A major limitation of the uniaxial tensile test (ASTM E8) is that after a certain amount of tensile strain, the sample eventually undergoes a mechanical instability - it forms a neck. Although the sample continues to plastically deform, the stress can no longer be unambiguously determined as load divided by area, as the area within the gauge length is no longer uniform. However, stress-strain data requirements for many forming operations often extends well beyond the ultimate tensile strain at the point of localization. Mathematical formulas have been devised to estimate the tensile stresses beyond this point, but they are based on dubious assumptions such as constant volume (neglecting the formation of voids at high strains) and that the state of stress is still uniaxial.
This project uses both digital image correlation (DIC) and xray stress measurements to continue to gather valid data from within the neck up to the true strain at failure. This data is being used to both extend uniaxial stress-strain curves to much higher applied plastic strains, but also to develop new mathematical methods to extract more data out of a tensile test beyond necking in a reliable and repeatable fashion.
More results to come - Hint: the stress state is NOT uniaxial inside the neck.