Why deformation gradients reduce the average length of line was discussed in the June column. In July the allowable peak (major) strain in the gradient was shown to depend on the minor strain created by the process design. This month we explore how the two surface strains (major and minor) combine to fix the amount of sheetmetal thinning. Sheetmetal forming is limited by a series of rules, mostly determined by the laws of physics. One interesting rule is the constancy of volume. Prior to deformation, the sheetmetal has a fixed amount of volume. Unless material is removed during the deformation (usually by scoring of the surface), the volume does not change but redistributes itself like putty. Increase the surface area to provide the shape required by the part design and the thickness must decrease. This is true of the sheet as a whole and for each small local element of material.
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