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How broken rails fail

RAILTRACK ADMITTED last week the cause of the rail break at Hatfield was 'gauge corner cracking' of the five year old rail.

This is caused by very high rolling contact stresses between the rail and train wheel and has emerged over the last decade as a major cause of rail breaks in the UK.

Gauge corner cracking usually occurs on the inside edge of the outer 'high' rail on banked curves of the track, where wheel loads can reach 100kN.

As the train goes through the curve at speed the wheels impart high lateral forces on the track head.

With repeated loads over time the resulting shear stresses can generate fatigue cracks on the top of the rail.

These near horizontal cracks can extend for up to 20mm before turning down to become vertical cracks. It is these vertical cracks that can cause the rail to break.

development of cracks is known to be accelerated by hammering from oval wheels.

Worn rail head profiles and poor lubrication also contribute to the problem.

The usual solution is to grind away the initial cracks before they become vertical. But a major problem is knowing when to do this. Ultrasonic testers are used to investigate the integrity of the rail, but are not always able to see though minor flaws which may hide the serious vertical cracks in their shadow.

In light of this, Railtrack has started to replace rails preferentially rather than grind the cracks away. 'Grinding is not now on the action list, ' Dennis Rapley, a Railtrack senior engineer, told NCE.

'We now replace rather than grind.'

Ultrasonic tests at Hatfield in January revealed the cracked rail needed attention and it was last ground in September before the planned replacement next month.

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