plastic properties from MTR

[jrosvold]

Hello, 

I have and Material Test Report (Mill Certificate) giving me Tensile Strength (Su), Yield Strength (Sy), % Elongation(e), and % Contraction of Area (q) for a steel (E).

I am having a little trouble with the help files description of strain hardening modulus.  Do I need to use true stess and strain for this number?  From the graph it looks like the strain hardening modulus is:

(Su-Sy)/(e/100 - Sy/E)  

Is that correct?  I am not sure where the Elongation at 2in comes from.  

Does anyone know if I should be using True stress?

Thanks.

[lesmfunk]

The strain hardening modulus is a the slope of a line between the Sy and the UTS (Su) that approximates the true curve in the plastic region. Autodesk Simulation calculates this number from the Sy, UTS and the % elongation at 2". (I once found an old Algor web lesson that solved this issue for me. See attachment)

Now, it is best practice to use the numbers from the actual standard (ASTM, AISI, etc.). The reason is this:

The standard is almost always only a minimum spec. An MTR is often much higher in strength than the spec. If you were to do an actual physical test that passes the criteria and then place that material spec on a drawing, the next run from the mill may be much lower strength. The lower strength material may not pass a mechanical test like the first one, even though it is within spec according to the drawing.

Considering that FEA is only an approximation, I have often used the MTR numbers in the absence of the standard. Sometimes you get lucky and the standard specs are written on the MTR.

Hope this helps anyone with this question.