Evaluation of fatigue strength and life in rail steel under variable amplitude load

The initiation and growth behavior of transverse crack in rail steel was experimentally investigated using the notched keyhole specimen under constant amplitude and variable amplitude loads. Fatigue limits of smooth and keyhole specimen in rail steel at R = 0 were about 198 and 59 MPa, respectively. The fatigue crack initiation length at notch root was about 1.3 mm. The fatigue lives under variable amplitude load predicted by introducing the modified equivalent stress (sigma(eq)') agreed closer to the experimental results than those predicted by using the conventional equivalent stress (sigma(eq)). As the characteristic stress intensity factor range (DeltaK(rms)) increases, the fatigue crack propagation rate (da/dN) under variable amplitude load derived from the actual load history is faster than that under constant amplitude load. Through the examination of fracture surfaces in each load condition, it was found that this behavior results from the transition of fracture appearance.