Multiaxial fatigue analysis of high-strength steel welded joints using generalized local approaches

The objective of this thesis is to improve methods for predicting the fatigue life, multiaxial or not, of mild- and more particularly high-strength steels using local approaches that have the capacity to be applicable to almost any cases, hence the term generalized local approaches. More specifically, the most suited and practical generalized stress-based approaches are determined and the framework for a 3D generalized approach based on an existing novel continuum strain-based one for 2D is developed. To achieve this objective, at first, several multiaxial fatigue tests are carried out on two representative welded details. These welded specimens are made of S690QL HSS and some of conventional S235JR structural mild steel. Advanced measurements techniques are used to quantify the parameters that drive the fatigue life and to determine the various stages of the fatigue damage process. Then, various multiaxial fatigue stress-based criteria are used in combination to the effective notch stress local approach to estimate multiaxial fatigue life of the tested specimens and of a database obtained from the literature. The results are statistically analyzed to help determine the most reliable multiaxial fatigue stress-based criteria and the ones that should be avoided, as well as their relative FAT categories based on various statistical approaches proposed in the EC3, IIW and in the literature. The von Mises equivalent stress expressed from stress ranges and the more complex and advanced Findley and Carpinteri-Spagnoli criteria have proven to be very well suited to study uniaxial and multiaxial fatigue under constant amplitude proportional and non-proportional loading, leading to safe estimation if combined to the proposed or updated FAT categories. On the contrary, the principal stress criterion has been shown to be inaccurate and leads to unsafe estimations as soon as multiaxial loading are involved. Thirdly, the novel continuum strain-based approach generalization in 3D is presented. This approach allows to precisely estimate each step of the fatigue damage process, including initiation, short crack propagation and propagation up to the final fracture, taking into account even more parameters than the previous local stress-based approaches. To estimate the fatigue effective stress in the damage process zone, the proposed generalized local approach is based on a so-called "area method". The equivalent fatigue effective strain time history is deduced from the fatigue effective stress time history using a continuum single element model (CSEM), validated under different individual loadings. The proposed approach allows to account accurately for complex elastic-plastic material behavior while avoiding effects of finite element singularities, mainly at the crack tip. The effect of mean stress is modelled while the effect of the residual stresses is also considered but in a simplified manner. Finally, the influence of the most important parameters that govern fatigue life, mentioned previously, is assessed by mean of parametrical analysis using the continuum strain-based approach. Estimated initiation fatigue lives and crack plane angles are shown to compare well with experimental data, determined by help of stereo Digital Image Correlation (DIC) measurements.