Patrick and Ed,
Thank you all for the help.
The strain is supposed to be bending strain, i.e.
along tubing axial direction, compressive on one
side and tensile on the opposite side. Our
machine has automatic 4 circle rotations. So
sample rotation is not a problem. The problem is
the limit of the sample height for our machine.
The reason I worked on the cross-section is that
only a short cross-section sample can fit in the
machine. Scanning the tubing surface is
impossible with our machine unless the tubing is
cut open which will relax the strain I am
measuring. So with our machine I have to use
cross-section. Any ideas how? I also worry about
axial strain relaxation of the cross-section sample.
Ed: Have you received my pictures? Does it help to explain my problem?
Patrick: If I can use powder to check, can I use
power as reference to do a single theta-2theta
scan and get strain along certain direction?
Jun
At 09:27 AM 2/13/2008, edward laitila wrote:
>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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>> >>SIGNOFF XRD
>>
>> >>
>>
>> >>
>>
>> >>
>>
>> >
>>
>> >--
>>
>> >______________________________________________________
>>
>> >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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>>
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>>
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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
>
>______________________________________________
>XRD mailing list
>[log in to unmask]
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>[log in to unmask] with the text
>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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