An important concern in forming is whether a desired process can be accomplished without failure of the work material. Forming limits vary with material for any given process and deformation-zone shape. As indicated in Chapter 10, central bursts may occur at a given level of Δ in some materials and not in others. Failure strains for a given process depend on the material.

DUCTILITY

In most bulk forming operations, formability is limited by ductile fracture. Forming limits correlate quite well with the reduction of area as measured in a tension test. Figure 11.1 shows the strains at which edge cracking occurs in rolling as a function of the tensile reduction in area. The fact that the limiting strains for strips with square edges are much higher than those with rounded edges indicates that process variables are also important. Similar results are reported for other processes.

METALLURGY

The ductility of a metal is strongly influenced both by the properties of the matrix and by the presence of inclusions. Factors that increase the strength generally decrease ductility. Solid solution strengthening, precipitation, cold work, and decreased temperatures all lower fracture strains. The reason is that with higher strengths, the stresses necessary for fracture will be encountered sooner.

Inclusions play a dominant role in ductile fracture. The volume fraction, nature, shape, and distribution of inclusions are important. In Figure 11.2, the tensile ductility is seen to decrease with increased amounts of artificial inclusions.