Analysis of the evolution of the Lankford coefficient in uniaxial tensile tests and validation with grain structure analysis

Modern materials mostly exhibit a low formability or a complex material behavior. For example, the common mild steel DC06 exhibits a strain state dependency in the yield stress and a strain dependency of the Lankford coefficients. Thus it is necessary to characterize the material properties from the onset of plastic deformation up to fracture. For example in the uniaxial tensile test, the flow stress and the r-values can be determined. According to the standards, the characterization is only applicable up to uniform elongation. In the case of DC06, the uniform elongation is with 24% comparatively low in contrast to the ultimate elongation with 60%. Research has proven, that the true stress can be characterized up to high strain in the uniaxial tensile test. For the strain dependent Lankford coefficient, this validation is missing. The aim in this investigation is the analysis of the strain dependent anisotropy up to high strain levels in uniaxial tensile tests and the validation. This is done with a comparison of optically measured r-values with the grain structure evolution of the pre stretched parts. These results prove the applicability of the standardized evaluation of Lankford coefficients out of DIC data above the uniform elongation.