Attached is a figure to illustrate the difference in the value of yield strength as observed from the true stress/strain curve and engineering stress/strain curve. While conducting FEA in ANSYS for a isotropic ductile material, I need to compare the maximum stress regions with that of the yield strength to know if that structure is undergoing plastic deformation or not. My question is which value should be used for yield strength; the one obtained from true or engineering?
UPDATE Based on comments
The question was:
After I obtain the maximum stress results from my simulation, should I compare the maximum stress that I observe in the simulation to the engineering or the true yield stress of the material?
IMHO, it doesn't really matter, at least for steel (there are other materials that will exhibit significant differences in yield stress, but steel is not amongst them IMHO).
The reason is that at small strains (i.e. in the linear region below the yield stress) the true stress and the engineering strain are almost identical.
The relationship between true stress ($\sigma_t$) and engineering stress ($\sigma$) and strain ($\varepsilon$) can be approximated by:
$$\sigma_t = \sigma \cdot (1 + \varepsilon)$$
E.g. For example, for steel, the yield strain is about 0.2%, therefore the change between true and engineering strain is about 0.2%.
In most cases of real life problems, - IMHO- if the error in the maximum stress is within 5% of the actual stress you've done a superb job. So checking a value that is 0.2% when your error is about 5% doesn't make sense IMHO
Again, what I wrote above is applicable to steel, or other materials that exhibit yield strain at really low values (less than 5%).
The choice is up to your task/interest and its application.
The ultimate strength is completely obscured in a true stress-strain curve. However, the engineering stress-strain curve hides the true effect of strain hardening. The true stress-strain curve is ideal for showing the actual strain (and strength) of the material.
The engineering stress-strain curve is ideal for performance applications (meant design). The true stress-strain curve is ideal for material property analysis (meant research or study). For everyone except (some) materials scientists, the engineering stress-strain curve is simply more useful than the true stress-strain curve.
However, the true stress-strain curve is ideal for showing the actual strain (and strength) of the material.
Some materials scientists may be interested in fundamental properties of the material. In this case, the true stress-strain curve is better. This curve tells the actual state of stress in the material at any point. It also shows strain hardening without being affected by the changing area of the sample.
For example, many metals show strain-hardening behavior that can be modeled as:
Where K is a constant and n is the strain-hardening exponent. n is always less than 1.