Critical volume approach in multiaxial fatigue assessment of notched specimens

Engineers are facing the problem of fatigue of materials since more than one century. This mechanical phenomenon is particularly challenging because it can lead to the catastrophic failure of mechanical parts without any visible warning signs. When notched components are considered, it becomes even more difficult to predict fatigue life. Presence of stress concentrators amplifies locally the stress field and makes its distribution across a load-carrying cross-section non-uniform. Due to the notches, the loaded component is subjected to multiaxial loading even if the external load is uniaxial. Multiaxial fatigue loading, which can result from the application of a combination of several forces along different load channels, requires sophisticated tools to be studied. Several approaches to multiaxial fatigue have been developed over last decades, and many multiaxial fatigue strength criteria based on them were proposed. This master’s thesis project, developed at the Faculty of Mechanical Engineering at CTU in Prague, proposes a new approach to deal with multiaxial fatigue which is based on critical volume. Since the statistical size effect plays a crucial role in fatigue strength of materials, the aim of the new approach is to involve concretely such an effect into the estimation of the damage parameter, which is performed by any multiaxial fatigue strength criterion. In particular, the Dang Van criterion has been taken into account for this study, while the experimental campaign has investigated the structural steel S355J2. Core idea of the proposed approach consists in modifying the material parameters, which are used as input in the multiaxial fatigue criterion, depending on the observed value of the critical volume. This insight originates from the fact that size effect turns out to not be effectively considered in widely used methods like Theory of Critical Distances (TCD) and Relative Stress Gradient approach, which are instead more focused on the stress gradient effect. TCD has been anyway implemented in the current work to get comparative results. The main outcome obtained from this work includes the evaluated dependencies between fatigue strength and critical volume for both cases of axial load and torsion load. Such dependencies can be used in conjunction with the Dang Van multiaxial fatigue criterion, since they allow the calculation of modified material parameters that should make the chosen criterion to perform more accurate multiaxial fatigue predictions. Final outputs from the method say clearly that further work is needed to improve how this approach is structured and to overcome some of its limitations, which represent boundary conditions fixed at the beginning to start with the study. However, the critical volume approach studied here provides valuable insights on how size effect should be properly integrated into multiaxial fatigue assessment.