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Research Papers

Stress-Strain Behavior of Perfluorosulfonic Acid Membranes at Various Temperatures and Humidities: Experiments and Phenomenological Modeling

[+] Author and Article Information
Ahmet Kusoglu, Yaliang Tang, Anette M. Karlsson

Department of Mechanical Engineering, University of Delaware, Newark, DE 19716

Michael H. Santare1

Department of Mechanical Engineering, University of Delaware, Newark, DE 19716santare@udel.edu

Simon Cleghorn, William B. Johnson

 Gore Fuel Cell Technologies, 201 Airport Road, P.O. Box 1488, Elkton, MD 21922-1488

Nafion® is a registered trademark of EI DuPont De Nemours and Company.

DuPont™ is a registered trademark of EI du Pont de Nemours and Company.

The use of an extensometer to calibrate the crosshead displacement showed consistent measurements between the crosshead displacement and the material deformation. Therefore during tensile testing the crosshead displacement was taken as the actual displacement in the specimen.

The engineering stress is the measured force divided by the original cross sectional area, the engineering strain is the change in length divided by the original length or the gauge length. For the specimens tested, the cross sectional area is determined from the width and thickness of the membrane.

1

Corresponding author.

J. Fuel Cell Sci. Technol 6(1), 011012 (Nov 07, 2008) (8 pages) doi:10.1115/1.2971069 History: Received May 03, 2007; Revised October 19, 2007; Published November 07, 2008

The constitutive response of perfluorinated sulfonic acid (PFSA) membranes based on tensile testing is investigated, and a phenomenological constitutive model for the elastoplastic flow behavior as a function of temperature and humidity is proposed. To this end, the G’Sell–Jonas (1979, “Determination of the Plastic Behavior of Solid Polymers at Constant True Strain Rate  ,” J. Mater. Sci., 14, pp. 583–591) constitutive model for semicrystalline polymers is extended by incorporating, in addition to temperature, relationships between the material constants of this model and the measured relative humidity. By matching the proposed constitutive model to the experimental stress-strain data, useful material constants are found. Furthermore, correlations between these material constants and Young’s modulus and proportional limit stress are investigated. The influence of material orientation, inherited from processing conditions, on the stress-strain behavior is also studied. The proposed model can be used to approximate the mechanical behavior of PFSA membranes in numerical simulations of a fuel cell operation.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Chemical structure of the Nafion® membrane

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Figure 2

Deformation of the Nafion® 112 membrane during a uniaxial tension test conducted at 25°C and 50% relative humidity, shown with grids, for three strain values: (a) ε=0 (initial state), (b) ε=0.3, and (c) ε=0.6

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Figure 3

The experimental true stress-strain data (markers) obtained for the machine direction, plotted with the proposed constitutive model (solid lines) at four temperature values for (a) 30%, (b) 50%, (c) 70%, and (d) 90% relative humidities

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Figure 4

The experimental true stress-strain data (markers) obtained for the transverse direction plotted with the constitutive model (solid lines) at four temperatures for (a) 30%, (b) 50%, (c) 70%, and (d) 90% relative humidities

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Figure 5

Contribution of the terms in the constitutive model (Eq. 3) to the stress-strain behavior of the material. In the plot, the following values have been used: K=7.53, W=41, and h=2.3.

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Figure 6

The material constant, K (Table 1), and the proportional limit, σp(23), plotted at each temperature and relative humidity for both transverse and machine directions. The slope of the fitted line gives the constant C1.

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Figure 7

The values of Wσp and E(23) are plotted at each temperature and relative humidity. The slope of the fitted line represents the constant C2.

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