Theory of the Peak Shift Anomaly due to Partial Burial of the Sampling Volume in Neutron Diffraction Residual Stress Measurements

Abstract

A theory is presented to describe the anomalous peak shift encountered in neutron diffraction residual stress measurements as the specimen is translated into and out of the sampling volume, which is defined by a pair of masking slits inserted before and after the specimen. Analytical formulae for the anomalous peak shift were obtained for both position-sensitive-detector-based diffractometers and conventional scanning diffractometers. The results indicate that the observed peak shift is a complex function of many variables, including the in-pile collimation, slit widths, slit-to-axis distances, mosaic spread of the monochromating crystal, and mismatch in lattice spacing between the sample and the monochromator. Calculations based on the derived analytical formulae are in good agreement with experimental observations. It is shown that by the choice of appropriate experimental conditions, this peak shift anomaly can be suppressed or, in some cases, eliminated altogether.

Quantifying residual stresses in resistance spot welding of 6061-T6 aluminum alloy sheets via neutron diffraction measurements

“…7 shows data acquisition for neutron scatter diffraction, including a data collection panel, a Gauss model for signal intensity, and a d-spacing illustration for measured location within the welded coupon and stress free combs. This technique was also used by Wang et al (1998) in developing the theory of the peak shift anomaly due to partial burial of the sampling volume in neutron diffraction residual stress measurements. The aluminum alloy (3 1 1) diffraction peak was used in this study.…”

Neutron Diffraction

“…Three effects [26][27][28] occur for monochromatic beam diffraction with an area detector and slit geometry. Firstly, the center-of-gravity of the diffraction from the sample is shifted away from the reference point of the instrument.…”

“…The uneven distribution of intensity across the counter can also lead to an error. The problem has been studied experimentally and also modeled [26][27][28]. Partial filling usually arises in attempts to measure near-surface stresses but can occur if the sample has been set up incorrectly or if slits have been bumped off alignment.…”

Analytical model for neutron diffraction peak shifts due to the surface effect

“…Neutrons are unique in probing materials nondestructively to great depths (Withers, 2004;Woo et al, 2011) but require a much larger diffracting volume compared to X-rays. Therefore, neutron measurements suffer from the surface effect when trying to approach shallow depths (Webster et al, 1995;Spooner & Wang, 1997;Wang et al, 1998;Hutchings et al, 2005), which complicates attempts to interpolate between neutron and X-ray data.…”