Browsing by Author "Liu, Dan"
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- Contact Loss beneath Track Slab Caused by Deteriorated Cement Emulsified Asphalt Mortar: Dynamic Characteristics of Vehicle-Slab Track System and Prototype ExperimentLiu, Dan; Liu, Yu-feng; Ren, Juan-juan; Yang, Rong-shan; Liu, Xue-yi (Hindawi, 2016-11-21)The contact loss beneath track slab caused by deteriorated cement emulsified asphalt mortar (CA mortar) has been one of the main diseases occurring in the CRTS- (China Railway Track System-) I Slab Track of high-speed railway in China. Based on the slab track design theory and the vehicle-track coupling vibration theory, a vehicle-track vertical coupling dynamic FEM model was established to analyze the influence of the contact loss length on the dynamic characteristics of vehicle and track subsystems at different train speeds. A prototype dynamic characteristic experimental test of CRTS-I Slab Track with CA mortar contact loss was conducted to verify the FEM model results. The train load was generated by the customized ZSS50 excitation car. The results showed that when the operation speed is less than 300 km/h, the contact loss with length smaller than 2.0 m barely affects the running smoothness ride safety of vehicle. The contact loss length effect on the dynamic characteristics of track subsystem is pronounced, especially on the track slab. Once the contact loss beneath the track slab occurs, the vibration displacement and the acceleration of the track slab increase rapidly, while it has little influence on the displacement and acceleration of the concrete roadbed.
- Durability study of proton exchange membrane fuel cells via experimental investigations and mathematical modelingLiu, Dan (Virginia Tech, 2006-07-11)In this dissertation, novel approaches to PEMFC durability research are summarized. These efforts are significantly different from most other studies on durability in that rather than focusing on chemical degradation, more attention is given to the mechanical aspects of the PEMFC system. The tensile stress-strain behavior of Nafion® 117 (N117) and sulfonated poly(arylene ether sulfone) random copolymer (BPSH35) membranes is explored under ambient conditions, with respect to the effects of initial strain rate, counterion type, molecular weight and the presence of inorganic fillers. A three-dimensional "bundle-cluster" model is proposed to interpret the tensile observations, combining the concepts of elongated polymer aggregates, proton conduction channels as well as states of water. The rationale focuses on the polymer bundle rotation/interphase chain readjustment before yielding and polymer aggregates disentanglement/ reorientation after yielding. In addition, the influence of uniaxial loading on proton conductivity of N117 and BPSH35 membranes is investigated. When the membranes are stretched, their proton conductivities in the straining direction increase compared to the unstretched films, and then relax exponentially with time. The behavior is explained on the basis of the morphological variations of hydrophilic channels, accompanied by the rotation, orientation and disentanglement of the copolymer chains in the hydrophobic domains, as illustrated with the help of our bundle-cluster model. Finally, the long-term aging of hydrogen-air PEMFCs is examined with a cyclic current profile and under constant current conditions. The end-of-period diagnosis is performed for both MEAs at 100h aging intervals, including a series of cell polarization, impedance and electrochemical experiments. The results demonstrate that hydrogen crossover is the most significant result of degradation for the MEA under cyclic aging mode due to the formation of pinholes at approximately 500-600h, and mass transport limitations are the major degradation sources for constant current mode. A phenomenological mathematical model is set up to describe the PEMFC aging process under both cyclic and constant conditions.