PSI - Issue 39

Giovanni Pio Pucillo et al. / Procedia Structural Integrity 39 (2022) 700–710 Author name / Structural Integrity Procedia 00 (2019) 000–000

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For more details see (Pucillo et al. 2020a; 2020b; Reid 1993; Houghton and Campbell 2012; Ball and Lowry 1998). These stresses have the effect of reducing the actual stress, as well as the Stress Intensity Factor (SIF), with a consequent delay of crack initiation and growth, being the total stresses the sum of residual and applied stresses. However, cold expansion is well known in the aeronautical industry and almost all the studies deal with aluminum, whereas no experimental studies exist on the Cold Expansion effects on the fatigue strength of railway structural steels. For this reason, RFI and the University of Naples decided to investigate on the effects of this technique on railway steel by mean of an extensive testing campaign. Thirty-eight (38) rectangular drilled specimens, previously submitted to stress relieving and to an accurate surface finish treatment, were extracted from a specific batch of rails. Successively, fifteen (15) where expanded at 2%, and ten (10) to 4%, where the degree of cold expansion is defined as the value of the interference between the maximum diameter of the mandrel and the hole, namely: [%] = + − × 100 (1) where R is the maximum radius of the mandrel, t is the sleeve thickness, and r is the radius of the hole. 2. Experiments 2.1. Specimens preparation For the experiments, rectangular drilled specimens, whose dimensions are reported in Fig. 2, were extracted from a specific batch of rails. Before drilling the specimens, a stress relief was executed, with the aim to remove all existing residual stresses induced during rail production, and to be sure that the only parameter affecting the fatigue crack growth rate was the residual stress field due to cold expansion. For the same reason, the surface finish has also been treated with detail, with three preliminary steps of diamond grinding and one tangential grinding.

Fig. 2 – Specimens dimensions.

Cold expansions were performed in FTI Inc., Seattle, but all the photos made during the process are under a confidentiality agreement, and for this reason they cannot be shown in this paper. For monitoring the crack extension during fatigue tests, two types of crack gauges were applied on expanded and not expanded specimens (see Fig. 3-a/b), and a crack gauge adaptor was used to connect the crack gauges into a common Whetstone bridge.