Jun,

Look at B.D. Cullity Elements of X-ray Diffraction a very good reference
book for practical diffraction.

Lots of things to be concerned with but a few to get you started;
diameter of the tubing, cutting it will usually change the stress state,
if you cut the surface should be polished to less than a micron and
etched, back to last polish then etch, again back to polish etch. This
ensures that deformation from polishing and cutting are removed (this is
not always necessary depends on what you are measuring). Most likely
you want to do the residual stress analysis on the outside of the
tubing. Since you must have some curvature I would use a spot focus but
these are speculations since I do not know your situation.

The basics of the sin2(psi) method: with Bragg-Brentano geometry
(typical XRD) only planes parallel to the surface of the sample
(assuming a flat sample) are observed by the detector. Therefore if you
change the angle of the sample (psi angle - rotate sample with respect
to mounting surface of goniometer) such that you now observe diffraction
from planes that are not parallel to the surface but at a psi angle.
Therefore if you have a tensile stress on a plate the planes parallel to
the surface are only slightly changed in spacing by Poisson's ratio but
the planes at some angle to the surface feel a large effect and the
spacing is increase substantially by the stress. Hence you have a change
in d-spacing divided by the normal d-spacing and you get a strain. It
gets more complicated since you now need x-ray elastic constants to
determine the stress.

Hope that helps,
Ed

Jun Lu wrote:

>Hi,
>
>I have a superalloy tubing which has been severely deformed and heat
>treated. I suspect there is large residual stress. I found from literature
>that XRD is a reliable way to measure the residual strain. I read about
>the sin2(psi) method, but the exact measurement procedure is still not
>clear to me.
>I have tried to run theta-2theta scans on the tubing cross-section and
>observed difference in lattice spacings at different locations. But I was
>not sure these results are reliable, because there may be considerable
>stress relaxation at the surface? It is also known that the surface
>finishing results in significant additional residual strain. Could anyone
>give me advice as how the surface should be prepared?
>
>Please advise on those issues or point me to the book or papers where I
>can find the answer.
>
>Thank you in advance!
>
>Jun
>
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______________________________________________________
Edward A. Laitila Phone : (906) 369-2041
Engineer/Scientist Fax : (906) 487-2934
Michigan Technological University
Department of Materials Science and Engineering

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