Residual stress evaluation using XRD and failure analysis

Abstract

An industrial component often contains residual stress due to processing or in-service conditions. Residual stress is important among other surface integrity parameters, because it can contribute directly to pre-mature fail¬ure of structures. The knowledge of the origin of residual stress, magni¬tude, type and distribution with respect to applied stresses is essential to prevent failures and to enhance the lives of the components. X-ray diffrac¬tion based technique for the evaluation of residual stress is non-destruc¬tive and determines the nature and magnitude of both macro and micro surface stress. It is based on the change in inter-planar spacings which causes a peak shift of an X-ray diffraction line. The challenge in this method lies in the determination of the X-ray elastic constants for a se¬lected crystallographic plane. These are used to convert the measured strain data into stress values. The X-ray diffraction work has some inher¬ent problems due to large grain size, presence of texture, strain gradient and shear stress. Ways are described to overcome these shortcomings. Special diffractometers are mentioned which are portable, fast and use position sensitive detectors. They can be utilized for carrying out in-situ work at plant sites. The latest research for measuring sub-surface stress, stress gradients, stress at elevated temperatures and in different environ¬ments are briefly outlined. Finally, some case studies dealing with the failure analyses of components are highlighted.