Optical Alignment and Sample Positioning in Neutron Residual Stress Characterization

Neutron residual stress characterization involves measurement at three principal directions of the sample. However, the measurement of the third principal direction usually requires rotation of sample manually. Inconsistent measurement points in different directions will lead to test failure and wrong result. Accurate optics alignment and precise sample positioning in the coordinate system of the sample stage are important procedures of neutron residual stress characterization, since optics alignment and sample positioning directly affect the reliability of the residual stress test. In this work, the sample stage coordinate system of the diffractometer was established based on the experimental optics and the sample stage. The center of the coordinate system was determined by a positioning hole machined in the center of the sample table and a positioning pin made by martensitic steel, austenitic steel, copper, nickel based high-temperature alloy, aluminum alloy, etc. to satisfy the requirement of conventional metal test. Aimed at the integral intensity of the diffraction peak of the positioning pin, the instrument optics including monochromator, the incident slit, the diffraction slit and the radial collimator was accurately aligned by step scanning. Based on the optics of the neutron residual stress diffractometer and the coordinate system of the sample stage, electronic total station, edge scanning using incident neutron beam and diffraction neutron beam, and sample contour scanning were used to accurately position the sample coordinate. Especially, the position precision was quantitatively analyzed. For samples with regular shapes, the coordinate position of the sample surface could be observed by a total station. According to the attenuation of the sample on incident neutron beam, a low efficient ³He tube detector located at the center of the incident neutron beam was used to record the relationship between neutron intensity and sample translation perpendicular to the incident neutron beam. Neutron intensity of the ³He tube follows a Boltzmann function distribution with the translation position of the sample. The coordinate position of the scanned side of the sample can be obtained through curve fitting. Edge scanning by diffraction beam is the most accurate positioning method. The integral intensity of the diffraction peak is affected by the combined effect of the effective sampling size (contributing diffraction intensity) and the neutron path (attenuating diffraction intensity), while the neutron path and effective sampling size can be obtained by calculating the surface position of the sample. Thus, the relationship between the integral intensity of diffraction peak and the sample translation could be established. The coordinate position of scanning edge can be obtained through curve fitting using transmission scanning optics and reflection scanning optics. For samples with complex shapes, sample contour scanning by coordinate measuring machine or laser scanning arm should be used to obtain the irregular outline drawing. This work will help users understand the optics alignment and sample positioning of neutron residual stress characterization, analyze the error source of residual stress measurement, guide users to accurately and quickly carry out neutron residual stress experiments.