Fatal failure of an axle in spite of compliance with mandatory safety factors

In December 1996 an accident occurred on a cableway which caused the death of one passenger and injuries to several others. The cause of the tragic accident was the unexpected failure of the axle that carries the bull wheels for the wire rope. During the course of the official investigation it was demonstrated that the failure was due to a fatigue crack that had already propagated through a large portion of the cross-section. This fatigue crack had been initiated by fretting. The crack had developed in a rather short time at stresses which according to the designer should have been below those necessary for crack initiation. Therefore it may be concluded that a simple compliance with the safety factors will not always provide the necessary margin of safety against failure. Safety can be guaranteed only if all the important mechanisms of possible damage have been fully accounted for in the design.