X-RAY DIFFRACTION MEASUREMENT OF RESIDUAL STRESS (STRAIN): ENERGY DISPERSIVE X-RAY RESIDUAL STRESS MEASUREMENT.

摘要

As a result of forming, joining, machining, grinding, shot peening, carburizing, nitriding, and heat treating processes, a solid material is often found to be in a state of internal stress (strain) even though all externally applied stresses have been removed. This internal stress state is commonly termed residual stress. The term, residual stress, is used in this thesis in order to conform to conventional usage, although what is actually measured in all cases is residual strain. Since these residual stresses (strains) can markedly affect the performance of a material, it is important to be able to measure them. In the present thesis x-ray diffraction techniques, the oldest of the nondestructive methods for measurement of residual stress, are thoroughly investigated.; This thesis is divided into two sections: (I) X-ray Diffraction Measurement of Residual Stress (Strain) and (II) Energy Dispersive X-ray Residual Stress Analysis. In Section I a comprehensive survey of the entire x-ray residual stress literature is made. The fundamental theory upon which the x-ray diffraction method of residual stress measurement is based is reproduced, all assumptions made are clearly indicated, and the effects of experimental deviations from these assumptions discussed. Consideration is also given to sources of error, precision of various experimental geometries, choice of radiation, sample preparation, grain size, x-ray penetration depth, Lorentz-polarization, absorption factors, and x-ray elastic constant determination. Recommendation is made as to optimization of the different measurement techniques, so that the best technique can be selected for use in practical applications.; In Section II, for the first time, the energy dispersive x-ray residual stress analysis technique is compared on a one-to-one basis with conventionally established x-ray angular dispersive and strain gauge measurement techniques. Stress measurements were performed on a special tapered steel cantilever specimen subjected to both tensile and compressive loads. A specially designed diffractometer arrangement was constructed so that a solid state energy dispersive Si(Li) detector could be interchanged with a conventional angular dispersive proportional counter for accurate residual stress measurement comparisons. It was experimentally demonstrated that the energy dispersive technique does have the precision necessary to measure residual stress (strain). However, the most critical factor in measuring residual stress is the problem with sample displacement that can occur in all small 20 angle diffractometry arrangements.