Through Thickness Residual Stress Measurements by Neutron Diffraction and Hole Drilling in a Single Laser-Peened Spot on a Thin Aluminium Plate

Residual stress measurements are very challenging in thin aluminium plates. Rollinginduced crystallographic texture can lead to an S-shape fit when using the sin²ψ method for surface X-ray diffraction. Peak broadening and missing peaks can also be observed for synchrotron X-ray diffraction with conventional θ/2θ scanning due to texture. In addition, when measuring near the plate surfaces, partially-filled gauge volumes in neutron and synchrotron X-ray diffraction experiments will lead to "pseudo-strains", an apparent shift between measured and actual positions for the diffraction peak. Obtaining a meaningful value of d₀ for strain calculations is another issue for diffraction experiments in thin plates. The low thickness also offers challenges for destructive methods including incremental hole drilling: there is no defined ASTM standard for measuring nonuniform residual stress profile for thin plates. In this work, 2-mm-thick Al2024-T351 plate was investigated for residual stress fields due to laser peening. Neutron diffraction measurements were carried out at POLDI (Pulse Overlap time-of-flight DIffractometer) at the Paul Scherrer Institute, Switzerland and the results are compared with incremental hole drilling.