Evaluation of DIC Based Forming Limit Curve Methods at Various Temperatures of Aluminum Alloys for Automotive Applications

Aluminum alloys are increasingly utilized in automotive body panels and crash components to reduce weight. Accurately assessing formability of the sheet metal can reduce design iteration and tooling tryouts to obtain the desired geometry in aluminum stampings. The current ISO forming limit curve (FLC) procedure is a position dependent technique which produces the FLC based on extrapolation at the crack location. As aluminum sheet metal use increases in manufacturing, accurate determination of the forming limits of this material will be necessary prior to production. New time dependent methods using digital imaging correlation (DIC) account for variations in material behavior by continuously collecting strain data through the material necking point. This allows more accurate FLC determination that is necessary for efficient design in the automotive stamping industry. Two different time dependent FLC analysis techniques using DIC were evaluated and compared with the position dependent ISO technique. All of the techniques were tested at room and elevated temperatures with aluminum 6xxx and 7xxx alloys to show the versatility of the techniques. The resulting FLC diagrams were compared after analysis of the data. Regardless of the testing temperature both tested time dependent techniques resulted in minor variation between trials. The DIC time dependent techniques proved to be more efficient without sacrificing accuracy and consistency. Additionally, time dependent methods displayed less conservative values than the ISO method and the FLC obtained using time dependent method was generally more practical than the ISO method.