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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electrical consumption, less heat generation, no noise, high cleanliness and almost no wear neither friction. The principal drawback is the limitation in load capacity, in which the most powerful machines goes up to 10 kN (with more than one linear motor in tandem), being much less than what is possible to achieve with hydraulics, but enough to test small samples for most engineering materials (M. Freitas et al. 2014). The combined axial–torsion test is a popular biaxial testing system, used for example, to simulate the stress states of shafts and so on. This is in most of the cases an accessory that can be included in single axis servo-hydraulic test machines. However, only some stress states can be simulated by this test type, (Manuel de Freitas 2017). It means that the biaxial tension stress state and the biaxial compression stress state cannot be simulated by the combined axial– torsion test system. Another disadvantage of the first method is that it requires the material to be in the form of a circular tube, being difficult to be applied to rolled sheet materials and most of the composite shapes, therefore cruciform specimens must be used. 2.2. Ultrasonic fatigue testing at Very High Number of Cycles Ultrasonic fatigue tests associated with improvements in piezoelectric devices, made these fatigue testing methods an attractive technique to establish S-N curves in VHCF. The piezoelectric technique started with Hopkinson in the beginning of the 20th century, and fifty years later Mason presents the first ultrasonic fatigue machine working at 20 kHz, establishing the frequency testing norm (Claude Bathias and Paris 2005). Other machines operating at higher frequencies were presented but the difficulties to correlate results and experiments slowed down the development of this technique. Nowadays (Lage et al. 2014), with the development of new sensors, new computational methods and faster and efficient control systems, some issues in VHCF were overcome.

Figure 1. Resonant system components

The resonant system is composed by a piezoelectric actuator, a booster, a horn and the specimen sequentially assembled together in series by screw connections. These four elements form the resonant system of the testing