Fatigue crack growth rate dependency on cold expansion degree in railway steel

The increase of railway track occupancy due to high speed trains introduction in the morning and cross-border transportation of hazardous and non-hazardous materials during late hours, require particular attention for the fatigue design as well as the maintenance planning of railway superstructures. For this purpose, strategies finalized to increase the fatigue strength of railway structural parts assume great interest for railway engineers. Cold expansion technique, for example, is less expensive than other mechanical and thermal treatments, but in the literature there are no data concerning its use in the railway field, being almost all available studies focused on aluminum parts. In this paper, fatigue crack growth response of drilled specimens extracted from rails submitted or not to cold expansion are presented. Two degree of cold-expansion, 2% and 4%, are investigated in terms of fatigue crack growth rate, and the results compared to those obtained with not-expanded specimens. It is found that cold expansion postpones fatigue crack initiation, and that this effect is more pronounced for increasing degree of cold expansion. A preliminary predictive model was developed, based on the sum of the Stress Intensity Factors calculated with the weight functions and concerning the residual stress field induced by cold expansion, and the one generated by the external loads.