Urban systems: Broken rails

Growing passenger numbers and more extensive services have placed infrastructure under increasingly heavy loads.

Under these circumstances, it is rails that are the most exposed to risk. Greater ridership boosts traffic and adds to the loads they have to bear on a daily basis, making it necessary for the railway industry to pay increasingly close attention to the problems of track fatigue and wear.

To assist operators and maintainers in coping with these problems, the Measurement & Test Laboratory (LEM) can put the substantial knowledge it has acquired in analysing rail and rail fastening defects to good avail.

The challenge is vital since, apart from the risks to people and property, a broken rail can bring traffic to a halt for lengthy periods on the line, making trains late and causing traffic to stack up on the network.

With its top-notch test resources, LEM is able to identify the phenomena that caused the damage and can therefore point its clients towards the remedial action necessary to improve infrastructure availability.

As an illustration of the role that LEM can play in relation to a specific case, take the example of a rail broken in the vicinity of a thermit weld. In cases such as this, LEM is, in particular, able to perform the following laboratory tests on samples of the damaged rail:

• Morphological investigations to identify the nature of the break (fatigue, brittle fracture, etc.) and estimate the types of stresses that caused the rail to break,
• Microfractographic investigations by means of Scanning Electron Microscopy (SEM) to locate the starting point of the crack and the method of crack propagation,
• Materials analysis and mechanical tests to check the conformity of the material used to produce the rail in relation to the requirements of both standards and spécifications,
• Micrographic examination to analyse the structure of the rail steel at different places close to the fracture point (thermit or other weld, solid matter).

Through further analysis of the configuration of the track in the area of the break, LEM may propose a scenario to explain the reasons for the failure from its robust knowledge of the problems connected with the rolling stock/infrastructure interface.

analysis of the causes of the broken rail shows that there may have been a excessive loading at a given point along the rail, LEM may also go to the site and fit the track in the area concerned with the necessary instruments to study its behaviour under the effect of passing trains.

Using movement detector sensors, video cameras and accelerometers, a complete record can be obtained of the dynamic behaviour of the track.

Various assumptions can be investigated  by cross-analysing the data recorded: train with wheel flats, abnormal track behaviour, local tamping defect, etc.

Conversely, if the evidence shows that the rail break was caused by fatigue, LEM can place strain gauges on the rail to study the distribution of the various stresses in the parts of the rail closest to the thermit weld.

This data will then be digitised to provide a precise analysis of the type of loads applied to the rail. The useful life of the rail can then be calculated using multiaxial fatigue criteria (Dang Van, Wang-Brown, etc.) and the geometric effect of the weld on the degree of deformation can be quantified in detail.

The effect of temperature variations may also be studied for sections of tracks located in the open air (thermal loads). On these bases, LEM is also able to assist design offices specialising in track in devising rails manufactured with more robust grades of steel, for example, or in inventing ways of laying the track to reduce potential loads or in suggesting a thermit welding process less vulnerable to fatigue phenomena, etc.