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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3. Results and discussion

3.1 Ultrasonic fatigue results

On Figure 5 are plotted the ultrasonic fatigue endurance of Nafion 115 corresponding to experimental results listed on Table 2. A clear tendency is observed on the S-N graph showing that fatigue endurance increase with the decrease of applied load. The logarithmic regression line is obtained from the experimental points and the corresponding equation is shown in the graph. In the last equation: N is the number of cycles of fatigue life and σ is the high applied stress on the Nafion 115 strip. The high stress applied to the strip was evaluated from the stress value of 0.46 MPa, adding the displacement induced by the ultrasonic vibration on the strip surface.

Fig. 5. Ultrasonic fatigue endurance of the Nafion 115 proton exchange membrane, under three points bending.

Experimental points plotted in Figure 5 show that fatigue life under the described fatigue modality is close to half millions of cycles when the applied load is near to 0.478 MPa; whereas the fatigue life increases to 5.5 million of cycles with the reduction of applied load to 0.468 MPa.

3.2 Fracture surfaces under three points ultrasonic fatigue testing

The fracture localization under three points ultrasonic fatigue loading for specimen 18 on Table 2, are shown in Figure 6a, 6b and 6c. The Figure 6a reveals that fracture occurs at the center line of the Nafion 115 strip, where awl was applied, and at one side of this center line. The Figures 6b and 6c show the amplification of a center crack observed on Figure 6a: the mechanism of damage under this modality of fatigue tests appear a combination of thermal and mechanical effects (Bhadra et al., 2010, Alavijeh et al., 2016). Furthermore, considering that tensile strength of Nafion 115 is 32-43 MPa, fatigue life under three points ultrasonic tests attaining the half millions of cycles, requires applied load of only 0.48 MPa, about the 1/100 of tensile strength for this membrane (Tang et al., 2007). Concerning the images of fracture surfaces, small bubbles are observed at the surface by the combination of mechanical and thermal loading, which coalescence during the experimental tests leading to pin-holes. The degradation of this polymer during the ultrasonic fatigue testing is clearly observed on the Figures 6b and 6c: tearing and material separation at the external layer of Nafion 115; whereas the internal zone is damaged as it is depicted in