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- Bonded joints of dissimilar adherends at very low temperatures - An adhesive selection approachPublication . Anes, Vitor; Pedro, R.; Henriques, E.; Freitas, M.; Reis, L.Maintenance, repair and overhaul companies have been reporting corrosion failure events in the Airbus A320 CFM56-5b intakes. These intakes are attached to the power plant frame by a dissimilar material bonded joint, where liquid shim adhesive is used to avoid the dielectric formation between dissimilar materials. In previous works, the authors reported that the A320 intakes corrosion is a result of the adhesive inability to avoid the dielectric formation, which is a result of micro-cracks formation within the adhesive layer. The main reason that lead to these cracks is the adhesive aging and thermal cycling at very low temperatures, which quite often reach values lower than -50ºC. This paper studies the effect of negative thermal loading on dissimilar materials bonded joints. Two epoxy adhesives are studied and compared, namely the Hysol EA-934, which is the adhesive currently used in the A320 Airbus intakes, and the Hysol EA-9394, a second generation adhesive candidate to replace the actual adhesive. A numerical study was performed in order to simulate the adhesive joint using a finite element analysis commercial package, where several hypotheses were explored. These hypotheses where focused on the effects of several factors on the adhesive layer stress distribution. Factors such as temperature range, boundary conditions, variation of the coefficient of thermal expansion with temperature, and interfacial cracks between the adhesive layer and dissimilar adherend materials were analyzed. Results show that very low temperatures have a negative impact on the adhesives strength and micro-cracks formation may result from thermal loads below zero degrees Celsius, even for adhesives without any aging. Moreover, low temperatures in dissimilar materials bonded joints may create stress states that surpass the adhesive lap shear strength. Some conclusions are drawn regarding adhesive selection for dissimilar materials bonded joints in order to overcome these issues.
- On the determination of J-resistance curve of metallic materials using the unloading compliance technique and normalization methodPublication . Leite, A.; Balhana, A.; Anes, Vitor; Reis, L.In the present work, a study is made about the experimental determination of the fracture resistance curve (R curve) of metallic materials, using the methods: resistance curve and normalization, guided by the ASTM E 1820 standard. The resistance curve method, used here, consists in obtaining the “R curve” (expressed in integral J) through a single specimen. Within this method, the elastic compliance technique was used to obtain crack extension values. The normalization method consists in obtaining the resistance curve (expressed in integral J) directly from the force vs displacement diagram and the initial and final crack lengths, measured at the fracture surface. wo sets of specimens of two metallic materials were tested. Two specimens from each set were submitted to the normalization method, while the remaining were studied with the resistance curve method, implemented in the Instron Wavematrix software, based on the procedure of ASTM E 1820. It was not possible to obtain the value of Fracture Toughness in plain strain, JIC, since ductile crack sizes did not comply with all mandatory items of the standard ASTM E 1820, namely a crack tunnelling effect was observed, being the crack front much deeper than its value near the free surface Within each material, the respective resistance curves were compared using both methods.
- 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.
- The damage scale concept and the critical plane approachPublication . Anes, Vitor; de Freitas, M.; Reis, L.Critical plane criteria seek the plane in which their damage parameter has its maximum value. On this plane, the fatigue damage of both normal and shear stresses is updated to the same damage scale by a constant, typically based on material normal/shear fatigue limits. This paper focuses on the damage scale concept under a stress-based critical plane standpoint, and the main objective is to evaluate its performance. To perform this study, data from the high strength steel 42CrMo4 were considered. Five proportional loading paths with different stress amplitude ratios and stress levels were analysed using 2 different multiaxial fatigue criteria, namely, the Findley critical plane model and the stress scale factor equivalent shear stress. Results show that the damage scale between normal and shear stress components varies accordingly to the loading plane direction considered.