where on and o are nominal and true strains; the true plastic strain rate decreases with increasing plastic strain and is never equal to the nominal strain rate except at the very beginning of deformation. Accordingly a change in the actually imposed plastic strain rate during deformation at a constant nominal strain rate, that is at a constant cross-head speed, should affect both the mobile dislocation density rm and the average velocity of mobile dislocations 6 since they are inter-connected in the equation of [5]