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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Figure 6b. In the last two figures, plastic deformation and polymer degradation at microscopic scale is shown, where the temperature effect would be the predominant factor for the fatigue failure (Khorasany et al., 2015, Khorasany et al., 2016).

(a)

(b) (c) Fig. 6. (a) Fracture localization under three points bending ultrasonic fatigue testing, (b) magnification at 200 µ m of a central crack, and (c) magnification at 50 µ m of the same crack.

4. Conclusions

The following conclusions can be drawn from the present work:

• Ultrasonic fatigue tests were carried out under the three points bending modality on the Nafion 115 proton exchange membrane fuel cell. • Higher testing temperature was keeping low (60° C), representing the 55% in regard the glass transition temperature of this polymeric material. • The experimental points present some dispersion; nevertheless, a clear tendency is observed for the fatigue life under this testing modality: a) fatigue life increase with the decrease of applied load and b) testing temperature icreases with the increase of applied load. • Fatigue endurance under this modality is close to half millions of cycles for 0.478 MPa of applied load; this mechanical property is close to 5.5 million of cycles when applied load decreases to 0.468 MPa. • Under external control of relative humidity (RH = 40 – 45%), crack initiation is located at the zones of high thermographic temperatures: at the center line of Nafion 115 strip (along the contact between the awl and the Nafion 115 strip), and close to clamped ends of the strip. The crack initiation and propagation under this modality of fatigue testing is induced by a combination of mechanical and thermal effects. • Initial degradation on the Nafion 115 strip is observed at the external layer of this polymeric material by tearing and material separation; further degradation is extended to interior zone with testing time.