The Generation of Residual Stresses in Metal-Forming Processes

Metal-forming processes commonly generate non-homogeneous plastic deformation in the workpiece so that the final product is left in a state of residual stress. These stresses may be detrimental or beneficial to the strength and reliability of the product in subsequent service (certain metal-forming processes are designed to produce beneficial residual stresses). In order to be able to evaluate the residual stresses in the product, a complete stress analysis of the workpiece throughout the process is needed and this calls for elastic-plastic analysis, for the more common rigid-plastic theory of forming cannot in principle yield stresses in the rigid regions which include the product. The analysis of stress and deformation distributions on the basis of elastic-plastic theory at finite strain is described and applied to the extrusion process using the finite element method. Examples of residual stresses generated, including the important steady state situation, are presented which illustrate the influence of the configuration of the die and the influence of large and small area reductions.