Browsing by Author "Singh, Y"

Now showing 1 - 6 of 6

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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. G
Interactions 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.
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. D
Fatigue-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.
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. D
The 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.
Embargo
In-situ simulation of membrane fatigue in polymer electrolyte fuel cells
(Pergamon, 2017-04-20) Khorasany, R. M. H; Singh, Y; Alavijeh, A. S; Rajapakse, R. K. N. D; Kjeang, E
Estimation of membrane fatigue lifetime under in-situ conditions involving cyclic hygrothermal stress is of particular interest in fuel cell durability research; however, conducting experiments to study the in-situ fatigue process within membranes is often expensive and in many cases, infeasible. Here, an in-situ numerical fatigue model based on the Smith-Watson-Topper (SWT) criterion is presented and validated against experimental results of membrane fatigue lifetime under humidity cycling in a fuel cell. The amplitude of strain oscillations is found to have a profound impact on the membrane fatigue lifetime. Importantly, it is also discovered that membrane fatigue failure occurs earlier under the channels than under the lands. The model is further used to simulate the membrane fatigue lifetime under operationally representative conditions of simultaneous temperature and humidity cycling where the lifetime is severely reduced due to increased amplitudes of strain oscillations.
Open Access
Numerical modelling of piezoelectric actuators exposed to hydrogen
(Springer Vienna, 2014-10) Sapsathiarn, Y; Singh, Y; Rajapakse, R. K. N. D
Modern fuel injectors have been developed based on piezoelectric stack actuators. Performance and durability of actuators in a hydrogen environment are important considerations in the development of hydrogen injectors. 2D plane stress and 3D models for analysis of coupled diffusion and thermo-electromechanical response of actuators are presented. Chemical potential, electric field and temperature gradients are taken as driving forces for hydrogen transport. The explicit Euler finite difference method is used to solve the nonlinear diffusion governing equation. The finite element method is used for time-dependent analysis of fully coupled mechanical, electric and thermal fields. The diffusion process and thermo-electromechanical deformations are coupled through the dependence of piezoelectric properties on hydrogen concentration. Experimental results for the piezoelectric coefficient d 33 of PZT ceramics exposed to different hydrogen concentrations are used. A comparison of a fully coupled 2D model with 2D and 3D models with reduced coupling is made to examine the significance of coupling and computational efficiency. Selected numerical results are presented for time histories of hydrogen concentration, temperature and stroke of an idealized actuator unit cell to obtain a preliminary understanding of the performance of actuators exposed to hydrogen.
Embargo
Performance of piezoelectric actuators in a hydrogen environment: Experimental study and finite element modelling
(Pergamon, 2015-03-02) Singh, Y; Rajapakse, R. K. N. D; Kjeang, E; Mumford, D
Significant improvements in fuel efficiency and emissions can be achieved in internal combustion engines (ICE) by electronically controlling the fuel injector opening valves with piezoelectric actuators. Hydrogen is considered an attractive alternative fuel with near-zero emissions at the point of use; however, the current understanding of the performance of piezoelectric actuators in a hydrogen environment is very limited. Variation in the performance of piezoelectric actuators due to their continuous and cyclic exposure to hydrogen at 100 °C and 10 MPa is experimentally investigated in the present work. The actuator's stroke-voltage relationship is evaluated under quasi-static as well as dynamic electric loading conditions within the ambient temperature range of 5–80 °C. A 3-D finite element model is also developed to simulate the behaviour of a single stack of an actuator exposed to hydrogen by using experimentally determined piezoelectric coefficients. The importance of coating technology to protect the actuator material from hydrogen is confirmed by the experimental study and numerical modelling.