Direct evaluation of sheet metal forming properties under various deformation conditions

Uniaxial tension tests are traditionally used for determining various material properties. Normally, extensometers are employed to measure the dimensional changes of the specimen. Such a measurement technique will be unable to establish local strains accurately since the exact portion of the specimen that actually contributes to deformation is uncertain, i.e., changes are not limited to the gauge-length portion alone, leading to inaccurate assessment of anisotropy value and the stress-strain relationship. Also, the conventional measurement technique has limitations in terms of test speed and its ability to reflect the changes that would occur over the entire deformation area. In this paper, a new vision-integrated methodology was used to evaluate the material properties. This method offered a most direct, continuous, non-interfering and accurate way of measuring the surface strains involved. With the establishment of actual strains, it is possible to estimate the stress accurately. By performing regression analysis on the stress-strain data, the strain hardening exponent and the strength coefficient of the material have been evaluated. The instantaneous anisotropy values were readily estimated and strain localization could also be easily and accurately monitored.