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

用间歇超声疲劳法测定了不同强度钢的S-N曲线，并与常规方法进行了比较。这种方法的有用性作为一个快速确定的疲劳强度得到了证实。间歇地施加超声波列（脉冲模式）的原因是为了避免由于非常高的循环变形而产生的热量使试样温度升高。在10^7次循环以上的极高循环状态下，只要试样中没有出现温升，就没有观察到占空比暂停时间对疲劳性能的影响，并且通过使用30 ms占空比和70 ms暂停时间获得了以下结果。用150个试样获得了高温回火的S35 C和SNCM 439钢的P-S-N图。所得的P-S-N图与JSMS或NRIM资料中的结果有很好的相关性。此外，马氏体时效钢的S-N曲线，表现出没有持久极限，得到了千兆周制度。在10^7次循环以下断裂的试样中，裂纹总是从表面萌生，而在10^7次循环以上断裂的试样中，裂纹是从夹杂物在表面下萌生的。通过制造过程中夹杂物尺寸的改进，实现了高周疲劳性能的改善。这样的结果，通常需要几年才能得到一个传统的方法可以在几个月内获得，它表明，这种方法是一个强大的工具，为开发新材料。并成功地应用于SUJ 2轴承钢、SCr 220和SNCM 420表面硬化钢以及缺口试件。

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.