Study on mechanism of cylindrical shallow shell surface deflection

The surface deflection is a remarkable disadvantage in the automobile outer panels forming. It is difficult to study because of the complexity shape of automobile panels. The finite element models of cylindrical shallow shells that could represent automobile panels were established. Simulations of forming and spring back of the cylindrical shallow shells were carried out. The consistency of simulations and experiments shows that the numerical simulation on research of surface deflection is accurate and feasible. From the characteristics of stress distribution and deformation, it is indicated that sheet metal can entirely flat punch over stamping and the surface deflection does not exist after forming. Surface deflection appears after unloading. Great residual stress exists along the cylindrical direction of shell, and the surface deflection increases with the increasing of residual compression stress. The residual stress is a necessary condition of the generation of the surface deflection. Mechanism for the initiation of the surface deflection is presented by numerical simulation analysis, and is verified by the model analysis of the surface deflection of engine hood.