Residual stress characterisation
There is a wide range of experimental techniques available to characterise the residual stress in a component or engineering structure. When selecting a particular method for any application there are 4 main criteria that need to be considered:
- The depth from the surface that needs to be characterised
- The spatial resolution
- The level of impact on the component (i.e. is it destroyed or still useable)
- The stress component or components that need to be characterised.
For example, Figure 1 shows the comparison of several common techniques when applied to steel with regard to the level of damage and the depth penetration.
Figure 1: Level of damage vs penetration depth for common techniques used to characterise residual stress in steels
Perhaps the most important point to consider first is if the component or artefact to be characterised can be destroyed, if it is required to be in service post measurement or if a small amount of superficial damage can be tolerated.
Of the non-destructive techniques, x-ray diffraction is probably the most important for crystalline, engineering components. It is based on the simple principal that the crystalline lattice is deformed by the presence of a stress and can be exploited as an atomic scale strain gauge. X-ray diffraction equipment can be laboratory based but is limited by the penetration of the radiation. Two other types of radiation, Neutrons and synchrotron x-rays can be produced at wavelengths suitable for the study of engineering materials. The depth penetration of these types of radiation is significantly greater than for laboratory based x-ray systems in most materials, however limited by their availability. Neutrons and synchrotrons x-rays are only available at large-scale facilities such as the Rutherford Appleton Laboratory.
Further particulars of non-destructive methods offerred by StressMap can be found at:
The other non-destructive techniques rely on the principal that residual stress affects some other physical property of the material. Commonly magnetic properties or ultrasonic wave propagation are used for this purpose. These techniques are less common but serve specific engineering applications.
Destructive and semi-destructive techniques
The destructive and semi-destructive techniques use the principal of deliberately removing material to create new free surfaces and as a consequence, relieving the residual stress. The relief of stress is accompanied by a distortion and by quantifying the distortion, the original residual stress can be determined. The extent to which the technique is destructive is essentially based on the method and amount of material moved.
Semi-destructive techniques, such as incremental centre-hole drilling, damages the component being examined but generally to an extent that allows the component to continue in service or to be characterised further by another technique.
Unlike the other techniques mentioned so far that generally probe the residual stress at a single discrete point (albeit with the possibility of repeatedly measuring the sample at many locations), the Contour Method is a unique technique in that it can provide full two-dimensional distribution of residual stress through the thickness of large engineering structures.
The Contour Method produces much greater richness of residual stress measurement data over any other known technique. Neutron and synchrotron scattering are perhaps the nearest rival methods; but their expense is considerably greater, components have to be taken to Central Facilities, they are very sensitive to microstructure and cannot deal with thick structures (for example the limiting thickness of steel is about 40 mm). Many studies have compared the contour method with other techniques, and all have confirmed the accuracy of the results it provides.
Further particulars of the destructive methods available at StressMap's laboratories can be found at: