Effect of strain-rate on forming limit in biaxial stretching of aluminum sheet

Abstract

High-speed biaxial stretching apparatus was developed, where the punch was driven by a drop weight. Effect of the strain-rate on the forming limit strain of thin aluminum A1050 and A5052 sheets was inspected for various strain ratios. Strain-rate was about 100 /s in the high-speed test. It was from 0.0003 to 0.1 /s in the low-speed test. Limit strain of A1050 was larger in high speed. The strain of A5052 in high speed was comparable with that in 0.1 /s, though there exists a slight difference under low-speed conditions. Multiple cracks were formed in high-speed stretching of A5052 attributed to the positive strain-rate sensitivity. Profile of the crack opening is rough in high speed. Growth of the ductile fracture dimple size and depth in microscopic view may play a role to increase the roughness. Sheet thickness at the crack becomes smaller in high-speed stretching, which accords with the tendency of the forming limit strain.