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- Evaluation and numerical modeling of phenomenological approach for AZ31B-F magnesium alloy under multiaxial fatiguePublication . Moreira, R.; Anes, Vitor; Freitas, M. De; Reis, L.Magnesium alloys have been attractive to use in structural components due to their high strength to weight ratio, low density and high damping capacity. However, magnesium alloys show peculiar plastic deformation mechanisms under cyclic loads (twinning and de-twinning) that causes the asymmetric material behaviour and limits their use in structural components. Recent researches indicate that this type of plastic deformation mechanism cannot be fully characterized using the typical tools used in steels. Therefore, the phenomenological Hypo-strain (HYPS) model has been developed to capture the asymmetric behaviour of magnesium alloys under uniaxial and multiaxial loadings. This study aims to evaluate the phenomenological Hypo-strain approach for AZ31B-F magnesium alloy and to implement the HYPS model on an external subroutine (UMAT) to run on Abaqus. The goal is to reach a numerical tool that can be used to accurately describe the cyclic elastic-plastic behaviour of magnesium alloys in synergy with finite element packages. In order to characterize the cyclic behaviour of AZ31B-F magnesium alloy, experimental tests were performed considering proportional and non-proportional loadings. To evaluate the implemented model in UMAT, these results were correlated with the experiments and with the analytical HYPS approach. Moreover, the estimates were also correlated with the Armstrong-Frederick model available on Abaqus/Standard 6.14 library. The results have shown that the HYPS model was successful implemented on the UMAT subroutine with a good correlation between experimental tests and the HYPS model. Some remarks between the HYPS and Armstrong-Frederick models are drawn.
- Fatigue damage assessment under random and variable amplitude multiaxial loading conditions in structural steelsPublication . Anes, Vitor; Caxias, J.; Freitas, M.; Reis, L.Fatigue damage assessment of multiaxial random loadings is a complex issue and a subject of actual interest in mechanical design. In this work, the performance of the stress scale factor (SSF) criterion is evaluated under variable amplitude loading conditions, and damage accumulation approaches. This evaluation is performed by taking into account two types of loading spectra, namely the loading block spectra (where the loading pattern is well identified and repeated until rupture), and the random loading spectra (where the stochastic behaviour of the axial and shear loading components do not allow a direct identification of the loading pattern). Moreover, the validity of the hypothesis in which the SSF damage map remains valid for any high strength steel under variable amplitude loading conditions is also inspected by analysing fatigue life correlation of the 1050 QT steel and the 304L stainless steel under a multiaxial loading block.