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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The physical and chemical properties of this polymeric material used on PEM have been studied intensively in the last years (Kumar and Khare, 2014, Amjadi et al., 2010, Bertoncello et al., 2002, Tan and Bélanger, 2005); nevertheless, their mechanical properties are not fully understood nowadays. Some investigations on this matter have been carried out in the last years: the effect of temperature and humidity on the mechanical properties of Nafion 117 (Bauer et al., 2005), mechanical properties of Nafion and Titania/Nafion membranes for fuel cells (Satterfield et al., 2006), stress–strain behavior and temperature sweeps on the Nafion membrane under hydrated conditions (Kundu et al., 2005), or the measuring of the mechanical properties of Nafion thin films during hydration cycles (Page et al., 2015). In this work is studied the mechanical behavior of the PEM Nafion 115 under ultrasonic loading and the three points bending modality. Especial attention was carried out on the environmental humidity and the increase on temperature during the ultrasonic fatigue testing. A simple equipment was designed to apply initial stress to the Nafion 115 film in the plane surface direction, and to positioning the specimen for the perpendicular ultrasonic loading.

2. Experimental procedures

2.1 Testing material and experimental set up

The principal physical and mechanical properties of Nafion 115 are listed on Table 1.

Table 1. Principal physical and mechanical properties of Nafion 115. Tensile Modulus (MPa) 50% RH, 23° C Tensile Strength, max. (MPa) 50% RH, 23° C

Elongation at Break, % 50% RH, 23° C 225 (MD) - 310 (TD)

Tear Resist. - Initial, g/mm 50% RH, 23° C

249

43 (MD)* – 32 (TD)*

6000 (MD and TD)

* MD = machine direction and TD = transverse direction.

Additional physical and mechanical properties are: specific gravity = 1.98, water content = 5 (% in weight compared to dry weight basis), thickness = 128 µ m, Poisson ratio = 0.4, glass transition temperature Tg = 110° C, basis weight (g/m 2 ) = 250, tear resistance - propagating (g/mm) >100 (MD), >150 (TD) (50% RH, 23° C, thickness used = 50 µ m). The dimensions of testing specimen are shown in Figure 1a, it is a rectangular strip with the dimensions: 10 x 50 mm. In order to carry out ultrasonic fatigue testing under the modality of three points bending, an awl of aluminum alloy 7075 was designed to apply ultrasonic vibration at the center of the Nafion specimen, as illustrated in the Figure 1b.

Awl

(a) (b) Fig. 1. (a) Dimensions (mm) of Nafion 115 testing specimen, and (b) Set up for the ultrasonic fatigue tests under three points bending.