Experimental investigation of sheet metal formability under various strain rates

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 AlSI 1045 sheets. In addition to determining the conventional FLDs at quasi-static condition ((epsilon) over dot <= 0.01/s), FLDs were acquired in the case of forming by low impact ((epsilon) over dot <= 50/s) as well as explosive free-forming ((epsilon) over dot <= 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 OWL) 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.