A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 2 2 6 maximum of 106 or 107 cycles. However, many applications require component fatigue life in excess of 107 cycles. Testing in the very high cycle regime in excess of 107 cycles has shown that failure can initiate due to internal inclusions rather than surface defects commonly observed in lower cycle testing. The basic components of an ultra- sonic fatigue system are shown in Fig. 2. A piezoelectric oscillator operating at 20 kHz is connected to a booster and amplification horn, which magnify the displacement of the oscillator. One end of the sample is connected to the horn while the other end is free to oscillate. A longitudinal wave vibration travels through the sample, which then stretches and compresses in resonance with the oscillator. For a resonating sample, maximum displacement occurs at the sample ends while maximum stress occurs at the sample midpoint, as shown in Fig. 3. The amplitude of the displacement is controlled by the power input to the piezoelectric oscillator. The load is not measured directly. System software allows stress to be calculated from sample di- mensions and displacement of the free edge of the sample, which is measured using a noncontact eddy current displacement sensor. The ultrasonic fatigue test can be designed to exert a mean stress on the sample by attaching the entire acoustic wave train to a universal testing machine (UTM) and connecting the sample to a second horn, which is fixed to the base of the UTM frame (Figs. 4 and 5). In this case, the sample displacement used to calculate the sample stress is measured using a strain gauge attached to the sample, as there is no oscillating free end to observe. The sample shape and length are chosen to allow resonance at 20 kHz. Tapered cylindrical, rod, or notched type samples are used. System software enables the correct sample length for resonance at 20 kHz to be calculated from inputs of the test material’s density and Young’s modulus. Fig. 3 — Illustration of the principles of a piezoelectric oscillator as wave vibrations move through a sample. Fig. 5 — The acoustic wave train can be mounted to a universal test machine with the sample connected to a second horn at the base of the frame to apply a mean stress. Fig. 4 — External view of the mean stress loading jig.
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