In recent years, laser peening has been attracting attention as a means of improving fatigue life. Generally, pulsed lasers from nanoseconds to femtoseconds pulsed laser are used in laser peening, but there are few reports that systematically discuss the laser irradiation conditions such as pulse duration and spot diameter. Therefore, an experimental study was conducted on the effects of the scanning pattern and the spot diameter on the application efficiency of compressive residual stress. In this study, frequency doubled Nd: YAG laser of 5ns pulse durations and 532 nm wavelength were used, and Gaussian beam was irradiated in a water chamber through a glass plate. The laser energy irradiating to the specimen was calculated by the transmitting ratio of laser beam, which was measured through the water and the glass plate. High-strength steel HT780, which is designed for large scale constructions, was used as a specimen, and 5.0 mm thickness specimens were annealed to avoid the influence of initial stress distribution. The residual stress was obtained by repeating the measurement by the X-ray diffraction method after removing the predetermined depth by electrolytic polishing.

The effect of scanning pattern on the application of compressive residual stress was investigated by two methods arranging the laser spot continuously or independently. The stability of compressive residual stress can be improved by continuously regular scanning pattern compared with that by independent scanning pattern.

Experiments by continuously regular scanning pattern were conducted by the spot diameters of 200, 400, and 600 μm. The maximum compressive stress was equivalent at the spot diameters of 200 µm and 400 µm, and the generation depth took maximum values at a spot diameter of 400 μm. Thus, it is expected that the applying effect of compressive stress can be improved by continuously regular scanning pattern of an appropriate spot size.