Automated Method for Pre-Straining of Steels for Bake Hardenability Testing

The bake hardening effect in steel, characterized by a boost in yield strength resulting from initial deformation and subsequent heat treatment, holds significant promise for the automotive sector. Bake hardening is characterized by the Bake Hardening Index (BHI), as outlined in the test standards EN 10325:2006 and JIS G 3135:2018. The suggested test procedure involves pre-straining the dogbone samples and subjecting them to thermal cycling. However, the current standards lack specificity regarding the pre-straining procedure and the estimation of elastic recovery in metals, leading to inconsistencies in results across different materials and laboratories. This study addresses this gap by proposing a standardized and fully automated method utilizing in-situ strain measurements to precisely pre-strain uniaxial tension samples to any desired level of plastic strain. This method offers precise control over strains and accurate estimation of strain overcompensation, which accounts for elastic recovery during unloading. In this paper, the proposed method is validated using optical strains using digital image correlation, however the proposed method is independent of the choice of strain measurement method and works with clip-on extensometers, optical extensometers and even the crosshead transducers in the load frame. The foundation and effectiveness of the proposed method are demonstrated through statistical data obtained from testing 11 different steel grades, including bake hardenable steels and Advanced High Strength Steels commonly used in the automotive industry. The proposed method is independent of the choice of steel grade and consistently delivers satisfactory results for all tested steel grades, producing test samples with plastic strains that adhere to the specifications of the testing standards.