Rapid in-line residual stress analysis from a portable two-dimensional X-ray diffractometer

Application of 'cosa' method using portable 2DXRD instruments has attracted considerable attention for rapid, in-line residual stress measurements. However, the validity of this technique has often been questioned, particularly under field conditions, because the mechanisms of how critical measurement parameters such as sample-to-detector distance and X-ray incidence angle affect the accuracy of measured residual stress is not adequately understood. To address this issue, near-surface residual stress in a shot-peened AISI 4140 alloy coupon was determined using 'cosa' method for a wide range of sample-to-detector distances and incidence angles - this data was then compared with the data from conventional 'sin(2)psi' technique. Results showed that there exists a window of incidence angles for which the values of residual stress from 2DXRD are reliable. However, outside of this window, data loses accuracy due to beam defocusing and errors in determining its orientation with respect to diffracting crystallites. Unlike incidence angle, the impact of sample-to-detector distance on the accuracy of residual stress was found to be less significant, as it only changed the size of the Debye ring and did not affect its distortion. Results also showed that increase in beam size can reduce the exposure time required for data collection, but it comes at the cost of sacrificing the intrinsic resolution and signal-to-noise ratio (SNR) of the diffraction peak. Insights from the experimental data presented in this work can establish proper measurement protocols to enable accurate and rapid residual stress analysis using portable 2DXRD instruments and improve the current practice of quality control in surface treatment industry. (C) 2020 Elsevier Ltd. All rights reserved.