Bakerloo and Jubilee lines defect data sheet

The defect data sheets analysed in this research contained the recorded information between the years 2013 – 2015. When the inspection methods in identifying the defects were compared in Figure 3.10, it was noticed that the ultrasonic testing was the primary technique used in these two lines. However, it was also noted that approximately 25% and 5% of the total defects on Bakerloo and Jubilee lines respectively were recorded as ultrasonically untestable. This means that the level of damage on the surface of the rails prevented ultrasonic detection.

Figure 3.10: Rail inspection method in the Bakerloo and Jubilee lines

LUL records the observed defects using a Code Number which is defined according to principles given in the UIC 712 Rail Defects Leaflet. This code gives three primary information about the defects;

1) Defect zone whether they are generated on the welds (if the rails are connected by a joint/fishplates; rail‐end), switch and crossings (S&Cs) and mid‐rails (plain track) which represents the intermediate sections between rail‐end/welds and/or S&Cs. 2) Defect position inside the rail, head, web and foot of the rail.

3) Defect type such as squats, shelling, corrugation and etc.

The code usually consists of three or four digits. Whilst the first two digits represent the defect zone and rail position, respectively, the last one/two digits give information about the defect type and further details. For example, the code 227 represents the squat type of cracks inside the railhead in the mid‐rail/plain track zone.

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Figure 3.11: Recorded defect zones in the Bakerloo and Jubilee lines

Figure 3.11 illustrates the recorded defect zones on the selected lines. As expected, cracks were predominantly observed on plain-track sections which had a longer distance compared to other zones on the track. Rail-end and S&C zones were also under high risk of damage especially on the Bakerloo line. Since this line is a relatively old metro line, the age of turnouts and the larger proportion of bull-head type of rails, which are connected by rail joints, might account for the increase in the number of defects.

The effectiveness of rail inspection depends on the efficiency and accuracy of the inspection device and the skill and experience of the inspector. The data presented here are the outputs from the defect reports prepared by LUL inspectors and hence it might occasionally contain misinformation. For example, the defect zone or the defect type may be typed incorrectly or no information may be provided. Due to this problem, approx. 16% of the total number of observed defects on the Jubilee line had no information regarding its occurrence zone.

The RCF cracks are mainly divided into two groups: surface and subsurface-initiated defects. Whilst, the defects in the first group are mostly generated due to repeated loads and high contact forces at the wheel-rail interface, subsurface-initiated cracks are often caused by metallurgical faults such as improper heating or cooling. As it can be seen in

Figure 3.12, the most prevalent type of rail damage was squats which can be frequently observed on both Bakerloo and Jubilee lines. They are often described in the literature as dark spots containing cracks with a circular arc or V-shape. Widening of the running band and localised depressions were also indicated as the by-product of this defect mechanism (E. Magel, 2011). On LUL, when the estimated length and depth of the squat defects exceed a certain value, they were recorded as squat with T/O (tache ovale) which corresponds to a transverse defect from RCF in Figure 3.12. The results indicated that approx. 10% and 25% of the total squats recorded in Jubilee and Bakerloo lines, respectively had a possibility to growth further, resulting in a transverse defect.

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Figure 3.12: RCF defects recorded in Bakerloo and Jubilee lines

The second dominant type of rail damage was shelling which was often seen on the gauge corner and the top of running surface of the railhead. The high contact stresses leading to surface and subsurface-initiated cracks merge together to cause localised loss of structural integrity which results in shelling of the surface material in the railhead (Olver, 2005). This shows that high contact stresses are not just limited to heavy axles in freight traffic, but metro lines also suffer from high forces generated at the wheel-rail contact in combination with frequent load passages which have a significant impact on the formation of damage. Longitudinal vertical and horizontal cracking were also recorded in the data sheets. These are progressive type of cracks which tend to separate the head into parts horizontally, parallel to the running surface or vertically through the head (UIC, 2002). Besides the surface-initiated cracks, the tache ovale type of defect which is a subsurface defect, was also reported by the maintenance team.

As expected from the finding of the stud study, a high number of squats were observed in LUL (Grassie et al., 2011). However, as aforementioned, some of these recorded squats could be stud type defects which was also declared by LUL staff. In fact, a relatively low number of wheel burns were also reported in the lines.