A Method of Determining Flow Stress under Forming Conditions

Summary For practical determination of flow stress under forming conditions, i.e., at large strains, high strain rates and at elevated temperatures, a method based on the upsettability test is proposed. The fundamental procedure of the method is measuring the load and reduction in height in upsetting of a cylindrical specimen between grooved platens. A formula converting the measured results into average flow stress-equivalent strain curve is derived by using the result of the rigid-plastic finite element method. This method is simple to carry out and is free from the effect of variation of frictional condition over the workpiece-tool interfaces. The method is confirmed to provide an accuracy of ±5% after eliminating the systematical error due to the finite element calculation up to a strain of about 2.0 as long as large cracks do not appear on the side surface. Some experiments are carried out for a 0.21%C steel at temperatures up to 700°C on a mechanical press. It is shown that the effect of temperature rise due to plastic deformation is decisive of the flow stress at large strains. The flow stress at equivalent strain of 1.5 has a speak due to blue brittleness at about 200°C. The flow stress determined from the final load in the upsettability test is suggested to be used as a representative value of the flow stress under the forming condition.