GAUGE CORNER CRACKING

Summary

  This note has been produced by the Rail Regulator for the Transport Sub-committee of the House of Commons. It is a response to the Committee's request for a written note about gauge corner cracking. The note states that gauge corner cracking (GCC) is a particular type of rail fatigue defect that has been increasingly observed in Britain during the last 10 years. Its precise causes are still being researched, but rail fatigue has been well understood for many years. Management and rail inspection processes should be capable of containing the problem. The GCC problem is not unique to the Railtrack network, but other railways appear to have been more successful in managing it.

Background

  Rails are said to be defective when they have to be removed from service prematurely ie before they would normally be replaced. Rail defects generally appear as cracks and/or surface damage, and they are typically caused by:

(a)  internal flaws within the metal arising from the manufacturing process;

(b)  physical damage that occurs during installation (eg rough handling);

(c )  physical damage that occurs in service (eg surface damage arising from wheel spin); or

(d)  metal fatigue resulting from the continual passage of traffic.

  Gauge corner cracking (GCC) is universally regarded as a metal fatigue problem. It appears as surface cracking on the running surface and inside edge (the running edge) of rails. This is the part of the rail that is in contact with train wheels, and is therefore subjected to repeated cycles of particularly high stresses as traffic passes over the rail.

Nature of gauge corner cracking

  There are several phases of development of GCC.

  The GCC cracks appear as a series of transverse cracks across the top of a rail (the running surface), and they may extend down the inner vertical face of the rail. In the early stage of development these cracks are fine, hairline fractures, that may not even be immediately visible to the naked eye.

  Continued loading under traffic causes these cracks to grow. Crack growth initially occurs at shallow depth (a few millimetres) below the rail surface. However, if left unchecked, two things tend to happen. The near-surface cracking can lead to significant spalling or "shelling", where the rail surface actually begins to break up and pieces of metal come away, leaving the rail surface pitted and corroded. This will be quite visible, but the internal cracking will also grow deeper into the "head" of the rail to form a transverse defect across the rail section. If that growth is allowed to develop, the head of the rail becomes less and less able to withstand the stresses imposed by passing trains and it will eventually fracture—the break usually being triggered by the passage of a train.

  This type of rail defect is particularly serious. In its report to the Regulator in October 2000, Transportation Technology Center Inc. (TTCI) explained that:

  "These types of defect are dangerous in that they appear in groups, and a single break may lead to consequential breaks and catastrophic rail failure."

  These words were written before the Hatfield disaster. The inquiry is yet to conclude and report its findings, but it appears likely that it will determine that this failure mechanism was indeed the cause of this particular accident.

Is gauge corner cracking a new phenomenon?

  Gauge corner cracking cannot be described as a new phenomenon. It is well known that fatigue defects do develop in rails, although it is true to say that the precise form of fatigue cracking known as GCC has been less well understood than other forms of rail defects. However, it has been known about in Britain for at least 10 years.

  Railtrack has informed ORR that:

(a)  the causes of gauge corner cracking are not well understood; and

(b)  the problem has recently been appearing at a rapidly accelerating rate.

  Whilst neither of these views is disputed, the nature of fatigue cracking in rails and the need to properly manage it, have been well understood by railway engineers for many years. The following extracts from "British Railway Track—Design, Construction and Maintenance" (the track engineers' handbook, published by the Permanent Way Institution in 1993) illustrate the point.

  Referring to rail fatigue defects known as "squats", the book describes a form of crack development that is virtually identical to GCC:

"These are fatigue cracks initiated at the running surface by wheel/rail contact forces. They are visible on the running surface...the crack propagates at a shallow angle to the horizontal until it reaches 3 to 5 mm below the surface. At this depth, it usually branches downwards to form a defect which is very similar in appearance to a Tache Ovale. If left in track, this would eventually produce a brittle fracture and a complete transverse break of the rail"

"Developments in steel technology have reduced and in some cases eliminated some traditional fatigue failure types, whilst improved ultrasonic test methods and a better understanding of fatigue mechanisms and fatigue life prediction, are helping to contain the problem in service...Nonetheless the constant drive for higher speeds and heavier axle loads will demand that a continued effort be maintained to keep rail fatigue under control" (my emphasis)

  ORR's view, therefore, is that although the precise manifestation of GCC may well be a more recently observed phenomenon, the industry should have been quite capable of detecting and understanding the consequences of such cracking in rails, even if it did not fully understand the causes.

  In fact, in its report to ORR in October 2000, TTCI's conclusions indicated that Railtrack has not made sufficient efforts to keep rail fatigue under control:

"An area of concern is automatic ultrasonic inspection, where it is believed Railtrack has not yet adopted best practice in technology.. It is also felt that inspection frequencies and minimum actions for defect removal may not have kept pace with increases in traffic. It is repeated that high levels of breaks indicate either that defects are not being found sufficiently by inspection, or that they are being left in track too long." (Ref. P46, "Rail Failure Assessment for the Office of the Rail Regulator", TTCI, October 2000)

Is the problem unique to Britain?

  No. Many other railway systems report problems with GCC. The TTCI report to the Regulator (October 2000) shows that fatigue failures of rail (including GCC and "shelling" of rail) are a major problem on some of the world's large passenger railway systems. East Japan Railways report that more than 60 per cent of rail defects are fatigue failures, whilst both SNCF (France) and DB (Germany) also list them amongst the most common causes of defects that require rail to be replaced prematurely.

  It is true that the causes of GCC are not yet well understood, although many theories exist about the nature of rail steel now being used, the criteria used for track and vehicle wheel/suspension design, and the quality of rolling stock maintenance. Railtrack has recently been drawing in leading academic and technical expertise from around the world (including TTCI) to study GCC. This group is due to report very shortly, and ORR awaits its conclusions with much interest.

  The possibility of comparison with overseas railways' experience is very welcome, because it is likely to pinpoint particular aspects of recent UK experience and practice that may have contributed to the scale of GCC problems now being suffered on Railtrack's network.

  Nevertheless, ORR is not aware of any evidence to suggest that other railways have suffered the same severity of the GCC problem and service disruption, and must therefore conclude that other railways have managed the problem more successfully than Railtrack.

  ORR is continuing to press Railtrack for its detailed responses to, and plans for implementing, the recommendations for the TTCI report. ORR sees this as an essential element of achieving a long-term network recovery.