X-ray diffraction (XRD) is a non-destructive technique to measure residual stresses on the surfaces of metal specimens. It can be applied on crystalline materials as ferritic and austenitic steels, aluminium alloys and cast iron.
Measurement depth is limited to some microns and for this reason this technique is generally used for surface engineering application, shot and laser peening or for quality control.
The cost for a single surface measurement is limited, but the solution of the complete field state requires the repetition of the x-ray diffraction measurements in 3 different independent directions.
In some applications, X-ray diffraction measurements can be repeated in the same location at different depth of analysis after an electrochemical process of electro polish of the surface: in this case the accuracy of the method is drastically reduced and both cost and required time are increased. The purchasing of the measurement hardware requires a higher investment comparing with the semi-destructive methods based on strain gage rosettes. The accuracy of this method can be influenced by the grain size and texture: for this reason, generally a good surface finish is required.
The X-ray diffraction method can measure residual stresses also thanks to its ability to measure the atomic distance between crystallographic planes in crystalline materials.
In case of linear conditions of stress, when the material is loaded, the distance between crystallographic planes increases or decreases in relation to the unloaded condition depending on the type of stress (tensile or compressive).
When x-ray radiations interact with the surface of a crystalline material, according to the Bragg’s law, radiations are absorbed and then reflected with a certain orientation and with the same frequency.
This angle is proportional to the lattice spacing. The measurements of the lattice plane can be performed using different techniques: the most used are the sin2ψ and the cosα.
Thanks to the long experience on stress analysis and residual stress measurements, the laboratory of SINT Technology is able to provide x-ray diffraction measurements for laboratory applications or integrated solutions for the complete analysis of the residual stress field in the specimens under test.
Further readings & Scientific Papers
- Fitzpatrick, M.E., Fry, A.T., Holdway, P., Kandil, F.A., Shackleton, J., and Suominen, L., “Determination of Residual Stresses by X-ray Diffraction”, Measurement Good Practice Guide No. 52, (Issue 2), NPL, UK, 2005.
- SAE Residual Stress Measurment by X-ray Diffraction, SAE J784, Society of Automotive Enginners Handbook Supplement, Warrendale , PA, 2003.
- Delbergue, D., Texier, D., Lévesque, M., Bocher, P. “Comparison of Two X-Ray Residual Stress Measurement Methods: Sin2 ψ and Cos α, Through the Determination of a Martensitic Steel X-Ray Elastic Constant”, 2016, 10.21741/9781945291173-10.
- Tanaka, K. “The cosα method for X-ray residual stress measurement using two-dimensional detector.” Mechanical Engineering Reviews, 2018. 10.1299/mer.18-00378.
- Schajer G.S., “Practical Residual Stress Measurement Methods”, Wiley, 2013.