CURRENT APPLICATIONS OF X-RAY DIFFRACTION RESIDUAL STRESS MEASUREMENT

A brief theoretical development of x-ray diffraction residual stress measurement is presented emphasizing practical engineering applications of the plane-stress model, which requires no external standard. Determination of the full stress tensor is briefly described, and alternate mechanical, magnetic, and ultrasonic methods of residual stress measurement are compared. Sources of error arising in practical application are described. Subsurface measurement is shown to be necessary to accurately determine the stress distributions produced by surface finishing such as machining, grinding, and shot peening, including corrections for penetration of the x-ray beam and layer removal. Current applications of line broadening for the prediction of material property gradients such as yield strength in machined and shot peened surfaces, and hardness in steels are presented. The development of models for the prediction of thermal, cyclic, and overload residual stress relaxation are described. X-RAY DIFFRACTION (XRD) STRESS MEASUREMENT can be a powerful tool for failure analysis or process development studies. Quantifying the residual stresses present in a component, which may either accelerate or arrest fatigue or stress corrosion cracking, is frequently crucial to understanding the cause of failure. Successful machining, grinding, shot peening, or heat treatment may hinge upon achieving not only the appropriate surface finish, dimensions, case depth or hardness, but also a residual stress distribution producing the longest component life. The engineer engaged in such studies can benefit by an understanding of the limitations and applications of XRD stress measurement. This paper presents a brief development of the theory and sources of error, and describes recent applications of material property prediction and residual stress relaxation. Application of XRD stress measurement to practical engineering problems began in the early 1950's. The advent of x-ray diffractometers and the development of the plane-stress residual stress model allowed successful application to hardened steels (1,2). The development of commercial diffractometers and the work of the Fatigue Design and Evaluation Committee of the SAE (3) resulted in widespread application in the automotive and bearing industries in the 1960's. By the late 1970's XRD residual stress measurement was routinely applied in aerospace and nuclear applications involving fatigue and stress corrosion cracking of nickel and titanium alloys, as well as aluminum and steels. Today, measurements are routinely performed in ceramic, intermetallic, composite, and virtually any fine grained crystalline material. A variety of position sensitive detector instruments allow measurement in the field and on massive structures. The theoretical basis has been expanded to allow determination of the full stress tensor, with certain limitations.