Laser Peening System Research Articles

Investigations for improving fatigue strength by low energy laser peening via bending fatigue test

Laser peening can introduce compressive residual stresses on the surface of various materials, thereby effectively prolonging their fatigue lives. In this study, the effects of laser peening with pulse energies of 20 and 70 mJ on residual stress and fatigue life were investigated on two types of HT780 box-welded specimens of different sizes, considering the realization of a portable laser peening system equipped with a small laser device with low pulse energy. As a result of such low pulse energy laser peening, it was found that the depth of compressive residual stress becomes shallower compared to the current laser peening with a pulse energy of 200 mJ, while the residual stress on the surface remains at the same level. Bending fatigue tests were conducted with the stress ranges at 100 and 150 MPa, which revealed that the fatigue life of the specimens with low pulse energy laser peening was at least 50 times and eight times longer than that of the as-welded specimens, respectively. Comparing the fatigue test results of different size specimens, it is clear that laser peening with low pulse energy is effective in extending the fatigue life of HT780 box-welded specimen regardless of its size.

Comparison between the improvements made to the fatigue strength of stainless steel by cavitation peening, water jet peening, shot peening and laser peening

In order to make a clear comparison between the various peening methods used to improve the fatigue strength of stainless steel 316L, the fatigue strength of specimens treated by cavitation peening, water jet peening, laser peening and shot peening were examined using a plate bending fatigue test. Both cavitation peening and water jet peening were carried out using a submerged water jet system, and these are distinguished by the distance from the nozzle to the specimen. A submerged laser peening system was used for laser peening. The impact with the surface due to laser cavitation was greater than that due to laser ablation. For each peening method, the optimum coverage was examined by measuring the fatigue life at constant bending stress. Then, the fatigue strength of specimens treated with the optimum coverage was examined. The fatigue strength of the non-peened specimen was 279 MPa, whereas it was 348 MPa for cavitation peening, 325 MPa for shot peening, 303 MPa for laser peening and 296 MPa for water jet peening. The fatigue strength of each peening method was affected by the surface roughness and work hardening.

Effect of Overlapping of Peen Spots on Residual Stresses in Laser-Peened Aluminium Sheets

In this study, two different laser peening systems were employed to investigate the effect of overlapping on the resulting residual stress fields. For each laser peening system, 2.0-mm-thick aluminium sheets were laser peened with different amounts of overlapping. The residual stresses after laser peening were measured by incremental hole drilling. It was shown for both laser peening systems, that an increase in amount of overlapping induced higher magnitude compressive and deeper residual stress fields.

비등수형 원자로 발전소에의 레이저 피닝 적용기술

Toshiba has developed a laser peening system for PWRs(pressurized water reactors) as well after the one for BWRs(boiling water reactors), and applied it for BMI(bottom-mounted instrumentation) nozzles, core deluge line nozzles and primary water inlet nozzles of Ikata Unit 1 and 2 of Shikoku Electric Power Company since 2004, which are Japanese operating PWR power plants. Laser pulses were delivered through twin optical fibers and irradiated on two portions in parallel to reduce operation time. For BMI nozzles, we developed a tiny irradiation head for small tubes and we peened the inner surface around J-groove welds after laser ultrasonic testing (LUT) as the remote inspection, and we peened the outer surface and the weld for Ikata Unit 2 supplementary. For core deluge line nozzles and primary water inlet nozzles, we peened the inner surface of the dissimilar metal welding, which is of nickel base alloy, joining a safe end and a low alloy metal nozzle. In this paper, the development and the actual application of the laser peening system for PWR power plants will be described. Key words: Fiber delivered laser peening, Pressurized water reactors, Primary water stress corrosion cracking, Reactor vessel, Laser ultrasonic testing