Fracture Properties of Catalyst Coated Membranes
Notice bibliographique
Résumé
Recent studies have shown that a suitable approach for analyzing the mechanical durability of fuel cell stacks is to study the mechanical behavior of catalyst coated membranes (CCMs) under hygrothermal loading conditions [1–4]. Previous studies have also indicated that under cyclic mechanical and hygrothermal loadings, cracks are initiated and propagated inside the membrane [5,6] eventually leading to the cell/stack failure. The crack propagation rate of pure membranes is found to be a strong function of applied stress, temperature, and humidity [7]. Given the significant difference in mechanical properties of pure membrane and CCM [1], their crack propagation characteristics can also be expected to differ. In an operating fuel cell, this implies that a crack could potentially propagate differently when it has penetrated through the entire CCM compared to when it is confined within the membrane. To investigate this, a series of experiments were conducted to characterize the rate of crack propagation in the CCM. Rectangular specimens with a width of 10 mm and artificially created double edge cracks were used. The initial crack length on each side was approximately 0.7 mm. Under pre specified levels of environmental conditions (temperature and relative humidity), the specimens were subjected to cyclic mechanical loading [3–6] which led to crack propagation as shown in Fig. 1. The rate of crack propagation as a function of loading and environmental conditions was measured and analyzed. It is found that at elevated levels of relative humidity and temperature, the sensitivity of propagation rate to the applied mechanical loading increases. In a parallel study, a fracture mechanics model based on Paris Law theory and capable of simulating the ex situ crack propagation in the CCM during typical fuel cell operating conditions is developed. The model incorporates the characteristic time, temperature, and humidity dependent elastic-viscoplastic mechanical behaviour of CCMs [1] through a sub model developed using the finite element method (FEM) in COMSOL Multiphysics® . The stress-strain relationship of CCM simulated by the FEM sub model is validated at all combinations of 23 ºC and 70ºC temperature, 50% and 90% relative humidity; and 0.0001 s-1 and 0.001 s-1 strain rates. Fundamental fracture mechanics parameters, viz. J-integral, stress intensity factor (K), and configuration correction factor (ccf) are obtained iteratively for incremental changes in the crack length. These parameters together with the experimental crack propagation data enable the construction of Paris Curves at various temperature and humidity conditions. Information from the Paris Curves is used to predict the time taken by a CCM crack to increase from initial crack length ai to final crack length af under typical fuel cell conditions. The CCM crack propagation data collected and simulation capability developed during this work are considered to be important contributions towards developing a holistic understanding of mechanical fatigue and fracture phenomenon which are active during fuel cell operation and which ultimately lead to its failure. Acknowledgements: This research is supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada (APC) grant. References [1] M.A. Goulet, R.M.H. Khorasany, C. De Torres, M. Lauritzen, E. Kjeang, G.G. Wang, et al., J.of Power Sources. 234 (2013) 38–47 [2] R.M.H. Khorasany, M.-A. Goulet, A. Sadeghi Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources. 252 (2014) 176–188. [3] A. Sadeghi Alavijeh, M.-A. Goulet, R. Khorsany, J. Ghataurah, C. Lim, M. Lauritzen, et al., Fuel Cells. (2015) 204–213. [4] A. Sadeghi Alavijeh, R.M.H. Khorasany, A. Habisch, G.G. Wang, E. Kjeang, J. Power Sources. 285 (2015) 16–28. [5] R.M.H. Khorasany, A. Sadeghi Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources. 274 (2015) 1208–1216. [6] R.M.H. Khorasany, A. Sadhegi, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources. 279 (2015) 55–63. [7] Y. Singh, R.M.H. Khorasany, A. Alavijeh, E. Kjeang, G. Wang, R.K.N.D. Rajapakse, Fracture Properties of Fuel Cell Membranes, in: 226th Meet. Electrochem. Soc., Cancun, 2014. Figure 1
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Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,001 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,001 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,002 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».