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Step tests reveal failure details

 

Destructive tests on two aged escalator steps were completed by TWI's Structural Integrity Technology Group. The components had been in service for London Underground for half a century and the company wished to establish the riskof failure, how they might fail, at what load and also to investigate the possible detrimental effects of latter day repair welding.

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The two MH type steps of riveted construction were first subjected to the British Standard 5656 fatigue test of five million cycles of load between a half and three kilo-Newtons. This they passed, and under the same loading configuration were statically tested. The load path passed through a steel ball and rectangular load distribution plate into the step's tread plate. As the load increased, the step collapsed at the centre of its rear edge with no obvious distortion or sign of distress elsewhere. The test was stopped at a load of 49kN when the tread plate was so deformed that it actually touched the upper axle. The actuator displacement at this load was 69mm and as it was unloaded a 30mm permanent deformation remained in the step under the load plate.

Closer examination of the failed step showed that collapse was confined to the sheet steel and the hardwood treadplate. The front edge riser and trailer wheel arms appeared undistorted. Throughout both tests the load and actuator displacements were recorded in real time. The results have proved very useful to London Underground who also asked for the entire test to be recorded on video.

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