Cavitation impact induced by a cavitating jet can be utilized to improve fatigue strength in the same way as shot peening. The peening method using cavitation impact is known as "cavitation shotless peening" as shot are not required. For practical purposes, enhancement of the cavitation impact is required in order to get a better peening effect and to shorten the processing time. In the present paper, intensification of the cavitation impact was successfully realized by injecting a low-speed water jet around a cavitating jet in water, and an improvement in the fatigue strength of stainless steel was demonstrated by subjecting treated materials to a fatigue test. The fatigue strength of stainless steel was improved by about 28% by cavitation shotless peening compared with a non-peened specimen. In order to clarify the mechanism for surface enhancement by cavitation shotless peening, the residual stress on the surface was measured using an X-ray diffraction method. It was shown that cavitation shotless peening using a cavitating jet with an associated low-speed water jet in water introduced compressive residual stress on the surface of the steel. Interestingly, it was also found that the full width at half maximum of the X-ray diffraction profile from the surface decreased, even though compressive residual stress of about 500 MPa had been introduced by the cavitation shotless peening.
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