Young's Modulus / Steel / Abnormal values

Good morning,
Our laboratory makes tensile testing on steels. We often observe Young's
Modulus (initial slope of the tensile curve) very different of the expected
value of 207 GPa (we observe 140 GPa to 230 GPa) on Carbon Steel materials
in different metallurgical states (Q, QT, N, ...).
This seems to occur since years (I have asked former workers), independently
of tensile testing machines, operator, sample geometry.
We have also asked INSTRON, the supplier of our testing machine, and they
have no ideas.
To live with the problem, it was decided to make an hysteresis loop after 2%
deformation and use that slope as Young's modulus. The value so obtained is
only slightly less dispersed, but a recent experiment with multiple
hysteresis loops (after 1% 2%, 3%, ...) shows that doing so, the Young's
Modulus seems to decrease with the deformation performed before the
hysteresis.
Do you have any ideas or references about :
- why the initial slope and the hysteresis slope differ so much from 207 GPa
- why Young's Modulus seems to decrease as the inital deformation increases.
I have done search on the Web with google and Copernic, but got no helpful
answer.
Thank you very much for your help,
Michel
Reply to
Michel PIETTE
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Young's modulus of polycrystalline materials depends on crystallographic texture. 207 GPa is just a typical value for bcc iron.
Michael Dahms
f'up2 sci.materials
Reply to
Michael Dahms
If you go here and look at the slide near the very end.... titled anisotropy....
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(yliu)/notes/chapt1. you will find listed the claim that the elastic modulus for Iron in the face edge 100 directions is about 135 Gpa and the modulus for Iron in the cell diagonal 111 direction is about 273 Gpa.
Very nice presentation.
Reply to
jbuch
Email the author......
snipped-for-privacy@polyu.edu.hk
Ask if he will give you the link to his excellent materials notes.
Reply to
jbuch
I fully agree that E depends on orientation. Nevertheless, our steels are mainly Quenched and Tempered types, and I don't expect them to be textured. But maybe I'm wrong... What's your opinion ?
Michel
"jbuch" a écrit dans le message de news: snipped-for-privacy@enews4.newsguy.com...
Reply to
Michel PIETTE
Sheets are usually rolled, and do generally have texture from the planar nature of the cold work.
You can partly eliminate texture by examining tensile specimens cut parallel to the rolling direction and 90 degrees to it, parallel to the axis of the rolls used to produce the sheet.
Tough to take specimens in the thickness direction, but if you were to be careful, you might be able to get a compressive Young's modulus from the through thickness direction.
You could perhaps even investigate ultrasonic modulus measurements to dig into the question of texture....... and some metallography would always be informative.
If it isn't texture, then your measuring techniques are called into question as otherwise potentially a waste of time and money.
You don't want to be in that position.
Jim
Reply to
jbuch
formatting link
(yliu)/notes/ try this, you can get all chapter listed there. I think link was missing .pdf
Reply to
tfagan
independently
obtained is
Mike,
Check out "Fundametals of Cyclic Stress and Stain" by Bela I Sandor
What you are experiencing is cyclic softening (low cycle fatigue), sometimes this is addressed as the "Bauschinger effect"for large strains (see Mechanical Metallurgy by G. Dieter).
As for the root cause of you poor Young modulus slope do a sensitivity analysis (uncertainty analysis - if your A2LA accedited you should know of this) on the modulus slope itself I think you will find the major errors are involved with measurement and determination of area (stress=F/A= F/(thickness*width) and strain e=dl/L (change in length/orginal: plud into error differential form of "Youngs modulus equation": E=F*dl/(A*L)
dE/E = [ (dF/F)^2+(ddl/dl)^2+(dA/A)^2+(dL/L)^2)^(1/2) where d = partial differential NOTE dA/A = [(dw/w)^2+(dt/t)^2]^(1/2) for flats
You would think the manufactures of the stress frame would know...HA HA they got rid of the ol' timers and saved money...make you and us pay the cost.
Ed
Reply to
Ed
Two years ago I did some tensile tests of aluminium and copper foils and found that the moduli were significantly smaller than quoted values. It is quite difficult to test foils though, so my colleague took the foils to another place and performed some sensitive three point bend instead. The values he got were similar to the quoted values. So perhaps you could perform a three point bend test (apparently you can design it to be very sensitive) to check.
At the same time, we were also interested in the yield values, and it was surprising to me that it was such a difficult measurement. Further depending on how the stress-strain curve is fitted, e.g. power law/bilinear, the yield value varies significantly. Now, I am more of a soft matter person and was not so experienced with stiff, plastically deforming materials, so I was generally surprised to find that those whom I was working with (whom I presumed were more well verse with the problem) were scratching their heads over this matter. Does anyone have similar experience on measuring yield stress/strain?
Reply to
Trina

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