ADVANCED MATERIALS & PROCESSES | APRIL 2025 22 material[7]. Peening was applied using a power density of 3 GW/cm2 in patches 30-mm wide that were located 25 mm from the central starter notch as shown in Fig. 4. The patches were peened on both faces of the sample and on both sides of the starter notch. Compared to the fatigue life of baseline unpeened samples (≈38000 cycles), the fatigue life of peened samples increased by a factor of 4 to approximately 148,000 cycles. As can be seen in Fig. 4, the growth rate of the fatigue crack reduced significantly as soon as the crack entered the peened patch. Also notable, the fatigue crack in the peened sample actually grew faster prior to entering the peened patch. This was due to tensile balancing stresses located outside the peened patches. Residual stresses are known to alter the trajectory of cracks in metallic structures. By coupling the versatile processing ability and unique aniso- tropic residual stress fields generated by LSP, structural crack trajectory modification can potentially be achieved with LSP-induced residual stress[8]. Figure 5a shows a fatigue crack propagating from a starter notch in a middle- crack tension sample. The fatigue crack is perpendicular to the applied loading direction with some minor wobbling of the crack trajectory, which is normal for fatigue crack growth. Figure 5b shows the trajectory of a fatigue crack as it propagated towards an LSP patch angled at 45° to the applied loading direction. The fatigue crack initially propagates at a trajectory of about 20° to the normal before growing perpendicular to the applied loading direction once it enters the peened region. This LSP application may result in a possible solution for reduction in the size of the fuselage crack arrestors, increasing the performance of the structure. FUTURE DIRECTIONS Laser shock peening is an effective and novel method of surface engineering, that has demonstrated significant impact in safety-critical applications. There has been much research into characterization of the residual stress fields produced by LSP, as it is the residual stress that predominantly provides the benefits for fatigue and corrosion resistance. With careful application of existing residual stress methods, the near- and sub-surface residual stresses can be determined with good accuracy, particularly if multiple methods are deployed. For deployment of LSP more widely in diverse industrial sectors, develop- ments are required to reduce cost and increase flexibility. Work in Japan on miniaturized laser systems is likely to be one part of the solution, alongside improved equipment for delivery of the beam to the component using fiber waveguides. Practically, water as a confinement medium is problematic on many manufacturing or assembly lines, and there is a need for development of novel plasma confinement methods. At the same time, improved modeling methods will give increased confidence in the application of the Fig. 4 — Effect of peening on fatigue crack growth rate. Fig. 5 — Fatigue crack trajectory in (a) baseline unpeened sample and (b) deviation due to laser shock peening patch at 45°. (a) (b)
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