Fracture Properties of Catalyst Coated Membranes
Bibliographic record
Abstract
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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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".