Abstract
Most rails in the track cannot avoid suffering defects and notches due to the contact rolling load of the wheels. These rails are often found to be broken by fatigue cracks originating from these defects. The most part of the fatigue lives of the members having sharp defects or notches are occupied by the propagating stage of fatigue cracks. Then, by using the specimens cut out from the experimentally melted rail steels (thick plates) and many kinds of rails, the characteristics of the fatigue crack propagation rate and the effect of the metallurgical structures on them were studied. Especially, the effect of residual stress on the crack propagation rate was investigated, because residual stress is inevitably produced in such a shapes as rails.
The main results obtained are as follows;
(1) The fatigue crack propagation rate is little affected by the chemical composition, pearlite block size and mechanical properties if the stress ratio is held constant.
(2) If the fatigue crack propagation rate is expressed by the equation of dl/dN=C(ΔKeq)m, where ΔKeq=√ΔK·Kmax, it can be well represented by one linear relation. irrespective of the stress ratio. In addition, the values of C and m can be evaluated immediately from the case of the zero stress ratio.
(3) The variation of the crack propagation rate at the constant stress ratio disappears by the annealing (520°C×30min, furnace cooling). This reason is considered due mainly to the relief of residual stress in the specimens.
