Load Interaction Effects in a Medium Strength Steel for Railway Axles

As is well known, an interaction effect arises on crack propagation when a specimen or a component is subjected to variable amplitude fatigue loading. Depending on the applied load sequence, a certain amount of retardation or acceleration can then be observed, on the fatigue crack growth rate, with respect 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. From this point of view, in the present research, load interaction effects in a medium strength steel for railway axles are analyzed. An experimental campaign was carried on this material, using SE(T) specimens, in order to understand and quantify the interaction effects arising from relevant load sequences derived from service. The experimental outcomes were then modeled adopting both a simple no-interaction approach and a more sophisticated strip-yield model in order to quantify the possible interaction effects. The modeling was carried out considering different experimental techniques for deriving the crack growth and threshold behaviors of the material, i.e., the traditional DK-decreasing technique and the compression pre-cracking one.