Investigating Fatigue Frequency Effects on Single Phase Ductile Materials

This paper aims at a deeper understanding of frequency effects in the fatigue response. Commercially pure polycrystalline copper was investigated. Fatigue tests were conducted at 20 kHz using an ultrasonic technique and at 100 Hz using an electromagnetic fatigue device. The specimens were designed to get the same stress amplitude gradient along the specimen axis during both types of tests. Quasi-isothermal conditions were considered. The amount of slip markings at the specimen surface was estimated by the surface area covered by slip markings. The number of cycles to failure was found much higher at 20 kHz than at 100 Hz while the amount of slip markings (for the same stress amplitude and number of cycles) was much lower. The slip markings amount increases with the number of cycles at the specimen surface. This increase is much lower at high than low frequencies. The reasons for these differences are discussed.