Effect of specimen geometry, gage length, and width measurement locations on plastic strain ratio (R-value) in sheet metals

The effects of gage length, width measurement locations, and specimen geometry on plastic strain ratio have been investigated for the AA8011 aluminum alloy and interstitial-free (IF) steel sheets. The specimens were ASTM E 517 to 92a subsize, type A, and type A alternative with, respectively, 37.5, 76, and 57 mm reduced parallel sections and with different fillet radii. In each specimen type, there were differences in axial strains of gage marks and changes in width strain over the reduced parallel section, which depend on the applied axial strain, reduced parallel section length, and fillet radius. Therefore, the magnitudes of the calculated R-values depend upon gage length, width measurement location, axial strain, and specimen geometry. These dependencies were more pronounced in the high R-value IF steel sheet relative to the low R-value AA8011 aluminum alloy sheet. The dependencies of R-value on gage length and width measurement location are negligible in all AA8011 specimens, while in IF specimens, these dependencies can be neglected only for type A specimens with 12.5-mm fillet radius. It is concluded that the observed differences in the measured R-values for specimens with different geometries can be attributed to the constraints imposed by the shoulders, which affect the width strain measurements and the resulting R-values.