I know there are some methods that can be used to nondestructively measure residual stress near the surface of material, such as XRD, but how to get the residual stress deeper in the material. I don't want to use the blind-hole method that is destructive. Or is there a method to calculate the inside residual stress based on the surface residual stress? Looking forward to your reply, thank you!
The diffraction methods measure residual lattice strain distribution in a crystalline microstructure. It cannot be used on amorphous metals. The X-ray diffraction method has mainly become used for research studies, but equipment for in-line production control has also been developed. There are several good textbooks about the X-ray diffraction method (4,6,8). The X-ray is generated in an X-ray tube in which high-energy electrons hit a target of a pure metal. Then X-rays with wave lengths characteristic for the target material are generated. For studies on steel, Cr Kα radiation with a V filter has normally been used. It gives fewer problems with Fe-fluorescens than the more intense Cu Kα radiation. Modern solid-state X-ray detectors exist nowadays that can suppress the Fe fluorescens so that Cu Kα radiation can be used. In a typical laboratory diffractometer, the specimen is mounted in the center of a circle. An X-ray detector can be moved around the periferi to scan and record the X-ray diffraction profile. The peak position is then determined from the shape of the peak. Then the specimen is turned a certain angle, ψ-angle, around the diffractometer axis. The important information is the peak shift as a function of the ψ-angle. Knowing the so-called X-ray elastic constants, one can calculate the residual stress, and there is ample commercial software that can perform this function. Large samples such as engineering parts may be impossible to mount on a diffractometer. Then portable equipment often using so-called parallel beam geometry is more suited in which only a parallel beam of X-rays is permitted using a long parallel beam collimator. Much X-ray intensity is lost, but the setup is rather insensitive to the exact specimen surface position.
Use neutron diffraction. Because neutrons have no charge, they can penetrate deeper into the material, hence determine residual stresses in thicker materials. The basic principle (diffraction) is the same as for X-rays.
You can try ultrasonic testing for residual stress estimation of bulk sample
You can do this by increasing the glazing incidence angle (GIA). In case of thin film, you can also do this by varying the film’s thickness. For further detail, please refer to the article given at link; (https://www.researchgate.net/publication/352830671 or http://dx.doi.org/10.13140/RG.2.2.27720.65287). However, this article is in non-peer reviewed form.
If you end up with some surface measurements, there is the method of Eigenstrains explored by Korunsky (Article Variational eigenstrain analysis of residual stresses in a w...
) whereby from limited measurements of residual stress, full fields can be found.It stems from the exact problem that you are facing. It's spiritually similar to solving a boundary value problem in my mind.
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