PSI - Issue 3

574 4 G.M. Domínguez Almaraz et al. / Procedia Structural Integrity 3 (2017) 571–578 Author name / Structural Integrity Procedia 00 (2017) 000–000 machine. In Figure 3a is shown the numerical simulation for the maximum displacement of 100 µ m at the middle of the strip induced by the awl; this displacement generates a maximum Von Mises stress on the strip close to 68 KPa, Figure 3b. The highest stress in Figure 3b is located close to the clamped sides of the strip; this location corresponds to the high stress for a clamped plate (membrane), undergoing a central load which is perpendicular to the free surface (Boresi and Schmidt, 2002).

(a)

(b) Fig. 3. (a) Numerical simulation of displacement on the strip induced by the awl, (b) Numerical stresses along the strip, induced by the displacement of 100 µ m of the awl. From the position of 0 gr. of the tension self-fabricated equipment (equivalent to 0.46 MPa, approximately), ultrasonic vibration is applied inducing displacements of: 13, 14, 15, 16, 18, 19, 20, 21, 22.5, 24 and 25 µ m at the end of the awl (corresponding to same displacement at the middle of the Nafion strip). In Table 2 are listed the ultrasonic fatigue tests (the average of 3 tests for each displacement), carried out under la modality of three points bending: all testing temperatures ranging from 30 to 60° C, the latest value representing 55% of the glass transition temperature for this polymer (Satterfield et al., 2006). In order to keep testing temperature along the Nafion strip below the glass transition temperature of this polymer, dry cooling air was injected; then, the thermomechanical effect is assumed during the ultrasonic fatigue testing under this condition (Khorasany et al. 2015, Xia et al., 2016). It is deduced from the experimental results in Table 2, that the ultrasonic fatigue endurance of Nafion 115 under the described modality ranges from the half millions of cycles for the lowest fatigue life, to 5.5 million of cycles for the longest fatigue life. Furthermore, testing temperature was keeping low in regard the glass transition temperature of this polymer: the high value attains only 60° C. The experimental results are somewhat scattered; nevertheless, some general tendencies can