Extracting mechanical properties from strain-stress plot (real test)

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freddie_mclair
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Hi everyone, recently I was working out an elasto-plastic model to make some residual stress predictions: see it here.
Recently I got some real tensile tests results from a lab, therefore I would like to extract a better stress-strain curve to plug into my elasto-plastic model using the Ramberg-Osgood equation. The problem is that I cannot get the Young's modulus that they claim to have obtained!

Here's the tensile test plot:
Load versus displacement plot of tensile test curves: load in kN from 0 to 20 against displacement in mm from 0 to 15, with two families of curves in the legend, T and C samples.


The stress-strain curve I would like to extract corresponds to the plots on the bottom part of the image (ultimate stress around 7.5kN).
The geometry of the samples is the following:
Dimensioned technical drawing of a tensile test specimen: overall length 60.694, threaded ends M6 with 15 mm thread lengths, reduced gauge section 24 long and diameter 4 with R4 fillets, and a 2.646 and 0.701 dimension.


The results reported by the lab are the following:
  • ##\mbox{- Yield strength: } S_y=480\mbox{ MPa}##
    • from the plot we get ##6{kN}/(\pi 2^2)=477.5\mbox{ MPa}##. OK here!
  • ##\mbox{- Tensile ultimate strength: } S_u=615\mbox{ MPa}##
    • from the plot we get around ##7.5{kN}/(\pi 2^2)=597\mbox{ MPa}##. OK here too!
  • ##\mbox{- Elongation: } 54\%##
    • from the plot, we see that it has been elongated up to around 12mm, which, if we consider the length of the sample to be 24mm, it corresponds to these 50%. Also fine!
  • ##\mbox{- Young's modulus: } E=187\mbox{ GPa}##
    • I unable to get this result... from the plot, the displacement corresponding to the Yield strength (##@6\mbox{ kN}##), ##S_y=480\mbox{ MPa}## is around 0.6mm, which then leads to a strain of ##(24.6-24)/24=0.025##. Consequentially, this gives me an order of magnitude lower for Young's modulus: ##480/0.025=19.2\cdot10^3\mbox{ MPa}##, i.e., ##19.2\mbox{ GPa}##.
What am I doing wrong here?
Thanks in advance!
 

Discussion

Chestermiller said:
Where did the 187 come from?
This is the value declared from the lab report (it's not mentioned how it was calculated).
I was just trying to calculate it (and cross-check it) by myself based on the plot results.

Ah, this is a 306LN stainless steel.
 
No, for sure that it is not a typo. Also, for stainless steel, 190GPa are the "normal" values... I just don't get what is happening in the elastic part of this Load VS Displacement plot.

They have done other tensile tests on similar samples, and the values are also similar.
 
Typical strain values for steel at yield might be in the order of
0.002 rather than .025. Something funny with the decimal point here. Factor of 10. Something is amiss.