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Title: The low adhesion problem due to leaf contamination in the wheel/rail contact : bonding and low adhesion mechanisms
Author: Ishizaka, Kei
ISNI:       0000 0004 8498 9847
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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Fallen leaves on the railway track in the autumn cause many safety and business issues, such as signals passed at danger (SPADs) and delays in a timetable, leading to a decrease in ticket sales and poor customer satisfaction. Low adhesion conditions due to leaf contamination cause these problems, which form a black and slippery leaf film. When the leaf film is present, a train wheel is likely to slip and slide, resulting in severe damages to the wheel and rail. Hence, proper adhesion management is necessary to offer safe and cost-effective train transportation. Railway industries currently conduct several countermeasures, and most of them attempt to remove leaf films. However, the removal is not straightforward as leaf films stick tightly to the rail. Accordingly, a different approach should be considered: prevention of the leaf film formation and mitigation of low adhesion. For the development of a new method of prevention and mitigation, it is important to understand the mechanisms of low adhesion and bonding of leaf contamination. Hence, the aims of this study were the clarification of the low adhesion mechanism and a better understanding of the bonding mechanism between leaf films and rails. Tribological tests using a ball-on-flat method were carried out to figure out which material in leaves plays an important role to lower the adhesion in wet conditions. A black material synthesised with leaf extracts and rail steels was identified to decrease the adhesion. Chemical and material analyses were performed, and it was found that the black material was a mixture of graphitised carbon, iron carboxylate or iron oxide and various ions, including metal phosphates. Therefore, the graphite-like carbon was concluded to be the primary cause of low adhesion due to leaf contamination. Chemical analyses of leaf extracts were conducted to elucidate the chemical reaction process. Organic acids in leaf extracts were likely to trigger the chemical reaction. Although the key organic acids are still unknown, a new direction of prevention and mitigation was proposed: heat application for the pyrolysis of the organic acids in leaf extracts. Following this idea, another tribological test using a twin disc machine was performed. It was found that the heat application was effective for mitigation of the adhesion and possibly prevention of the leaf film as well. Moreover, a scratch test found that the bonding energy of leaf films on the disc seemed to decrease as the creation temperature increased, suggesting that heat energy could help weaken the bond between leaf films and rails. Based on the observation in the field test and findings in the experiments, a continuous tread braking was proposed as a potential measure. It can prevent the leaf film formation by removing leaf residue on the wheel surface, mitigating the low adhesion and weakening the bond by raising the surface temperature of the wheel.
Supervisor: Lewis, Roger ; Slatter, Tom ; Lewis, Stephen Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available