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

采用间歇超声疲劳法测定了不同强度钢材料在千兆周范围内的S-N曲线，并与常规方法进行了比较。验证了该方法快速测定疲劳强度的有效性。间歇式（脉冲式）施加超声波列的原因是为了避免试样因非常高的循环变形而产生的热量使温度升高。在10^7次循环以上的高循环状态下，只要试样不发生升温，则空载暂停时间对疲劳性能没有影响，并且使用30ms空载暂停时间和70ms空载暂停时间获得了以下结果。用150个试样对高温回火的S35C和SNCM439试样进行了高温回火处理，得到了高达千兆循环的P-S-N图。由此得到的P-S-N图与JSMS或NRIM数据手册中的结果有很好的相关性。此外，马氏体时效钢的S-N曲线在千兆循环状态下没有表现出持久极限。在10^7次循环以下的试样中，裂纹总是从表面起裂，而在10^7次循环以上的试样中，裂纹是由夹杂物在地下起裂。通过加工工艺改造夹杂物尺寸，提高了合金的高周疲劳性能。传统方法通常需要几年才能获得的结果可以在几个月内获得，这表明该方法是开发新材料的有力工具。在SUJ2轴承钢、sc220和SNCM 420淬火钢以及缺口试样上也进行了成功的应用。

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.