A key problem that railroads have faced since their earliest days is the prevention of defects within the track. There are many effects that influence rail defects and rail failure, including bending and shear stresses, wheel/rail contact stresses, thermal stresses, residual stresses and dynamic effects.
A contributing factor that can cause crack propagation is the presence of water and other liquids. When fluid fills a small crack and a train passes over, the water becomes trapped in the void and can expand the crack tip. In addition, the trapped fluid could freeze, which can potentially lead to crack expansion, or initiate a corrosion process.
Parts of a rail where defects can be found include the head, web, foot, switchblades (crossovers), welds and bolt holes. While the majority of the flaws are located in the head, the entire rail needs to be inspected.
In any country operating a passenger or freight rail network, the key issue is how to carry out inspection of a large amount of track. For example, in the UK there are > 40,000 miles of track and > 19,000 switch and crossing units. Whilst the principle of NDT inspection is train-based ultrasonic testing, liquid penetrant inspection using visible penetrants is ideal for the examination of identified faults, both prior to and post corrective action being taken. It is also possible to use magnetic particle inspection where the use of portable Yokes lends itself to inspection using visible magnetic particle inks.