ADVANCED MATERIALS & PROCESSES | APRIL 2025 21 into the material at high strain rates[3,4]. The creation of the residual stress field (i.e., by plastic deformation of the near-surface layer) is comparable to how the residual stress field is created using the more traditional shot peening method (i.e., indentation of the component surface by bombardment using metal or ceramic balls). However, the residual stress field created using LSP is generally greater in magnitude and extends further from the surface than after shot peening. Laser peening is often categorized in terms of its power density, which is the power per unit area of the laser beam with units of W/cm2. Power density is calculated by dividing the laser energy in Joules, by laser pulse time, in seconds, times laser spot area in centimeters squared. Characterization of the profile of residual stress from a surface engineering process is not straight- forward. Table 1 gives an outline of the key techniques, and their advantages, disadvantages, and costs. The team’s expertise at Coventry University spans the main methods used for the experimental determination of residual stress. For laser peening, no method is ideal, because often the stresses of interest are those from the surface to 3 mm depth, or even further. Hence in practice, the team deploys multiple methods to determine residual stress and provide cross-validation between methods. This might include a combination of the contour method, surface x-rays, and incremental hole drilling; or neutron or synchrotron x-ray diffraction with surface x-rays for components or assemblies that are not suited to the other methods. A comparison of laser-peen- induced residual stress measured using synchrotron x-rays, incremental hole drilling and the contour method is shown in Fig. 2. In this study, laser peening was applied to a rounded-edge stepped coupon[5]. Laser peening produced compressive residual stress on the surface of the coupon on the order of 250 MPa, that transitioned to tensile balancing stress of approximately of approximately 150 MPa peak was measured in the center of the samples. This tensile stress is a balancing stress required to maintain equilibrium with the induced compression fields. The magnitude of the tensile stress is somewhat high due to the limited volume in the thin sheet to maintain equilibrium. EFFECT OF LSP ON FATIGUE CRACK GROWTH As stated, the main application for laser peening is to extend the fatigue life of components. Laser peening was applied to 6-mm thick-middle-crack tension fatigue samples of 2524 aircraft wing-skin 50 MPa in the center of the coupon. As can be seen, there was good correlation between all three measurement techniques, which increases confidence in the measured values. In another study, laser peening was applied on both sides of 2-mmthick 2xxx series aluminum sheet[6]. The residual strain field was measured using synchrotron x-rays at the Argonne Photon Source (APS) at the Argonne National Laboratory in Lemont, Illinois. A 3D representation of the stress profile determined is shown in Fig. 3. Compressive stress of approximately 200 MPa was measured on both peen surfaces. However, a tensile balancing stress field Fig. 2 — Comparison of laser-peening-induced residual stress measured using multiple techniques. Fig. 3 — 3D contour map of residual stress field measured using synchrotron x-rays.
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