PSI - Issue 3

G.M. Domínguez Almaraz et al. / Procedia Structural Integrity 3 (2017) 571–578 Author name / Structural Integrity Procedia 00 (2017) 000–000

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2.2 Numerical simulations

In order to induce ultrasonic vibration along the awl, a finite element modal analysis was carried out to obtain the natural longitudinal vibration of this element close to the frequency of excitation source: 20 KHz. This numerical simulation was carried out with the purpose to fit the resonance conditions, which imply: a stationary elastic wave along the awl inducing high displacement at the ends (Dominguez Almaraz, 2015). In Figure 2a is shown the result of this modal simulation for the awl (aluminum alloy 7075); the obtained natural frequency of vibration in longitudinal direction was: 20060 Hz. Furthermore, the Nafion 115 strip was clamped at the two ends by a selfdesigned equipment, which allows to apply a tension stress by a tension system, Figure 2b.

(a)

(b)

Fig. 2. (a) Modal analysis result for the natural vibration of the awl in longitudinal direction, (b) Self-fabricated equipment to apply tension stress to Nafion 115 strip.

All tests were carried out at room temperature (20 - 23° C), with environmental humidity comprised between 40 – 45%, and with initial tension of 60 gr. (equivalent stress on the Nafion 115 strip close to 0.46 MPa). The sequence for the test starting was as follows: applying a tension of 60 gr. on the Nafion 115 strip by the tension designed equipment; then, the awl is approached at the middle section of the strip and pressure is exerted at this zone until the applied tension by the self-designed equipment indicates zero in the display (the original tension by the selfdesigned equipment is relaxed at the same time that the awl induces concentrated stress on the Nafion strip). All ultrasonic fatigue tests on Nafion 115 have followed the described sequence and after the last step, ultrasonic vibration of different amplitude is communicated to the strip by the awl, which is under resonance with the ultrasonic fatigue