About the Importance of X-ray Elastic Constant Determination in the Precise Measurement of Residual Stress Profiles

Author:  D. Delbergue, M. Lévesque, P.Bocher
Source:  ICSP-13
Doc ID:  2017074
Year of Publication:  2017
Abstract:  
Introduction: Shot peening is a surface treatment widely used in the aerospace industry in order to improve the fatigue performance of manufactured parts by introducing compressive residual stress at their surface. The residual stresses are actually found in a shallow depth under the surface, delaying both crack initiation and propagation [1]. Thus, accessing the in-depth residual stress profiles with accuracy is essential to calibrate the simulations of the shot peening process [2] or for adequate fatigue life predictions of the components [3]. Any error would lead to the improper calibration of the models. A large number of techniques can be used for residual stress measurement [4], but not all can quantify the stress gradient introduced by the peening process. X rays having a penetration of few micrometers in metals [5], X-ray diffraction (XRD) techniques are highly suitable for the measurement of residual stress gradients. Consequently, this technique is widely used in industries and laboratories in this regard. With XRD technique, the residual stress is determined from the measurement of the interplanar spacing through the use of X-ray Elastic Constants (XEC) [6]. They are generally extracted from the literature. Some are determined from single crystal specimens; others are extrapolated from known macroscopic values (Young’s modulus and Poisson’s ratio) resulting in possible artefacts as demonstrated by the present paper. The stress tensor being measured at the surface of the specimen, the stress in the irradiated layer is biaxial; therefore, only one XEC is required (equivalent to the ½S2=(1+ν)/E in the stress tensor) [6]. As the stress measured by XRD is a linear function of the XEC, any error in the assessment of the ½S2 value will lead to a proportional error on the measured stress value.