Research Papers - Department of Civil Engineering
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Publication Embargo Ex situ characterization and modelling of fatigue crack propagation in catalyst coated membrane composites for fuel cell applications(Pergamon, 2019-05-03) Singh, Y; Khorasany, R. M. H; Kim, W. H. J; Alavijeh, A. S; Kjeang, E; Rajapakse, R. K. N. D; Wang, G. GInteractions between catalyst layers and membrane are known to influence the mechanical properties of catalyst coated membrane (CCM) composites used in fuel cells, and can further affect their fatigue-driven mechanical fracture — an important lifetime-limiting failure mode in automotive applications. Here, the fracture propagation phenomenon in CCMs is characterized through a series of ex situ experiments and microstructural investigations conducted across a range of stress, temperature (23-70 °C), and relative humidity (50–90%) conditions relevant to low-temperature polymer electrolyte fuel cells. In comparison to pure membranes, the crack propagation rates are slightly arrested in CCMs through mechanical reinforcement offered by the catalyst layers; however, the membrane layer still controls the overall crack growth trends through its temperature and humidity dependent ductile fracture characterized by confined yielding around the fracture surface. Local interfacial delamination and severe electrode cracking are found to accompany the CCM crack propagation, which aids membrane fracture by loss of local reinforcement. A Paris law based fracture modelling framework, incorporating the elastic-viscoplastic mechanical response of CCMs, is developed to semi-analytically evaluate one-dimensional crack growth rate during cyclic loading, and provides reasonably accurate predictions for the present ex situ problem.Publication Embargo Ex situ measurement and modelling of crack propagation in fuel cell membranes under mechanical fatigue loading(Pergamon, 2017-07-27) Singh, Y; Khorasany, R. M. H; Alavijeh, A. S; Kjeang, E; Wang, G. G; Rajapakse, R. K. N. DFatigue-induced membrane fracture due to dynamic stresses is an important lifetime-limiting failure mode in automotive fuel cell applications. Here, a series of ex situ experiments are first conducted to measure the rate of crack growth in Nafion NRE211 membranes for a range of stress, temperature (23–70 °C), and relative humidity (50–90%) conditions relevant to automotive fuel cell operation. The crack growth rate is found to be ∼1–10 nm per load cycle and strongly depends on the stress intensity: the rate increases by an order of magnitude for a mere 10–30% increase in stress, which suggests that improved stress uniformity and avoidance of high stress points is important for durability. Moreover, the sensitivity to applied stress doubles from room conditions to fuel cell conditions, where the temperature has 2–3x stronger impact on the fracture propagation than the relative humidity. Microstructural analysis indicates that plastic deformation (60% localized thinning) at the crack tip accompanies crack growth. A semi-analytical model based on Paris law is then developed to simulate crack growth as a function of cyclic loading. The model incorporates elastic-viscoplastic mechanical behaviour of ionomer membranes and provides crack growth predictions in agreement with ex situ data up to 100% strain.Publication Embargo Fatigue properties of catalyst coated membranes for fuel cells: Ex-situ measurements supported by numerical simulations(Pergamon, 2016-06-08) Khorasany, R. M. H; Singh, Y; Alavijeh, A. S; Kjeang, E; Wang, G. G; Rajapakse, R. K. N. DThe interactions between catalyst layers and membrane are known to have significant impact on the mechanical properties of the composite catalyst coated membrane (CCM) materials used in fuel cells. The mechanical fatigue durability of such composite CCM materials is investigated herein, and compared to the characteristics of pure membranes. Ex-situ uniaxial cyclic tension tests are conducted under controlled environmental conditions to measure the fatigue lifetime, defined by the number of stress cycles that the specimen can withstand before mechanical failure. The sensitivity of the CCM fatigue lifetime to the applied stress is determined to be higher than that of the pure membrane, and varies significantly with environmental conditions. The experimental results are then utilized to develop a finite element based CCM fatigue model featuring an elastic–plastic constitutive relation with strain hardening. Upon validation, the model is used to simulate the fatigue durability of the CCM under cyclic variations in temperature and relative humidity, which is critical for fuel cells but cannot be effectively measured ex-situ. When combined, the experimental and numerical methods demonstrated in this work provide a novel, convenient approach to determine the CCM fatigue durability under various