8 - Flow Stress Data

Summary

The flow stress of a material is indeed an important parameter in metal forming. To create a good FEM model of a forming process, for example, it is required that one be able to specify the flow stress of the workpiece material with good accuracy.

Because of this, the main processing parameters affecting the flow stress of commonly used metals and their alloys will be considered in this chapter, as regards both cold and hot forming applications. When the flow stress has been measured, so one knows how the processing parameters affect the flow stress, it is common to search for an equation that can describe the actual flow stress dependence well. When one has arrived at such an equation, one says that one has obtained a good material model for the material. It is also common to call the material model the constitutive equation for the actual metal. Commonly applied procedures to determine constitutive equations for metals will be described and material models for some commonly used metals will be reviewed in this chapter.

The Flow Stress

In Fig. 7.8(b), flow stress curves were shown as they commonly appear for a metal at room temperature. As depicted in this figure, the true flow stress often increases with increasing strain when the metal is tested at room temperature.

If we consider metals, at cold, warm, and hot forming temperatures, a number of parameters can affect flow stress, not only strain.