PhD Oral Exam - Xingwen Wu, Mechanical Engineering
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
The wheel polygonalization, regarded as a periodic radial defect on the wheel circumference, has been observed in high-speed rail vehicles during the recent year. Such periodic defects of the wheel tread, especially the high-order wheel polygonalization, cause high magnitude and high-frequency variation in the wheel/rail contact forces, considerable axle box vibration, and stresses in the vehicle and track components, and thus in-service fatigue failures. The mechanism leading to high-order polygonal wear of the wheels, however, is not yet fully understood, although its adverse effects on dynamic responses of the vehicle have been reported in a few studies. This dissertation research focuses on the mechanisms leading to evolution of wheel tread polygonalization and its effects on dynamic responses of the coupled vehicle/track system.
A coupled vehicle/track dynamic model was initially developed to investigate the influences of discrete wheel defects as well as wheel polygonalization. Owing to the high-frequency nature of the wheel-rail impact force, the wheelset and slab track were modeled as flexible bodies, which were integrated into the vehicle model using the modal superposition method. The multi-body dynamic model was formulated for a typical high-speed train rail car while the track was modeled considering the rail as a Timoshenko beam discretely supported on a flexible track slab. The validity of the proposed coupled model was demonstrated by comparing the simulation model results with those available from the reported studies and acquired from a long-term field test program. The results obtained under excitations arising from a wheel flat or a polygonal wheel revealed substantial effects of wheelset flexibility on the peak wheel/rail creepage, axle box acceleration and stress in the wheelset axle shaft. The high order polygonal wheel wear also resulted in substantially higher frequency variations in the wheel-rail contact forces, which could excite vibration modes of the wheelset and the axle box.
A long-term field test campaign, involving measurements of changes in the wheel surface irregularities and the axle box acceleration responses, was undertaken on a high-speed rail vehicle to characterize wheel polygonalization and its growth rate. The wheel re-profiling process was found to be the main casual factor of initial wheel irregularities, which could be characterized by the third harmonic order. The measured data revealed that initial wear can rapidly propagate into polygonal wear of order 18, likely due to high-frequency wheel-rail impact loads. The results also revealed strong positive correlation between the axle box acceleration and roughness level of the wheel polygonalization. The simulation results showed that high-frequency wheel-rail impacts excite bending deformation modes of the wheelset axle shaft and the track at speeds exceeding 210 km/h.
A long-term wear iteration scheme integrating the coupled vehicle/track dynamic model and an Archard wear model, was subsequently formulated to identify main contributors of formation of wheel polygonalization. The simulation results were analyzed to establish a better understanding of evolution of wheel polygonalization over a relatively long period. The three half wavelength rail bending vibration mode within the wheelbase length (near 650 Hz) was identified as the primary contributor to high magnitude wheel/rail contact forces in the 500 ~ 700 Hz frequency range, and the high order wheel polygonalization. The wheelset flexibility was judged as the likely contributor to the lower order wheel polygonalization. Increasing rail pad support stiffness intensified the wheel/rail coupled vibration leading to lower order wheel polygonalization. The results further showed that a higher rail pad damping could suppress the formation of high order wheel polygonalization. In addition, eliminating the initial wheel irregularities and frequent variations in the operating speed could mitigate the formation of wheel polygonalization.