Digital Image Correlation Technique Applied to the Study of the Plastic Behavior of an Interstitial Free Steel

In this paper, the technique of Digital Image Correlation (DIC) is applied for analyzing the plastic behavior of an interstitial-free (IF) steel sheet usually used for automotive body stamping. The specimens were subjected to uniaxial tensile testing for measuring in-plane surface strains, which, in turn, are used for calculating the plastic anisotropy coefficient or the Lankford value. The IF steel sheet has a nominal thickness of 0.78 mm and the specimens were machined to gauge width and length dimensions of 20 and 146.5 mm, respectively. Six specimens were taken in the rolling direction of the sheet and three other from the transversal direction. We applied the DIC algorithm of the Automated Strain Analysis Environment Software (ASAME), in which the calculated in-plane surface strains based on the deformation of the speckle pattern from the digital images that were recorded during the tensile testing. For comparison purposes, the longitudinal strains were also recorded with a 50 mm-length gauge extensometer. For the same crosshead displacement, we verified a good correlation between DIC results and extensometer measurements, both for engineering and true-strain values. Based on DIC measurements, an average value of 2.2 was obtained for the plastic anisotropy coefficient, which is consistent with the standard properties of the IF steel sheet. We conclude that the DIC technique can be successfully used for evaluating the plastic behavior of thin metallic sheets in laboratorial tests, mainly for production control of uniaxial tensile mechanical properties.