Keywords: rail, fracture, perlitic steel, life time.

Public expectation of rail-guided mass transport systems is the perfect, consumer friendly and uninterrupted service with zero probability of danger to the daily user. From the rail and wheel manufacturers' point of view safety is achieved if neither the wheels nor the rails will be damaged by for example excessive wear, fracture, etc., or deteriorate prior to an expected service time. In any case, the optimization of maintenance costs is a very important task for every operator of rail-infrastructures. Decreasing time intervals between trains, a rolling stock of a variety of different types of vehicles and an aging infrastructure make it essential to continuously improve the railway systems. With regard to rail-wear and life cycle, the quality of the rail material becomes crucial. Hence, heat-treated rails with very high hardness are now systematically replacing conventional types of rails. The aim is to decrease the ratio between side- and headwear which directly leads to higher life cycles and lower life cycle costs based on longer maintenance intervals.

During periodic track inspection of the Vienna subway a new type of surface damage failure was discovered which in some cases led to total failure of the rail. Special attention is focussed on a new type of defects: square beam type break-outs which occur on the central section of the top running surface of the railhead. Subway rails are subjected to various loadings which induce a combination of mechanical, thermal and residual stresses.

The mechanism of these beam type break-outs could be clarified by micro-photographic investigation. A thorough macroscopic investigation revealed that many tiny surface cracks develop within the running area of the rail together with and in competition with contact-rolling induced sub-surface shear damage. Failures of this kind seem to have come into existence with the exchange of softer rails by head-hardened rails fabricated of perlitic steel. Fatigue crack propagation for head-special hardened rail steel can be best described by the well-known fracture mechanics based Paris law. Application is, however, limited to cases where, after exhaustive plastification, the post-shake down behavior of the material leads to purely elastic conditions and material behavior.

Although the lifetime calculation of a single shell crack is straight forward, lifetime calculations of groups of surface flaws and specifically of break-outs are extraordinarily complex and require sophisticated numerical modeling techniques.

The investigation of beam-type break-outs in the rail surface of subway lines showed that there is a very complex interplay between edge crack propagation and shear damage development just underneath the running surface in the railhead. If, in light rail service on head-hardened rails, the subsurface shear damage develops slower than any possible edge crack, the chances for a through-crack to develop are very high. On the other hand, if subsurface shear damage develops faster beam-type break-outs will be formed which lead to excessive damage of the rail surface. It was found that, under certain conditions, the lifetime of these new rails was drastically reduced.

An important role is played by residual and thermal stresses which need to be taken into account. With increasing tensile residual and thermal stresses the stress level is shifted towards larger tensile stresses and, therefore, the R-value rises. However, when the axial normal residual and thermal stresses (the summer period is of interest only) are superimposed, one may end up with a grossly increasing number of cycles, up to a factor of 2! This, of course, drastically lowers the lifetime of the structure. This effect has not been recognized in the past and properly addressed in earlier lifetime calculations.

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