What Is Residual Stress? Which Residual Stress Analyzer Is Good?

What is residual stress?
Independent of the external load, there are internal stresses in the structure and materials. These stresses are called residual stresses. The residual stress causes the strain corresponding to the change of lattice spacing. The residual stress spontaneously reaches equilibrium, because the tensile residual stress is known as the harmful compressive residual stress which is considered beneficial.
Shear stress
Why is residual stress measurement important?
Machining, welding, shot peening, heat treatment, grinding and other manufacturing processes will produce residual stress. Residual stress measurement is a basic method to determine whether the components can withstand severe load and stress conditions in service life. They can also be intentionally introduced in an appropriate way.
Residual stress measurement method
There are many ways to measure residual stress. These methods are usually divided into non-destructive, semi-destructive and destructive or diffraction based methods, strain relaxation based methods and other methods. All residual stress measurement methods are indirect. Residual stresses are calculated or derived from measurements such as elastic strains or displacements.
Residual stress measurement method based on diffraction:
In the diffraction based method, Bragg's law is used to measure the elastic strain, and the stress is calculated according to Hooke's law, elastic modulus (E) and Poisson's ratio (V).
X-ray diffraction
Compared with visible light, X-ray has high energy and short wavelength, which makes it very suitable for detecting crystal surface distance (= residual stress) in crystal materials. X-ray diffraction technology provides reliable and unparalleled data for quality control evaluation. This technology is suitable for all crystal materials, including ceramics. The X-ray diffraction method can measure absolute stress without calibration of stress-free samples. The measurement results of residual stress are expressed in absolute MPa.
Neutron diffraction
Neutron diffraction (nd) provides complete residual stress tensors σ 11 (parallel to the surface), σ 22 (parallel to the surface) and σ 33 (perpendicular to the surface) for the analysis of thick components. Like XRD, nd uses Bragg's law to measure elastic strain and calculates stress according to Hooke's law, elastic modulus (E) and Poisson's ratio (V). Because of the expensive fixed diffractometer for neutron generation, neutron diffraction for residual stress measurement has not been widely and easily obtained.
Synchrotron diffraction
Synchrotron diffraction is a higher level of X-ray diffraction. For components with complex geometry, synchrotron diffraction can be used, but the size of components is usually limited. There are only many synchrotron facilities around the world, which makes the method not portable and cost-effective.
Measurement of residual stress by drilling
Drilling is a common stress relaxation technique for measuring residual stress. By drilling a small blind hole in the target area to remove the stress material, the material around the hole spontaneously finds a new stress balance. This causes the surface near the hole to shift.
Borehole strain gauge method
Using the stain gauge method, install a special strain gauge on the surface to be measured. The residual stress was measured on a relaxation strain gauge.
ESPI drilling method
Using the ESPI method, the displacement near the surface is measured by using an optical interferometer. Then, the measured displacement is used to calculate the stress in the volume before drilling.