Process design and failure analysis of three-dimensional sheet metal forming using simplified numerical and analytical models

Sheet metal forming is an important process employed for many industrial products. The advanced technologies to efficiently and effectively design the forming process parameters, such as tooling geometry, material specifications, lubrication condition, and restraining force, are critical to the reduction of development time and cost for a desired part. In this work, we will address the problem from two different views: how to efficiently predict formability at the early design stage and how to accurately predict the tearing when detailed strain path is known from a complete 3D simulation.; The traditional practice of die tryout and today's complete finite element modeling of the forming of a 3D sheet metal part are expensive and time consuming. The simplified and accurate numerical models, which can be used to design the forming process time and cost effectively, are of great significance. In this work, a simplified axisymmetric finite element model with a center offset is proposed to predict tearing failure in the comer sections of 3D parts. Numerical simulation and experimental works were performed to develop and verify the empirical and analytical formulations for finding the center offset. A design algorithm is provided to enable engineers to rapidly specify the right amount of the restraining force in the comer section based on the desired center strains and forming depth.; One of the most critical elements in utilizing numerical simulation for the purpose of design is the ability to predict failure. It is found that the characterization of material behavior plays an important role in the failure prediction. A new method for determining the mixing factor and the stress exponent in a general anisotropic yield criterion is proposed. The optimum combination is determined by matching the calculated and the experimental measurement of the right hand side Forming Limit Diagram (FLD) of the material. Furthermore, the backstress and stress exponent are induced to accommodate the evolution of yield surface due to prestraining. Using the proposed method, very good agreement was obtained between analytical results and published experimental FLD data under both linear and nonlinear strain paths.; The proposed numerical and analytical models provide the designer with powerful tools to rapidly and accurately assess the manufacturability of the desired part.