Model for instabilities during plastic deformation at constant cross-head velocity

We consider the case of mobile dislocations; with a finite velocity there is then a phase displacement between the time of loop nucleation and the time at which the strain is recorded. With this hypothesis applied in an equation analogous to the Mecking and Lucke relation, we can explain the appearance of instabilities on the strain-stress curves, the influence of stiffness of the loading system, the formation processes of dislocation loops, and the phase displacement which appear in the criterion of stability are studied On attribue aux dislocations mobiles une vitesse finie; par consequent, un dephasage a lieu entre le temps ou les boucles sont creees et le temps ou la deformation est realisee. Cette hypothese appliquee a la relation de Mecking et Lucke permet d'expliquer l'apparition d'instabilites sur les courbes effort-deformation. Le role de la durete de la machine, les processus de formation des boucles, le dephasage apparaissent dans le critere de stabilite