Abstract:The basic characteristics of strain state at the instant that the deformation of material element bearing every kind of tensile loading enters the instable phase from the stable deformation phase were analysed firstly by the principle of minimum strain energy. It was found that at this instant the strain path of this element is not monotonous. The original strain path ends at this moment and a new strain path begins and becomes effective at the same instant. These analyses indicate, from the physical concept, the scientific basis to distinguish the critical state of instability as well as the analysis method which should be adopted for the kind of problems of tensile instability.

Key words: tensile instability　critical state　deformation characteristic