PSI - Issue 25

Pedro R. da Costa et al. / Procedia Structural Integrity 25 (2020) 445–453 Author name / Structural Integrity Procedia 00 (2019) 000–000

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machine, Figure 1. The mechanical vibration generated by the piezoelectric actuator is meant to reproduce a pure sine wave with a frequency of approximately 20 kHz. This wave is transmitted from element to element down to the bottom of the specimen with the displacement and stresses schematically shown on Figure 2. The principle of operation of the vibration system is based on free vibration resulting in a minimum of force contact between the elements.

Figure 2. Stress and displacements in the specimen

2.3. Multiaxial Tension/Torsion Ultrasonic Fatigue at Very High Number of Cycles In the region of very high cycles, research and development under multiaxial fatigue has not been carried out, mainly because of the inexistence of appropriate machinery to perform these tests in a reasonable time and cost.

Figure 3. Horn and specimen for tension/torsion testing (P. Costa et al. 2017)

In recent work (Vieira, De Freitas, et al. 2016), a device designed to produce axial/torsional loading fatigue testing using a single piezoelectric axial actuator, a specific horn and a new specimen has been proposed as shown in Figure 3 following the automation and ultrasonic methodology as described in (Lage et al. 2014). The horn is designed in order to transform the axial displacement in both axial and rotational displacement to the specimen. The specimen design follows specific rules, Figure 4, and is composed of three throats being the central one the main throat where maximum tension/torsion is applied (P. Costa et al. 2017) and where fatigue failure is expected. In this research, several tension/torsion specimens were machined from a railway wheel’s rim following the research (Soares et al. 2018). The specimens were designed so to achieve a stress relation close to the von Mises relation of 0,57. When testing specimens in ultrasonic fatigue machines the specimen does not achieve complete failure. It is required to take the specimens afterwards to another machine for the complete failure with the purpose to have the fatigue fracture unveiled. Such procedure is required because in such tests the specimen works in resonance, and with the growth of the fatigue crack, leading to loss of stiffness, it will be no longer possible to correctly excite the specimen in resonance. Such procedure is required in all ultrasonic specimens, uniaxial and multiaxial ones. The obtained ultrasonic multiaxial fractures were studied using a scanning electron microscope (SEM) and compared to ultrasonic uniaxial specimens.