Quick estimating method of giga-cycle fatigue properties by an intermittent ultrasonic loading

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S-N curves of the materials with various strength steels were determined up to the giga-cycle regime by an intermittent ultrasonic fatigue method, and the results were compared to those obtained by a conventional method. The usefulness of this method as a quick determination of the fatigue strengths was confirmed. The reason of applying ultrasonic wave train intermittently (pulsed mode) was to avoid temperature raise of the specimen by heat generation due to very high cyclic deformation. No effect of the duty-pause period on the fatigue properties in the very high cycle regime above 10^7 cycles was observed as long as no heat raise occurs in the specimen, and the following results were obtained by using 30 ms duty period and 70 ms pause period. P-S-N diagrams up to the giga-cycle regime were obtained on the S35C and SNCM439 tempered at high temperature by using 150 test pieces. Thus obtained P-S-N diagrams show quite good correlation with the results in the data book by JSMS or NRIM. Furthermore, the S-N curves of the maraging steels, which exhibit no endurance limit, were obtained up to the giga-cycle regime. In the specimens fractured below 10^7 cycles, crack initiation was always from the surface, while it was at subsurface initiation from the inclusions in the specimens survived more than 10^7 cycles. Improvement of the high cycle fatigue properties was achieved by reforming the inclusion size through manufacturing process. Such results that usually take a few years to obtain by a conventional method can be obtained within a few months and it is shown that this method is a powerful tool for developing new materials. Success applications were also made to the bearing steel of SUJ2 and the case hardening steels of SCr220 and SNCM 420 and to the notched specimen.