Investigations of rail fractures at vienna underground and measures to reduce them

At Vienna Underground, fractures of rails have turned up to a high extent, so it was necessary to search for methods to fight them. For this reason a catalogue concerning classification and coding of rail defects in addition to a general map for finding out accumulations of rail defects were drawn up. This special coding system developed in Vienna has been already applied at other Underground networks. Also load on the underground rail caused by the underground car with regard to frequency and axle load (wheel load) was determined. It has been amazing that the parameter tons/year is nearly in the same range as at normal railways (comparison of a section of the Federal Austrian Railways with the Vienna Underground). Sudden fractures of rails (fractures in the welded joints are not considered) which turned up because of reasons that were not recognizable at visual controls showed a lack of criteria. Also by means of questionnaires sent to the other metro networks of the world it could not be found out, when a fatigue crack, depending on load and load cycles, has reached a critical extension. This led to the decision to carry out measurements by means of strain gages at a high-loaded spot overrun by trains of the Vienna Underground. These investigations showed that the knowledge of the K Ic-value of the rail steel and of the specific loads of the rail have made it possible to reduce the frequencies of fractures. If the critical crack length in the rail is known, a safety factor can be determined and the intervals of inspections can be calculated.