Jun,
As Patrick mentioned you need to do this on the surface not on the cross
section. Your instrument should be able to do a residual stress scan (if
you have a theta 2 theta you cannot go to negative angles of psi and you
may be limited to an angle less than 45 degrees) and hopefully has the
software included to do the analysis. Also how are you mounting the sample?
Patrick Weisbecker wrote:
> Hi Jun,
>
> To use the sin²psi method you don't have to work on the cross section
> but on the outer or inner surface of the tube.
>
> Depending of the stress/strain you want to measure :tangential, radial
> or along the tube axe you have to acquire theta/2theta scan with psi
> (or khi) changing from -45° to +45° the stress can then be calculated
> using the following formula:
>
> Thus plotting 2theta versus sin²psi give you straight line (or an
> ellipse if there is some shearing) whose slope allow you to determine
> the stress.
>
> PS: theta0 refers to the stress free state (you can use theta à khi=0
> as an approximation), you don’t need to know it precisely using this
> technique.
>
> PS2: It is usually assumed that as XRD is a surface analysis the
> radial component (sigma r or 33) is equal to zero.
>
> To avoid being disrupted by the geometry of the sample (peak
> displacement due to the cylindrical geometry) you can scatter a powder
> on the surface and check you don’t determine “false stress”
>
> Hope this can help
>
> Patrick
>
> -----Message d'origine-----
> De : Support and use of X-ray diffraction instrumentation
> [mailto:[log in to unmask]] De la part de Jun Lu
> Envoyé : mardi 12 février 2008 22:00
> À : [log in to unmask]
> Objet : Re: Look for advice on residual stress measurement
>
> Hi Ed,
>
> Thank you for your help. The strain along the length of the tubing is
>
> most important to me. The tubing has near square cross-section with
>
> outer dimensions of ~ 15x15 mm2 and 1.6 mm wall thickness. It was
>
> bent and heat treated. Our XRD machine (Phillips X'pert) can only
>
> accommodate samples <10 mm in height. So I cut a ~ 8 mm long section
>
> and did theta-2theta scans on the cross-section using 1x1 mm2 beam
>
> size. When I compared the positions of the (111) peaks obtained at
>
> different spots on the cross-section, I found difference. The problem
>
> is that I did not have a reference. Can I use the powder ground from
>
> the sample as reference, and get absolute strain with sufficient accuracy?
>
> For the cross-section sample, the strain is out of plane. I wonder if
>
> the sin2(psi) method is still applicable. Is the sin2(psi) the only
>
> reliable method to measure residual strain? Another related question
>
> is, is the strain somewhat relaxed at cross-section surface where the
>
> XRD took measurements?
>
> Thanks!
>
> Jun
>
> At 03:06 PM 2/12/2008, edward laitila wrote:
>
> >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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>
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> >>
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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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>
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> >SIGNOFF XRD
>
> Jun Lu PhD
>
> Div. of Magnet Science and Technology
>
> National High Magnetic Field laboratory
>
> Tallahassee, FL 32310
>
> Phone: 850-644-1678
>
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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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