From Metal Forming: Mechanics and Metallurgy, Third Edition
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 ? in some materials and not in others. Failure strains for a given process depend on the material.
11.1 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.
Figure 11.1: Correlation of the strain at which edge cracks were first observed in flat rolling with the reduction of area in a tension test. From M.G.Cockcroft and D.J.Latham, J. Inst. Metals, 96 (1968), pp. 33 39.
[*] Wire drawing is an exception; the maximum reduction per pass is limited by the ability of the drawn section to carry the required drawing force without yielding and necking. Once the drawn wire necks, it can no longer support the required load, so the subsequent fracture strain is of little concern. It is also possible, though not common, that brittle fracture limits...
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