hygrothermal loading conditions of relevance for fuel cell design and operation.Publication Embargo On the constitutive relations for catalyst coated membrane applied to in-situ fuel cell modeling(Elsevier, 2014-04-15) Khorasany, R. M. H; Goulet, M. A; Alavijeh, A. S; Kjeang, E; Wang, G. G; Rajapakse, R. K. N. DThe elastic–viscoplastic behavior of catalyst coated membranes (CCMs) used in polymer electrolyte membrane fuel cells is investigated in this work. Experimental results reveal significant differences between the mechanical properties of a pure perfluorosulfonic acid ionomer membrane and the corresponding CCM under uniaxial tension and cyclic loading. An elastic–viscoplastic constitutive model that is capable of capturing the time dependent response of the CCM at different humidity and temperature conditions is developed and validated against ex-situ experimental results. The validated model is then utilized to simulate the in-situ mechanical response of the CCM when treated as a composite object bonded through the ionomer phase. When compared to a conventional membrane model, the CCM model predicts considerably lower maximum stress and higher plastic strain under typical fuel cell operating conditions and improved plastic strain recovery during hygrothermal unloading. These results reflect the weaker nature of the CCM material which yields at a lower stress than the membrane and may lead to elevated plastic deformation when exposed to hygrothermal cycles in a constrained fuel cell environment. Hence, coupled CCM implementation is generally recommended for finite element modeling of fuel cells.Publication Embargo Decay in Mechanical Properties of Catalyst Coated Membranes Subjected to Combined Chemical and Mechanical Membrane Degradation(Wily, 2014-11-28) Rajapakse, R. K. N. D; Wang, G. G; Lauritzen, M; Kjeang, E; Lim, C; Ghataurah, J; Khorasany, R. M. H; Goulet, M. A; Alavijeh, A. SThe mechanical stability of catalyst coated membranes (CCMs) is an important factor for the overall durability and lifetime of polymer electrolyte fuel cells. In this article, the evolution of the mechanical properties of degraded CCMs is comprehensively assessed. A combined chemical and mechanical accelerated stress test (AST) was applied to simulate field operation and rapidly generate partially degraded CCM samples for tensile and expansion experiments under both room and fuel cell conditions. The tensile results indicated significant reductions in ultimate tensile strength, toughness, and fracture strain as a function of AST cycles, accompanied by a mild increase in elastic modulus. The increased brittleness and reduced fracture toughness of the CCM, caused primarily by chemical membrane degradation, is expected to play an important role in the ultimate failure of the fuel cell. The expansion tests revealed a linear decay in hygrothermal expansion, similar in magnitude to the loss of mechanical strength. The decline in CCM sensitivity to environmental changes leads to non-uniform swelling and contraction that may exacerbate local degradation. Interestingly, the hygrothermal expansion in the late stages of degradation coincided with the fracture strain, which correlates to in situ development of fractures in chemically weakened membranes.Publication Embargo Simulation of ionomer membrane fatigue under mechanical and hygrothermal loading conditions(Elsevier, 2015-04-01) Khorasany, R. M. H; Kjeang, E; Wang, G. G; Rajapakse, R. K. N. DUnderstanding the fatigue lifetime of common perfluorosulfonic acid (PFSA) ionomer membranes under fluctuating hygrothermal conditions is essential for the development of durable fuel cell technologies. For this purpose, a finite element based fatigue lifetime prediction model is developed based on an elastic–plastic constitutive model combined with a Smith-Watson-Topper (SWT) fatigue formulation. The model is validated against previously reported experimental results for a membrane under cyclic mechanical loadings. The validated model is then utilized to investigate the membrane fatigue lifetime in ex-situ applications under cyclic humidity and temperature conditions. The simulations suggest that the membrane fatigue lifetime is shorter under fluctuating humidity loadings than for temperature loadings. Additionally, the membrane fatigue lifetime is found to be more sensitive to the amplitude of the strain oscillations than to the mean strain under hygrothermal cycling. Most notably, the model predicts that simultaneous humidity and temperature cycling can exacerbate the fatigue process and reduce the fatigue lifetime by several orders of magnitude compared to isolated humidity or temperature cycling. The combination of measured mechanical fatigue data and the present numerical model provides a useful toolkit for analysis of membrane fatigue due to hygrothermal variations, which can be costly and time-consuming when addressed experimentally.