Hi all,
..following a bit the last posting on the topic...
> << I start to be a little bit concerned about all those people claiming and
> pointing out that diffraction don't measure a residual stress but a
> residual strain.
> ..............
>
> So, my own idea is that people who don't know how to transform strain in
> stress will measure only strain and the other are measuring stresses.
>
> Let me stress the audience about that; may be is that I am an engineer and
> not a physicist or chemist.>>
>
>
> Well, there is nothing wrong being an engineer but I cannot share your
> concerns. Not to bore the audience, I just claim that you are just short of a
> few quantities to measure stress directly from a diffraction experiment i.e.
> of the elastic constants (which one hopes to find conveniently tabulated
> somewhere and prays that they apply to the material under investigation).
in any case it is in principle possible to measure some "elastic
constants" using diffraction: it's just a matter of performing an in-situ
3 point bending or tensile or whatever kind of mechanical test on your
sample and evaluate the response of the material itself.
Since this is rarely possible, tabulated values as you say are the next
choice... but they're useful only if you know what kind of grain
interaction model is applicable to your particular case. I mean: you find
tabulated the single-crystal elastic constants or the macroscopic
mechanical elastic constants, but you need the X-ray elastic constants for
the polycrystalline under study... you're not measuring all
crystallites... you're just sampling some of them.
Sometimes (eg the case of thin films, especially if they're textured),
having the tabulated values for the single crystal elastic constants and
getting a "physically reasonable" stress value are not synonyms!
> For
> measuring strain, on the other hand, the diffraction experiment provides all
> the quantities necessary, you don't even have to know the material.
are you really sure?? If you care about relative values I agree, otherwise
you need the unstressed lattice spacing....
I agree on the fact that what you measure is something "easily" related to
the interplanar spacing; from that you can "easily" get the integrated
relative displacement in the laboratory system (with no "personal
assumptions"). What follows is then just a matter of ASSUMING a
behavior for the material and modelling. From my point of view it's a huge
integral problem, so the solution is not unique... we just need better
models
As for the fact that someone is able to measure strain and some others are
able to measure stresses, well, even if it is impossible to demonstrate
that someone knows THE residual stress present in a sample, it's rather
easy to prove that different values can be obtained from THE SAME set of
measured points. Moreover, if variations of the sin2(psi) method are used,
it can be shown even without recurring to Shannon's theorem, that in
most cases the result depends also on the sampling in the (psi -
2theta/theta space)
Ok ok... this is supposed to be the Rietveld mailing list ;o) oops.. Alan
is gonna kill me... well.. but there are some analogies with the Rietveld
method... and stress models have already implemented inside it... and
well.. at least here neutron and X-ray fans cannot fight ;o) The two
techniques are "more complementary each other" than for structure
determination... to get more info (surface and bulk)... you need both! Now
I'll get a lot of ... ehm on me.. but in this case really X-rays become a
"surface" technique and neutrons a "bulk" one... otherwise I'd give anyone
a thin film and a 30cm-thick "heavy" sample and ask to get reasonable
strain values for both using only one technique!
Have fun...
Mat
PS. In any case better to know the material.. and the instrument: residual
macrostrain is not the only source for peak shift!
wwwww
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MPI fuer Metallforschung
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Matteo leoni, PhD
Department of Materials Engineering
University of Trento
38050 Mesiano (TN, Italy)
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