X-ray diffraction (XRD) is a nondestructive technique designed to probe the properties of crystalline materials. More commonly associated with the analysis of powders and single crystals for compounds and proteins of pharmaceutical interest, the technique is also used for residual-stress measurements in metals, advanced ceramics and other rigid samples.

Residual stress is a measurement of the stresses within a material without any impact from external forces. Often detrimental to the intended function of a particular component, it can develop from accumulative exposure to mechanical, thermal or metallurgical forces. XRD can measure this property through the interactions between the wave front of an x-ray beam and the sample’s crystal lattice. Since a crystal is composed of atoms arranged in a 3-D recurring pattern, it has atomic planes that cause constructive or destructive diffractions upon exposure to light. The nature of this interference depends on the spacing between these planes and the wavelength of the beam hitting the surface.

As stress creates an alteration in the crystal lattice, the spacing between the atomic layers will change, either increasing when the material is under tension or decreasing when the material is under compression. Thus, when the beam from an x-ray is directed onto the surface of a crystal, the angle of the diffracted beam will be affected by the presence of residual stress. Using elasticity theory and Bragg’s law, which correlates the angle of scattered light with the distance of atomic planes within a crystal, the residual stress can be calculated.

Samples whose residual stress is measured by XRD typically have grain sizes of 10–100 µm. To take a proper measurement, the sample is first washed to remove dirt or grease, and a flat region of the material is targeted. To avoid further stress on the sample, temperature is kept constant during measurement. While XRD can only probe the surface layers, this limitation can partially be overcome by using electro-polishing or other destructive methods, as long as they do not cause further strain.

Residual-stress analysis by XRD is applied to components of ships and aircrafts, as well as commercial vehicles. It is also used in the analysis of welds and materials used in infrastructure elements, such as bridges, pipelines and power utilities. Major markets for residual-stress analysis include the public, aerospace and automotive sectors. But most analyses occur in academic institutions or a few commercial labs.

The market for XRD residual-stress analysis was over $30 million in 2015, just over 5% of the total XRD market. The top players include PANalytical, Rigaku, Bruker and Shimadzu, although it is not a major focus on their businesses. Proto Manufacturing and Technology for Energy Corporation (TEC) are also suppliers but have a strong concentration on the technology. Already an established market, this technology’s growth is expected to be modest.

Residual–Stress XRD at a Glance:

Leading Suppliers

• PANalytical (Spectris)

• Rigaku

• Proto Manufacturing

Largest Markets

• Academia

• Government

• Aerospace &Automotive

Instrument Cost

• $50,000–$200,000