A criterion of instability and a critical condition of snap-back of testing machine-specimen system are put forward. The research work shows that the elastic parts of testing machine-specimen system are the origin of the failure or the snap-back of testing machine-specimen system. It leads to the discretional results of complete stress-strain curve in direct shear,which differ from shear stress-strain constitutive relation. The analysis of the relationship between the shear stress-strain curve and the shear band thickness shows that the thicker the shear band thickness is,the less brittle the material becomes and the earlier failure the specimen happens during loading. The analysis of the relationship between the shear stress-strain curve and the elastic parts length shows that the longer the elastic parts length is,the more brittle the material becomes and the earlier failure the specimen happens during loading.