Sorry, I guess I didn't explain my question very well. I was referring to how design codes treat listed Ultimate Stresses--Ftu, for instance, say for a sample of 6063-T6 aluminum, which is 35 ksi. Is this 35 ksi an engineering stress value (as in, force divided by original cross-sectional area) or TRUE stress (as in, force divided by instantaneous area).
As a sample is tested, in regards to a stress-strain curve (think of a simple tensile test specimen), usually all we are interested in is the engineering stress--even though the material deforms throughout its load cycle until failure. We do this by taking the force divided by the original cross-sectional area, and likewise, the strain is a measure of some change in length divided by original length. However, true stress is a measure of force over actual area of the necking part and true strain is the rate of increase in length divided by the instantaneous length. I have attached an example curve (I found this on academic.uprm.edu)
I think I have answered my own question, but had a moment of doubt concerning whether or not design codes use TRUE STRESS values for ultimate strengths or ENGINEERING stress values. I believe they use engineering stress values.
I am sorry for the confusion and thanks for the interest.