Formability of AA5182/polypropylene/AA5182 sandwich sheets

A sandwich sheet, AA5182/polypropylene/AA5182 (AA/PP/AA), has been developed by the roll bonding of two AA5182 aluminum alloy skin sheets with a pre-rolled polypropylene core sheet at 140 °C. The strain-hardening exponent of the aluminum skin was higher than that of the sandwich sheet, while the strain rate sensitivity of the sandwich sheet was higher than that of the aluminum skin. In order to optimize the sandwich sheet, the formability has been analyzed by constructing the forming limit diagram (FLD) based on the modified Marciniak–Kuczynski (M–K) theory. To account for the planar anisotropic property of the aluminum alloy skin sheet, Hill’s and Barlat’s new yield functions, published in 1948 and 2000, respectively, were employed. When compared with experimental results, the calculated FLD showed reasonably good qualitative agreement. Further analysis showed that the positive effect of the higher strain rate sensitivity of the sandwich sheet is compensated for by the negative effect of its lower strain-hardening exponent so that the greater thickness was the main cause of the higher formability of the sandwich sheet compared to that of the skin sheet.