Fatigue Behaviour and Lifetime Prediction of Cold-Formed High Strength Steel

Nowadays, high strength steel (HSS) grades serve as multipurpose materials since they combine several advanced mechanical properties, such as an excellent specific strength and enhanced fatigue performance. In recent years the implementation of HSS for various mobile applications and lifting equipment has increased significantly. The design and performance of these applications can be optimized by using high strength steel components, in thinner, more compact configurations. As service loads are typically cyclic in nature, the fatigue behaviour is of major concern for the final design. Furthermore, HSS components often require a closed cross-section achieved by a secondary forming process. This can induce a significant amount of residual plastic strain, strongly affecting the mechanical behaviour compared to the undeformed base material. In this study, a new specimen geometry and a set of modular tools have been developed to determine the fatigue behaviour of moderately thick air bent HSS plates. Since most applications employ this steel grade in long configurations, under plain strain conditions, a wider specimen has been designed. Firstly a forming process is modelled and fully validated through a state-of-the-art validation method that compares the residual strains after forming. Secondly a tensile fatigue load is modelled and used to predict the lifetime. This numerical prediction is then compared with an experimental fatigue test campaign. By means of numerical and experimental techniques, the fatigue behaviour of a cold-formed high strength steel S500MC is predicted and assessed. The present study is concerned with setting up a general framework for modelling and fatigue testing of bent samples where the strain evolution can be monitored at consecutive intervals of the expected fatigue life. Therefore, a full SN-curve has been developed for the abovementioned specimen. In addition, a predictive finite element model has been created, capable of assessing the fatigue life based on multi-axial fatigue criteria, used to assess the expected total life and compare numerical with experimental observations.