Formability of sheet metals is often assessed by means of forming limit diagrams (FLD). In this paper, performing three different series of experiments, the effect of forming velocity on FLDs is investigated experimentally for Al 6061-T6 and AISI 1045 sheets. In addition to determining the conventional FLDs at quasi-static condition ( _ " 6 0.01/s), FLDs were acquired in the case of forming by low impact ( _ " 6 50/s) as well as explosive free-forming ( _ " 6 1000/s). Samples were deformed in different strain states, thereby generating data for both sides of the FLD. Numerical simulating of stand-off explosive forming tests, the dies and specimens were designed accurately. The Johnson-Cook constitutive model for metal sheets and the Jones-Wilkins-Lee (JWL) model for the explosive charge were used. In order to optimize the sheet and explosive mesh size, a sensitivity analysis was carried out by using the response surface method.The results show that while a substantial improvement in high strain rate formability of the aluminium sheet can be obtained, this improvement is not considerable for the steel sheets. Also, under the effect of contact pressure between tool and sheet, the formability increases for both steel and aluminium sheets on impact.
Forming limit diagrams (FLDs) appear as one of the most applicable methods used for prediction of the instability in sheet metal forming. In this paper, the effects of strain rate on FLDs are analytically investigated over a wide range of strain rate (0.01/s-500/s). In order to obtain the strain-rate-dependent FLDs with the aid of imperfection concept, the motion equations (instead of equilibrium equations) are employed. Furthermore, in this study the critical strains, in plane strain loading, are numerically determined with respect to the principal strain rates at different material constants. For materials following the widely used Johnson-Cook constitutive law, the thermal softening due to the adiabatic conditions is taken into account. The results show that after a certain critical strain rate, by increasing the strain rate, localized necking is retarded and formability of sheet improves monotonically because of inertia effects. Finally, the comparison between numerical and available experimental FLDs for oxygen-free high conductivity (OFHC) copper is presented. This comparison shows that the theoretical results are in good agreement with the experimental observations.
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