Diffraction based residual stress analysis
Diffraction techniques, using both X-rays and neutrons, enables residual stress profiles of varying depths to be developed for the component in question. Although the cost of these processes and difficulties associated with their application generally limit these techniques to scientific applications, they are viable in certain industrial applications.
Although residual stress is often overlooked in industrial applications, the superposition of applied loading and residual stress and the effect these stresses have on fatigue performance of the component is often dominated by the nature/sign and magnitude of the residual stresses. As the residual stress distribution in materials and components can be varied by the choice of the processing parameters, and the order in which they are applied, it is possible to optimize the residual stress distribution with respect to the service induced stress state. Obviously such an approach requires knowledge of the residual stresses present in the component in question. Although the experimental determination of residual stresses using hole drilling techniques is well known, relatively inexpensive and can be applied to large components; diffraction techniques offer some unique advantages as they give a detailed profile of the residual stresses, are non-destructive and can be applied to all crystalline materials.
Diffraction techniques exploit the fact that when an applied or residual stress is induced in a material, the resulting elastic strains cause the atomic planes in the metallic crystal structure to distort. Diffraction techniques measure this inter-planar atomic spacing, which can be used to calculate the total stress in the material by comparing the lattice spacing in a stressed component to those in a similar unstressed reference sample. Two of the more common diffraction methods include X-ray and neutron diffraction.
X-ray diffraction for residual stress measurement is generally used to probe near surface regions of components. X-rays are generated in a cathode ray tube and directed onto the surface of the sample. Rays diffracted from the sample surface are collected by an X-ray detector which enables diffraction patterns to be observed and it is from these patterns, changes in lattice spacing and hence strain can be calculated. X-ray diffraction is a relatively rapid and cost-effective solution for residual stress measurement, with its main limitation being its penetration depth which is typically in the order of 0.025 mm. Greater penetration depths can be obtained by removing thin surface layers of the test specimen in-between measurements. However, this approach is tedious and may result in a change of the residual stress state within a component.
The application of neutron diffraction in solving engineering relevant problems has become widespread over the past two decades. The advantage of the neutron diffraction over X-ray diffraction is its larger penetration depth. Neutron diffraction enables the measurement of residual stresses at near-surface depths around 0.2 mm down to bulk measurement of up to 100 mm in aluminium or 25 mm in steel. With high spatial resolution, the neutron diffraction method can also provide complete three-dimensional maps of the residual stresses in the component. However, the relative cost of the application of neutron diffraction method is much higher, mainly because of the equipment cost and need for a neutron source. As it is too expensive to be used for routine process quality control, it is generally limited to scientific research. However, although it is necessary to take the component to the neutron diffraction detector which is obviously not always possible, the use of neutron diffraction can offer significant benefits in failure investigations and fracture mechanics studies of critical components.
Published in Technical Tips by Origen Engineering Solutions on 1 October 2016