In this investigation, two double-stage forming processes are used to study the effects of prestrain on sheet metal formability. The processes chosen are the bulge test and the plunger test using a die throat to punch diameter ratio of 2.42. It is seen that during the second stage of forming, deformation and shape of the deformed shell are influenced by the level of prestrain and strain gradients on the prestrained workpiece. An improved formability is observed when the workpiece for the plunger test is prestrained by hydroforming, but the formability is reduced when the prestraining process becomes the plunger test. The present results also imply that care should be taken in utilizing the forming limit diagram even if it is constructed from double-stage forming, in which strain gradient on the prestrained workpiece is generally absent.

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