XRD is the only technique that provides a direct measurement of lattice strain, which is the amount of compression or expansion of lattice parameters along a specific sample direction. Stress is an excellent indicator of performance and failure. Cracking, conductivity, film peeling, corrosion rate and longevity are related to residual stress in a material. Residual stress is a measure of the degree of elastic deformation due to sample processing or preparation. We use specialized microdiffraction equipment to measure curved and small parts especially small diameter wire.. Typical line source equipment cannot be used to measure stress on small areas or curved parts because the large beam illuminates areas where the stress is in different direction( radial stress in wire) or may be altogether different( mechanical failure).
X-ray diffraction provides the most direct measurement of strain. The residual stress is calculated by applying stress constants, Young’s modulus and Poisson’s ratio, to the measured strain for the specific material. Residual stress (also called macro stress) is uniform stress along a particular direction in a material. Bi-directional stress and stress tensors can also be determined by XRD.
Strain and the associated stress can be determined with the traditional Sin2Ψ method or with an advanced 2D algorithm that can determine multi-axial stress, including sheer stresses. For this method, multiple sections of the Debye ring are sampled, and the multiple profiles increase the reliability of the measured strain. Better estimates of the errors in the derived values are also obtained by sampling large areas of the Debye ring. It is possible to measure in-situ strain as well, by applying a known force to the sample while the data is being collected.
Micro-stress, related to non-uniform strain or stress gradients are also measured by XRD. Micro-stress is typically related to metal cold work or deposition technique for thin films.