Last week's NCE article on the Hatfield crash detailed damage to the head of the rail but did not explain complete tensile failure. I spent many years investigating failures in buried iron pipes and would suggest there are some important similarities.
Buried pipes can survive for years with inadequate bed support but will eventually fail due to a combination of superimposed loading and cyclical seasonal temperature and soil moisture effects. The broken rail at Hatfield occurred in October when ambient air temperatures begin to fall and temperature induced tensile strains will be on the increase.
The break occurred in the outer rail on a curve. It is possible to envisage a ratcheting mechanism where with the passage of every train, the track is gradually pushed outwards, passive resistance in the ballast preventing the sleepers returning to their original position. Enlarging the radius of the curve will increase the circumferential tensile strain in the outer rail, adding to any installation strains and reducing the ability of the rail to carry wheel loads.
Add to this defects in the metallurgical structure of the rail and strain levels will eventually rise to an unacceptable level causing failure.
The NCE 500 survey said that it did not matter that the chief executive of Railtrack had no engineering background. This is perhaps only satisfactory in a technology based organisation where there is an adequate core of experienced technical expertise to provide analysis and guidance.
My own observations suggest that all too often the technical experts have been pensioned off to make the newly privatised companies slimmer and more attractive to their shareholders. If this is the case this trend must be reversed.
Raymond C Owen Raymondcowen@new scientist. net