Last month's column described how the allowable increase in length of line is highly dependent on the uniformity of the deformation. Any localization or gradient of deformation causes a higher peak stretch, which will reach the failure limit at a reduced increase in length of line. Also important is the numerical value of the maximum (peak) stretch at which sheetmetal will neck and fail. How much can an AKDQ steel sheet, 0.039 in. thick with a yield strength of 27 ksi, stretch before failure? Tensile-test data for this sheet show that uniform elongation is 24.5 percent, n value is 0.22, and total elongation is 42 percent. Many would reply that 24.5 percent stretch is the maximum stretch because the deformation goes unstable after the maximum load is reached in the tensile test. However, the tensile test is only one of many modes of biaxial deformation.
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