Calculation of Isothermal Flow Stress by the Combination of FEM and a Simple Compression Test

Abstract

In forging of parts with complicated shapes or of high strength materials, it is important to predict the accurate load and the tool pressure, and thus it is essential to evaluate the exact flow stress of materials. In the practice, nominal flow stress calculated from a simple compression test, which is derived by diving the load by the cross-sectional area, is often used for approximate evaluation of forming load. This nominal flow stress contains the effects of the frictional force, the change in temperature caused by plastic deformation, and the heat transfer between the tool and the material. These effects are especially significant in a high strain range. To obtain an accurate flow curve, the rigid-plastic FEM-analysis is carried out by using the approximate value of flow stress obtained from a simple compression test and the effects of friction and heat transfer are eliminated. Then, isothermal flow stress which corresponds to the strain, strain rate and temperature at each point of material is derived from this result. It is concluded that the ratio of nominal stress to isothermal flow stress ranges from 0.75 to 2.0. © 1998, The Japan Society of Mechanical Engineers. All rights reserved.