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The fatigue crack propagation behavior of new generation Al alloys developed for aeronautical applications is studied at moderate ΔK and in the near threshold domain. The crack growth rate and the crack path are shown to depend on alloy composition, aging condition and atmosphere environment, and to be governed by the slip morphology. In absence of environment assistance, a crystallographic (1 1 1) faceted cracking leads to a slow stage I-like propagation in Al-Li-Cu alloys and underaged Al-Cu-Mg alloys with microstructure consisting of shareable precipitates or solute cluster structures that promote heterogeneous slip-band formation, which is in contrast with a ductile transgranular featureless stage II crack path in overaged Al-Cu-Mg. In air at moderate ΔK, an adsorption assisted propagation mechanism is assumed to prevail in both Li and Mg bearing materials, water vapor assistance inducing a transgranular stage II regime associated to homogeneous slip generating a flat-facet and step-like features; in the near threshold domain, the same mechanisms is operating for Al-Cu-Mg alloys while even more accelerated growth rates and lower effective threshold for Al-Cu-Li alloys are attributed to an assistance of hydrogen produce from the dissociation of adsorbed water vapor.  相似文献
2.
Microstructural evolution and mechanical properties of friction stir welded AA7075‐T6 aluminum alloy were examined. Grain structure and precipitate evolution in the stir zone and heat‐affected zone were evaluated using optical microscope and differential scanning calorimetry. A significant grain refinement and dissolution of η′ precipitates in the stir zone were found, but chromium‐bearing dispersoids remained nearly unchanged. The main particles in the stir zone and heat‐affected zone were η precipitates as well as Guinier‐Preston zones formed during post‐weld natural aging. The small recrystallized grains were observed in the thermo‐mechanically affected zone next to the stir zone. A W‐shaped hardness distribution where soft region was produced in the heat‐affected zone at a short distance from the stir zone were obtained. Hardness profiles of the welds were explained by precipitate distributions. Friction stir welding resulted in the reversion and coarsening of η′ precipitates. The formation of Guinier‐Preston zones in the stir zone and some parts of the heat‐affected zone during post‐weld natural aging increased the hardness. In transverse tensile specimens, fracture occurred in a location with the minimum hardness at either advancing or retreating side randomly. Further, influences of welding parameters on mechanical properties were investigated.  相似文献
3.
The main objective of this study was to predict the tensile relaxation modulus of asphalt mixes, without having to perform the common relaxation modulus tests, by developing a predictive model based on the mix characteristics, ageing condition, temperature and loading time. To this end, cylindrical asphalt mixture specimens containing crushed stone aggregates with 60/70 penetration asphalt binder were fabricated using two aggregate gradations, two binder contents, two air void levels and three ageing conditions with four replicates. Uniaxial tensile relaxation modulus tests were conducted on the specimens at four temperatures using the trapezoidal loading pattern at a low level of strain. Tensile relaxation modulus master curves of all the experimental combinations were constructed by the sigmoidal model. Statistical analysis of variance and regression analysis was performed on the test data and a predictive model was developed. Finally, the predictive model was verified using a group of measured values other than those used for the development of the model, and it was found that the predicted values correlated well with the measured ones.  相似文献
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