The thickness of the stess layer generated by machining is very thin, the changing rate of the stress value along the depth direction is very high, conventionally, the stress values are measured by XRD method and layer removal, anybody know how to measure the stress values changing along the depth direction without any layer removal?
I think with extensometry is a good possibility. But you need to make a stress relaxation for example drilling a hole.
the two main non destructive methods are X diffraction and Ultra Sonor measurements.
You also can make inverse method calculation based on simple structure and observed distortion to evaluate the initial residual stress.
Yous should look in casting or welding references.
- Handbook of residual stress and deformation of steel (G. Totten, M. Howes, T. Inoue)
- Methods of measuring residual stresses in components (Rossini).
Measurement depth in steel using X Rays ranges between 0.001 to 0.01 mm. Ensure that the sample is flat and free from surface artefacts.
This may help:
http://www.sciencedirect.com/science/article/pii/S0261306911005887
We employ a variety of techniques for residual stress determination, but when we need a non-destructive technique we tend to employ an X-Ray method, i.e. using the Stresstech 3000
see
http://www.stresstechgroup.com/content/en/1034/74696/Xstress%203000%20G3%20%20%20G3R.html
The Stresstech 3000 x-ray residual stress analyser provides:-
- a non-destructive method.
- surface stress analysis and subsurface analysis by stress free electropolishing removal methods.
- fast analysis (5 minutes from set-up).
- small irradiated areas, 5mm to 1mm diameter.
- improved access by the use of a miniature X-ray tube.
- high accuracy from 2 detectors.
- portability for use on site.
It has its limitations, like any of the techniques for residual stress determination.
The X-Ray method is the best to measure residual stress in the surface. Many of commercial equipments include a module to determine the stress state in the material subsurface, with application in substrate film systems, metals and alloys with surface modification by heat treatment, materials with plastic deformation in the surface caused by machining processes.
If you can create a small representative sample, neutron scattering may be an option offering a measurement along the thickness direction.
You can use white beam facilities and carry out an energiedispersiv XRD measurement. In reflecitivity mode you can measure a full stress depth profile down to 60µm. At synchrotron beamlines it is a standard method, i would say. But their are also possibility's under lab conditions.
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