Unstable neck formation in a ductile ring subjected to impulsive radial loading

Formation and growth of di€use necks in thin metal rings undergoing high-rate radial expansion under plane strain conditions is studied numerically. The material is described by an elastic±viscoplastic internal variable constitutive relation, so that failure by nucleation and void-growth is taken into account. Adiabatic heating due to plastic dissipation is also incorporated. The in¯uence of imperfections in the form of non-uniform wall thickness and of variations in constitutive parameters is studied. The computations show that long wavelength thickness imperfections gradually increase in amplitude after some initial inelastic deformation but do not result in highly localized neck formation. Instead, a critical, nearly periodic mode with short wavelength appears at rather large overall strain levels. It is from this critical mode that complete ductile failure develop