Formability evaluation by novel specimen designs in sheet metal forming with two-step strain paths

Abstract

Limit dome height test is commonly used for evaluating the formability of sheet material. Through altering the geometry of specimens, different strain paths could be obtained to establish forming limit diagram of the material. By incorporating finite element analysis with ductile energy criteria, engineers can also predict the formability of material. The forming limit diagram or constants in ductile energy criteria are usually determined by experiments with linear strain path. However, the predictions may lose their accuracy when evaluating products with complex strain paths, which is commonly seen in sheet forming processes. Therefore, a better method for evaluating formability of material under complex strain path should be developed. In this study, novel specimen designs for limit dome height test are applied to generate different strain paths with two-step strain effect on the specimens. Different geometric parameters of the novel specimen design can alter the slope of strain paths and create different strain paths similar to the two-step deformation conditions occurred in actual sheet forming processes. Three different two-step strain path conditions are created experimentally, and the changes in strain path are verified with simulations. From the results, the predictions of forming limit based on linear strain path forming limit diagram could be overestimated or underestimated under two-step strain path conditions. Thus, the formability of material under two-step strain path conditions can be determined by corresponding experiments using novel specimen designs, instead of predictions made by linear strain path conditions.