“…Most of the time, their approaches are included in the framework of the thermodynamic of the irreversible processes (Germain et al, 1983;Halphen and Nguyen, 1974;Lemaître and Chaboche, 1994;Chaboche 2003;Voyiadjis and Al-Rub, 2003) and are divided into two kinds. On the one hand, one can find non-unified models (Cailletaud and Sai, 1995;Blaj and Cailletaud, 2000;Charkaluk et al, 2004;Velay et al 2006), which considers a partition between the viscous phenomena and the plastic ones. On the other hand, there are unified constitutive models which take account of a unique viscoplastic strain.…”