Residual Stress Measurement
Residual stress measurement is one essential way to find out if the component can withstand the demanding load and stress conditions in its service life.
What is residual stress?
Independent of external loads, there are internal stresses inside structures and materials. These stresses are called residual stresses. Residual stresses induce a strain which corresponds to changes in lattice spacing. Residual stresses are spontaneously in equilibrium as
- tensile residual stresses which are considered detrimental
- compressive residual stresses which are considered beneficial
- shear stresses
Why residual stress measurement is important
Manufacturing processes such as machining, welding, shot peening, heat treatment, and grinding generate residual stresses. Residual stress measurement is one essential way to find out if the component can withstand the demanding load and stress conditions in its service life. It is also possible to introduce them intentionally with suitable processes.
Residual stress measurement methods
There are many methods to measure residual stresses. The methods are commonly grouped as non-destructive, semi-destructive, and destructive or diffraction-based, strain relaxation-based, and other methods. All the residual stress measurement methods are indirect. Residual stresses are calculated or derived from a measured quantity such as elastic strain or displacement.
Diffraction based residual stress measurement methods:
In diffraction-based methods, the elastic strain is measured using Bragg’s law and calculation of the stress is done with Hooke’s law together with elastic modulus (E) and Poisson’s ratio (v).
X-rays have high energy and short wavelength when compared to visible light making them ideal for probing the interplanar distances (=residual stress) in crystalline materials.
X-ray diffraction technology provides reliable data that is unmatched for quality control assessment. This technique is applicable to all crystalline materials including ceramics.
The X-ray diffraction method enables measurement of the absolute stress without the need for an unstressed sample for calibration. Residual stress measurement results are presented in absolute MPa values.
Neutron diffraction method
Neutron diffraction (ND) provides full residual stress tensor, σ11 (parallel to surface), σ22 (parallel to surface) and σ33 (normal to surface), analysis on thick components. As in XRD, ND also measures the elastic strain using Bragg’s law and calculates the stress with Hooke’s law together with elastic modulus (E) and Poisson’s ratio (v). Neutron diffraction for residual stress measurements is not widely available and easily accessible due to expensive stationary diffractometers for neutron generation.
Synchrotron diffraction method
Synchrotron diffraction is a higher energy version of X-ray diffraction. It is possible to use synchrotron diffraction for components with complex geometries, but usually the size of the component is limited. There are only a small number of synchrotron facilities around the world which makes the method less practical and cost-effective.
Residual stress measurement by hole drilling method
Hole drilling is the most commonly used stress relaxation technique for measuring residual stresses. Stressed material is removed by drilling a small blind hole in the area of interest and the material around the hole spontaneously finds a new stress equilibrium. This leads to a displacement of the surface near the hole.
Hole drilling strain gauge method
With the stain gauge method, special strain gauges are installed on the surface to be measured. Residual stress is measured on the relaxed strain gauges.
With the ESPI method, the displacements near the surface are measured by using optical interferometry. The measured displacements are then used to calculate stresses which existed in the volume prior to drilling.
How do Barkhausen noise values correlate with residual stress?
Measurement of residual stresses with Barkhausen noise is not a straightforward application since Barkhausen noise does not directly produce any MPa value results for stress state determination.
Barkhausen noise gives a response to the stress level of specimens. Barkhausen noise can be used for the evaluation of stress state of materials. Tensile stress increases the Barkhausen noise signal amplitude and compressive stress decreases Barkhausen noise signal amplitude.
With Stresstech products, residual stress measurement and detection can be done effortlessly in production lines, laboratories, and even in the field.
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