  Background: The determination of yield stress curves for ductile metals from
uniaxial material tests is complicated by the presence of tri-axial stress
states due to necking. A need exists for a straightforward solution to this
problem. Objective: This work presents a simple solution for this problem
specific to axis-symmetric specimens. Equivalent uniaxial true strain and true
stress, corrected for triaxiality effects, are calculated without resorting to
inverse analysis methods. Methods: A computer program is presented which takes
shadow images from tensile tests, obtained in a backlight configuration. A
single camera is sufficient as no stereoscopic effects need to be addressed.
The specimen's contours are digitally extracted, and strain is calculated from
the contour change. At the same time, stress triaxiality is computed using a
novel curvature fitting algorithm. Results: The method is accurate as
comparison with manufactured solutions obtained from Finite Element simulations
show. Application to 303 stainless steel specimens at different levels of
stress triaxiality show that equivalent uniaxial true stress -- true strain
relations are accurately recovered. Conclusions: The here presented computer
program solves a long-standing challenge in a straightforward manner. It is
expected to be a useful tool for experimental strain analysis.
