........ published in NEWSLETTER # 49
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........ published in NEWSLETTER # 49
MEASUREMENT OF RESIDUAL AND APPLIED STRESS USING NEUTRON DIFFRACTION
by Dr. M.T. Hutchings, AEA Technology, Didcot (U.K.)
It was just over ten years ago that diffraction measurements of residual stress were extended from X_rays to neutrons. During the ensuing decade measurements commenced at most steady state reactors and pulsed neutron sources around the world. So swift has been the development of the field that for some years now commercial measurements have been made on industrial components as well as the more scientific studies. The small but active neutron stress community convened in Oxford from March 18_22, 1991 to discuss in detail for the first time the problems, progress and future potential of the method. The volume (NATO ASI SERIES E216) is the result of that meeting, and as such forms a definitive statement of the current status of the field.
The accurate, absolute, and non_destructive measurement of residual stress fields within metallic ceramic, and composite engineering components is one of the major problems facing engineers. The extension of X_ray methods to the use of neutrons represents a major advance in such measurements. The technique utilizes the unique penetrating power of the neutron in most engineering materials, combined with the sensitivity of diffraction, to measure the separation of lattice planes within grains of polycrystalline engineering materials, thus providing an internal strain gauge. The use of neutrons is ideally suited to the determination of triaxial macrostress tensors, macrostress gradients, and microstresses in composites and multiphase alloys as well as deformed, plastically anisotropic metals and alloys. To date, it has been used to investigate welded and heat_treated industrial components, to characterize composites, to study the response of material under applied loads, to calibrate more portable methods such as sultrasonics, and to verify computer modelling calculations of residual and applied stress.
The contents of this volume can be summarized as follows. An overview of the method is presented followed by background presentations on the industrial need for such measurements, examples of industrial X_ray applications, and various analytical perspectives. Fundamentals are extensively addressed, including extraction of stress tensors, separation of macro_ and microstresses, errors in strain tensor determination, problems caused by anisotropy, large grains, and plasticity, and grain interaction and relaxation issues. Measurement details deal with the stress_free reference problem, spatial resolution, thick components, large grains, and sources of analysis errors. Instrumentation is a major theme and discussion is given of the optimized configuration, the use of position_sensitive detectors, microbeam methods, alignment methodology, and Fourier techniques. The use of pulsed sources is explored and their virtues relative to steady state sources discussed. The study of composites and the measurement of microstresses is discussed and examples given. Examples of applications and problems in the measurement of bulk components are presented. Several papers discuss a comparison of the neutron method with other stress probes, in particular with the closely related X_ray methods.
Both applied and basic problems are discussed. Though answers to practical industrial problems will probably be the main impetus for future developments in the field, the use of the neutron method for more fundamental studies will continue to evolve.
Reference books: B188, C320, E30, E113(a and b), E216