Interesting Chuck. ... But where does the bulge lie on the stress deformation axis, right at yield or could it be way to the right and how would you know?
Buzz,
Typical stress strain charts show elongation, often in percentage. If you had such data for your steel or felt you had such data applicable for your steel, you could measure the bulge and calculated the elongation to a comparable value and make your assessment.
The issue is really about getting applicable stress/strain data. Different alloys and hardness of steels will have different amounts of elongation.
That's what some of us were trying to do when we had a member going by "Zircon", or similar handle a few yrs ago, who was intending to do these tests on a variety of barrel samples he collected from members. I volunteered to make the test fixture and he was going to use it in a tensile test machine where he worked. It's been a few or more years since we last heard from him. The value of the data he was going to generate would be very valuable to making assessments of strength. As you also just discovered, it would be helpful for this question as well.
