I have a formed tube with reduction in diameter, i need to find residual stress through XRD. For sampling I can either cut a small section of hollow tube or make powder of it. If i turn my sample into powder, will there be same residual stresses or will it change?
Guys, I am sorry but you have to think what you want measure and what residual stress means!
Residual stress in caused by anisotropic strain e.g. by loading a surface by rolling, forging, shot peening, grinding, polishing, drawing... or thermal treatment like cooling, solidifying, local heating, local melting (e.g. laser melting), cladding, coating etc. which finally causes a gradual shrinking as typical in welded materials like tubes. The stress state can be quite complex, laterally but also in depth.
How to determine the strain for a certain depth? You simply look how for the "same" lattice plane, e.g. (211), the respective distance is changing with the angle to the loading direction. In that case you need to tilt the sample, or the analytical equipment, i.e. tube as well as detecting system. This means with tilting sample you measure the same peak again and again and you will observe a positive (compression, usually good) or negative strain (tension, usually bad). Of course...when you observe tension there is also something under compression (in depth) since everything has to be in equilibrium! This needs to be characterized...mainly in depth!
If you make a powder, all the strain is changed (mainly totally relaxed), i.e. your measurement is absolutely useles. Therefore, people use powder XRD for phase analysis, even if they produced the powder by very "brutal techniques". In first approximation even these brutal techniques do not dramatically affect the peak position, only pea width.
In other words, you need to measure the surface as it is. Any preparation like grinding will change everything. The only opportunity is chemical or electrolytical etching if necessary (e.g. for a depth profile which is important for a general characterization of he stress state).
There are mobile diffractometers available which can be mounted at the sample (big tubes, turbine blades, air planes etc.). Destructive techniques enable to use standard diffractometers, but usually they have to have an additional tilt axis in order to measure a sample from 0 (vertical) up to 70° tilt out of the diffraction plane and more....and this in both directions. The alternative tilt around omega (theta) is possible but then the absorption correction is more difficult, and the tilt is limited which decreases the reliability of the derived values. Usually, 2theta is in the scale of 140-160°. This looks strange but the higher the angle the higher the accuracy (detectable shift of the used peak). For Fe-containing materials usually Cr radiation is used. Cu is "bad" because of fluorescence.
A big challenge is the grain size. It is in the scale of several 10 µm or even more you are running into problems regarding the statistic. Then you can move your sample during measuring by a lateral translation or a gentle panning. If the crystals are then still to big you only can switch to single crystal analysis but this is a separate science and from my point of view only worth to do in the frame of a PhD work! This is definitely no routine job!
Hi Pranav,
cutting and powdering parts of your sample is done by local 'brute forces'. So you introduce excessive strains in your material which very most probably exceeds the limits of Hooke's law. So you not only will change your residual stress but also will introduce new stresses to your sample. In the case of powdering you will most probably remove (or at least change) your residual stress, which you want to measure. Your measurement then will be not really meaningful; sorry for that answer.
For finding the residual stresses through XRD, a separate setup and experimental procedure are there which is called solid XRD. This setup is different than powder XRD. Here you need not to make powder.
Guys, I am sorry but you have to think what you want measure and what residual stress means!
Residual stress in caused by anisotropic strain e.g. by loading a surface by rolling, forging, shot peening, grinding, polishing, drawing... or thermal treatment like cooling, solidifying, local heating, local melting (e.g. laser melting), cladding, coating etc. which finally causes a gradual shrinking as typical in welded materials like tubes. The stress state can be quite complex, laterally but also in depth.
How to determine the strain for a certain depth? You simply look how for the "same" lattice plane, e.g. (211), the respective distance is changing with the angle to the loading direction. In that case you need to tilt the sample, or the analytical equipment, i.e. tube as well as detecting system. This means with tilting sample you measure the same peak again and again and you will observe a positive (compression, usually good) or negative strain (tension, usually bad). Of course...when you observe tension there is also something under compression (in depth) since everything has to be in equilibrium! This needs to be characterized...mainly in depth!
If you make a powder, all the strain is changed (mainly totally relaxed), i.e. your measurement is absolutely useles. Therefore, people use powder XRD for phase analysis, even if they produced the powder by very "brutal techniques". In first approximation even these brutal techniques do not dramatically affect the peak position, only pea width.
In other words, you need to measure the surface as it is. Any preparation like grinding will change everything. The only opportunity is chemical or electrolytical etching if necessary (e.g. for a depth profile which is important for a general characterization of he stress state).
There are mobile diffractometers available which can be mounted at the sample (big tubes, turbine blades, air planes etc.). Destructive techniques enable to use standard diffractometers, but usually they have to have an additional tilt axis in order to measure a sample from 0 (vertical) up to 70° tilt out of the diffraction plane and more....and this in both directions. The alternative tilt around omega (theta) is possible but then the absorption correction is more difficult, and the tilt is limited which decreases the reliability of the derived values. Usually, 2theta is in the scale of 140-160°. This looks strange but the higher the angle the higher the accuracy (detectable shift of the used peak). For Fe-containing materials usually Cr radiation is used. Cu is "bad" because of fluorescence.
A big challenge is the grain size. It is in the scale of several 10 µm or even more you are running into problems regarding the statistic. Then you can move your sample during measuring by a lateral translation or a gentle panning. If the crystals are then still to big you only can switch to single crystal analysis but this is a separate science and from my point of view only worth to do in the frame of a PhD work! This is definitely no routine job!
Dear Steven,
if I consider the question I am quite sure that they don't have a neutron reactor in their lab. I am even not sure whether their X-ray equipment is suitable (in case they have one).
However, alternatively they should perhaps think about the drill-hole technique which is from my point of view the typically used method for tubes in industry.
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