Load interaction effects in medium and high strength steels for railway axles

It is well-known from the literature that an interaction effect on crack propagation arises when a specimen or a component is subjectedtovariableamplitudeloading.Independenceoftheappliedloadsequence,acertainamountofretardationoracceleration onto fatigue crack growth rate can then be observed if compared to the constant amplitude case. In the case of structural ductile materials, the interaction phenomenon is mainly addressed by the local plasticity at the crack tip and can be explained, from a global point of view, by adopting the crack closure concept. Considering the applicative case of railway axles, a good correlation between crack growth interaction effects under variable amplitude loading and the amount of plasticity-induced crack closure has been previously derived by the authors, relatively to the standardized EA1N steel. The other standardized European steel for railwayaxles,a25CrMo4gradenamedEA4T,isinsteadconsideredinthepresentresearch,aswellasanhighstrenghtsteelgrade. Anexperimentalcampaignwascarriedonthismaterial,usingSE(T)specimens,inordertounderstandandquantifytheinteraction effects arising from relevant load sequences derived from service. Firstly, tests were performed directly applying the acquired load time history. Eventually, the load history was transformed into an equivalent block loading sequence and applied to different specimens varying the number of cycles of each single block. Finally, the experimental outcomes were modeled adopting both a strip yield model and a simple no-interaction approach, in order to quantify the possible interaction effects. The modeling was carriedoutconsideringdifferentexperimentaltechniquesforderivingthecrackgrowthandthresholdbehaviorsofthematerial,i.e. the traditional ΔK-decreasing technique and the compression pre-cracking one