Implementation of a Phased array Rail Weld Inspection program for London Underground.

Buckley, Joseph; Level X NDT; United Kingdom

Buckley, J.M.; Level X NDT; United Kingdom
Willats, P.; London Underground; United Kingdom
Harrison, D.; London Underground; United Kingdom

ID: ECNDT-0501-2018
Download: PDF
Session: Transportation, railway & automotive UT 2
Room: R2
Date: 2018-06-13
Time: 10:50 - 11:10

The London Underground network (LUL) is the oldest public transit system in the world and has over 400 km of track. New and replacement sections of rails are predominantly joined using aluminothermic welds, of which there are estimated to be 50,000 on the network.

With the recent introduction of Night tube, a 24 hour running service on the busiest lines on Fridays and Saturdays, engineering hours, and thus the time available for inspection and maintenance, has been reduced. Furthermore, issues with previous thermic welding processes, along with increases in annual tonnages due to new, heavier trains and more ambitious timetables, has led to an increase in rail breaks and defects, a significant number of which have been located at aluminothermic welds.

While modern welding techniques are extremely reliable, there still exists the possibility of entrapped porosity, particularly on older aluminothermic welds.
This can lead to weaknesses in the joint and increases the probability of a full rail fracture, particularly when track stresses arising from other factors such as repairs, re- railing or wheel flats are experienced.

To improve safety, reliability, and assurance, London Underground decided to investigate the possibility of using a phased array ultrasonic inspection device to verify the internal integrity of welded joints, both in the head and the foot of the rail, in addition to the current visual inspection.

This paper discusses the initial investigations, the decisions that were made and the implementation of the equipment and inspection process.

New processes have now been in use since the start of the year and continues to give greater confidence in the integrity and reliability of both new and existing welds. These have been implemented into TFL documentation, and both ongoing and site specific inspections are being rolled out across the business, giving additional assurance to stakeholders throughout LUL.