PSI - Issue 42

Rita Dantas et al. / Procedia Structural Integrity 42 (2022) 1676–1683 Rita Dantas / Structural Integrity Procedia 00 (2019) 000–000

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2. Ultrasonic Fatigue Testing

The conventional fatigue testing machines operate at low frequencies, which results in very time consuming ex perimental campaigns and in the impossibility of testing beyond 10 7 cycles. For example, in the case of a rotating bending machine operating at a maximum frequency of 30 Hz to achieve 10 9 or 10 10 would be necessary years to test a single specimen, while in an ultrasonic machine, working without interruptions, a couple of days would be enough (Costa et al., 2020) (Anes et al., 2011). Thus, due to these di ff erences in the operating times and technologi cal advances observed in piezoelectric devices, the ultrasonic testing machine interest and popularity increased. The testing frequency of 20 kHz for ultrasonic testing machines has became almost a standard, since higher frequencies of testing rise some problems related to the correlation of results, overheating or the need of even smaller specimens to accomplish a certain frequency of resonance (Bathias and Paris, 2004) (Costa et al., 2020). However, the fatigue tests performed in this kind of machine are limited to the elastic region and to a range of displacement amplitude, and consequently the stresses present in the middle section, that need to be considered during the specimen design. Furthermore, a more conventional ultrasonic machine can only perform fatigue tests under a stress ratio of -1 and under uniaxial loading. Nevertheless, during the last decades some authors, such as Costa et al. (2020) and Mora (2010), developed new ultrasonic machines that can apply multiaxial loading cases and stress ratios di ff erent than -1. Regarding the working principle, an ultrasonic fatigue testing machine induces the specimen to vibrate in resonance at its first longitudinal free mode of vibration and usually includes the following components (see Fig. 1): • a piezoelectric transducer, which transforms the 20 kHz electrical signal from the generator into a longitudinal mechanical vibration of equal frequency. Thus, at the end of piezoelectric actuator can be measured a mechani cal wave of 20 kHz with a certain amplitude of displacement; • a booster, which gives a structural support to the machine and amplifies the maximum displacement of the mechanical wave generated by the piezoelectric; • a horn, which works as a second amplification element and applies the mechanical vibration to the specimen by a threaded connection. Besides these main components, a computer, a control system and a data acquisition device are also required elements in the setup of this machine. Another important aspect is the temperature monitoring, which is accomplished by a measuring device (e.g. pyrometer), and the air cooling system to ensure that the specimen does not overheat during the fatigue testing, due to the high frequency (Ilie et al., 2020). • a signal generator, which transforms the voltage signal of 50 / 60 Hz into an ultrasonic electrical sinusoidal signal of 20 kHz ;

Fig. 1. Ultrasonic fatigue testing setup