ANTUNES, A. M. B. S. Study of the high cycle fatigue behavior of the AA6351 and AA7050 aluminum alloys for aeronautics applications. 2017. 195 p. Thesis (

The study of fatigue crack growth (FCG) is aimed at residual life estimations in order to apply the damage tolerant criterion. Usual approaches are based on semi-empirical models that consider the stress intensity factor range of fracture mechanics,ΔK, as the governing driving force for crack propagation. An alternative approach is the use of predictive theoretical schemes arising from damage mechanics. Although they havent achieved a reliability level high enough to be used in design, predictive models may be important in some situations like material selection. In the present work, a predictive FCG method based on the cumulative damage of volume elements along the crack path is employed. The development of the work includes considerations about the stress distribution in the cracked body and the stress-life and strain-life relations used in the computational procedure. A previously developed analytical expression for the stress distribution ahead of the crack in a finite width plate, based on the numerical analysis performed by the Finite Element Method, is used in the predictive method. The stress field is determined for both upper and lower limits of cyclic loadings. The fatigue crack growth behavior of three Al-Mg-Si alloys: AA 6005, AA 6351 and AA 6063, tempered and aged for the T6 condition, was analysed for positive and negative R-ratios. In order to check the model results, constant amplitude FCG tests with load ratios ±0,5 were carried out in M(T) specimens. The experimental results, compared to the computational simulations, show that it is possible to obtain predictions of FCG behaviour for both positive and negative load ratios.

Al-Mg-Si alloys (6xxx series) are medium strength structural alloys, with good corrosion resistance, good weldability and high damping capacity. They represent a high volumetric fraction of extruded aluminium alloys which are produced for commercial use and have been increasingly applied in the automotive industry. For structural materials, the fatigue strength is the most important factor to ensure a long-term reliability. Engineering structures such as aircrafts and automobiles usually undergo complex multiaxial loadings, which lead to changes of the principal stresses and strains directions in components during a loading cycle. In this study, fatigue tests were performed in three Al-Mg-Si alloys, namely AA 6005, AA 6351 and AA 6063, tempered and aged for the T6 condition. A comparative study was undertaken by assessing their Low Cycle Fatigue (LCF) properties and multiaxial fatigue behaviour using round smooth specimens. Strain-controlled fully reversed axial loadings and distinct combinations of axial-torsional fully reversed stress cycles, including in-phase and 90o out-of-phase loadings were adopted for the tests. The collected data are discussed in relation to some well-known multiaxial fatigue models.

Over the past years, Al-Mg-Si alloys have been largely applied in automotive industry, which has required a deep knowledge of their mechanical properties and the influence of precipitates distribution on their mechanical behavior. This work evaluated the main mechanical properties of AA6005, AA6063, and AA6351 alloys by means of tensile and low cycle fatigue tests with 0.005 seg -1 deformation rate and 0.3% < ε at <1.2% strain amplitudes. Besides, the hysteresis loop and internal stress analysis were investigated to analyze hardening and softening phenomena and to evaluate the friction and back stresses, respectively. Macro and microstructural were performed focusing in intermetallic distribution. Concerning the low cycle fatigue behavior, AA6351 presented shorter lives for strain amplitudes higher than 0.5%, and AA6005 showed the highest fatigue strength and fatigue ductility. AA6063 showed the lowest fatigue strength due to the presence of coarse particles (Fe,Mn)SiAl. During internal stress analysis, the highest value of friction stress for AA6351 indicates the effect of hardener precipitates are the most relevant role for cyclic loadings and the lowest back-to-friction stress ratio indicating that deformation is controlled by particles. In the other hand, AA6063 showed the lowest friction stress due to low amount of fine precipitates.

The mechanical properties of age-hardenable aluminum alloys are strongly influenced by the volume fraction, size and spacing of hardening precipitates. In addition, researches have shown that interrupted ageing (T6I4) could benefit the hardening response of these alloys. Interrupted ageing is a term generally used for the microstructural development process of an alloy aged at a reduced temperature after partially aged at a higher temperature. Thus, this process produces a change in the mechanical properties of the alloy. In this study, the precipitate structures and mechanical properties of AA6351 and AA7050 alloys were investigated. Transmission electron microscopy analyses of the hardening precipitates were carried out. Hardness and tensile properties were performed to compare the effect of disrupted ageing on AA6351-T6 / AA6351-T6I4 and AA7050-T7451 / AA7050-T6I4 alloys. AA6351-T6I4 showed a higher volumetric fraction of hardening precipitates with heterogeneous size and resistance lower than AA6351-T6. In addition, AA7050-T6I4 showed a higher volumetric fraction of hardening precipitates with smaller size and similar resistance to AA7050-T7451. However, in both cases, interrupted ageing contributed to increase ductility and toughness.

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Zirconia ceramics (ZrO2) are bioinert materials with excellent biocompatibility, high resistance to corrosion and to wear, high toughness in comparison with other advanced ceramics, and suitable for various structural applications. These properties are related to their microstructure and effects caused by crystalline phases transformation, intrinsic of zirconia. In this work, stabilized zirconia ceramic (ZrO2 with 3 mol % yttria) was produced using the synthesized powder obtained by the sol-gel process, in which citric acid was chosen as complexing agent and maize starch as gelling. The zirconia ceramic was characterized with respect to relative density (99.75±0.10 %), crystalline phases (predominantly tetragonal), microstructure (homogeneous and small grains), flexural strength (510±60 MPa), Vickers hardness (11.6±0.3 GPa) and fracture toughness (Niihara = 11.8±2.9 MPa.m1/2 and Evans = 10.9±1.2 MPa.m1/2). It can be concluded that the sol-gel process is an attractive route to obtain zirconia ceramics with good mechanical properties.