Multiaxial Notch Fatigue: from nominal to local stress-strain quantities

Real mechanical components not only contain geometrical features resulting in stress concentration phenomena, but they are also subjected to in-service multiaxial fatigue loading. It is evident that, to efficiently address the above problem, structural engineers need sound and reliable methodologies which allow an adequate margin of safety to always be reached. The present book then attempts to summarise the methods we have devised to design real components against multiaxial fatigue by taking full advantage not only of nominal but also of local stress/strain quantities. This book begins by reviewing those definitions suitable for calculating the stress/strain quantities commonly used to perform the fatigue assessment. Subsequently, those pivotal concepts which should always be kept in mind when designing components against fatigue are briefly summarised (mainly considering uniaxial and torsional situations). By taking as a starting point the above concepts, the way of using the Modified Wöhler Curve Method is then explained in great detail, by focusing attention on both the high- and the medium-cycle fatigue regime. The existing links between the above multiaxial fatigue criterion and physical reality are also critically discussed. Subsequently, the procedure suitable for employing our method to estimate fatigue damage both in notched and in welded components is explained, by focusing attention on the most efficient way to estimate fatigue strength by post-processing Finite Element results. In light of the fact that the Modified Wöhler Curve Method as it stands can not be applied to estimate lifetime in the low-cycle fatigue regime, the main features of the so-called Modified Manson-Coffin Curve method are investigated in depth, by also reviewing those concepts playing a fundamental role in the so-called strain based approach. Lastly, the problem of performing the fatigue assessment of composite material is addressed by considering not only those design parameters influencing composites’ behaviour under complex cyclic loading paths, but also those criteria suitable for designing real components against multiaxial fatigue. The present book contains also two Appendices summarising several experimental results taken from the technical literature. In particular, about 4500 experimental fatigue results generated by testing plain, notched and welded specimens under constant-amplitude proportional and non-proportional multiaxial fatigue loading are listed in a systematic and organic way.