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The effect of strain rate and temperature on forming limit diagram for DKP-6112 and AZ31 materials

Goksen, Seckin
Darendeliler, Haluk
© 2020 The Authors. Published by Elsevier Ltd.Forming limit diagrams (FLDs) are used in sheet metal operations widely for predicting blank fracture and forming characteristics of materials. There are three approaches for building forming limit diagrams which are the experimental, theoretical and numerical methods. Experimental method, which includes Nakazima formability test, is generally preferred for determining forming limit diagrams, although it requires complex experimental setup and effort. This study, firstly, compares the experimentally determined FLD results with the numerically obtained ones by using the constitutive models formed through the use of von Mises criteria with isotropic, kinematic and combined hardening models, and Hill48 yield criterion at quasi-static loading and room temperature conditions. The stress-strain relations are obtained by applying the Johnson-Cook phenomenological model for DKP-6112 and AZ31 materials. Then, the constitutive relation that gives closest results to experimental data is chosen to evaluate the effects of the variations of strain rate and temperature values on the FLDs for both materials. Nakazima tests, with 8 different blank specimens, are simulated by using a finite element software to present and compare the numerical forming limit diagrams. For determining the necking time in numerical FLDs maximum strain acceleration strain localization method is used. It is observed that forming limit diagram shifts downwards with strain rate increase and shifts upwards with temperature increase for both materials.