Analysis of track rope damage in serious incident on Schilthorn aerial ropeway

Damage analysis of the serious incident that took place on 29 December 2004 on the Murren-Birg section of the Schilthorn aerial ropeway identified the cause of failure as hydrogen-induced stress corrosion cracking. The necessary preconditions for this damage mechanism, which was previously unknown in the aerial ropeway industry, were established by the complex interaction of various ancillary conditions. During periodic rope relocation in 1977, galling occurred between the track rope and a deflection saddle as a result of inadequate lubrication. The friction or galling processes resulted in changes in the grain structure (friction-induced martensite) as well as scale-like smearing on the wire surfaces. The formation of friction-induced martensite results in local stress changes (peak tensile stresses on the surface). Over time, corrosion processes took place in the area of the surface defects under dissociating conditions (absence of air), producing diffusible hydrogen. The hydrogen diffuses preferentially to points of peak stresses and gives rise to embrittlement of the material and intergranular cracks. The results of the investigation were immediately distributed to the licensing authorities and aerial ropeway operators in the form of specially designed training materials to enable appropriate inspections of systems of similar design. Detailed evaluation of this preventive measure is still in progress.