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# 3.2 Properties for processing – forming

Forming processes involve applying forces to the material being shaped. A good way of telling how a given material responds to applied force is to look at diagrams representing its stress-strain behaviour. Figure 28(a) shows the stress-strain curves, at room temperature, for two different metals. The two important things for the feasibility of squeezing-type processes are the point at which the solid starts to flow and the extent to which it can be persuaded to flow before it separates (i.e. fails). This is described by two properties, yield stress (or flow stress) and ductility. Remember that the yield stress is a good measure of the strength of a ductile material.

Figure 28(a) Schematic stress-strain curves for: A – a steel, B – a ductile metal such as lead

If you look at the curves, you can see that the one for the steel (curve A), after the elastic region, shows plastic deformation up to a strain of about 40 per cent. This provides a measure of the ductility of the steel, and the extent to which it can be squeezed, stretched or bent at this temperature. Curve B (for lead) shows much higher ductility and a much lower yield stress.

However, 40 per cent is not a lot of strain for many manufacturing purposes. Being able to change a material's dimensions by only 40 per cent would mean that forming would be virtually a waste of time. In addition, once the steel in Figure 28(a) has been strained plastically, it has also increased in yield stress and become harder (Figure 1.28(b)).

Figure 28(b) The effect of progressively straining a material