Even atmospheric moisture is recognized to have a little corrosive effect on fatigue performance of aluminum alloys. Much high-cycle S- N testing has been carried out in uncontrolled ambient lab air environments, thereby contributing to scatter in existing data. This factor should be recognized when comparing results of different investigations. Unlike sustained-load SCC, fatigue degradation by environment may occur even when the direction of principal loading with respect to grain flow is other than short-transverse.
Fatigue response to environment varies with alloy, so final alloy selection for design should address this important interaction. When accumulating data for this purpose, it is recommended that any testing be conducted in a controlled environment, and preferably the environment of the intended application. However, an environment known to be more severe than that encountered in service is often used to conservatively establish baseline data and design guidelines. Because environmental interaction with fatigue is a rate-controlled process, interaction of time-dependent fatigue parameters such as frequency, waveform, and load history should be factored into the fatigue analysis Ref 39, 40, Corrosion fatigue performance of 7XXX alloys may, in general, be upgraded by overaging to the more corrosion- resistant T7 tempers Ref 42, 43, 44, 45, 46, 47 , as indicated by results shown in Fig.
With 2XXX alloys, the more corrosion-resistant, precipitation-hardened T8-type tempers provide a better combination of strength and fatigue resistance at high endurances than naturally aged T3 and T4 tempers. However, artificial aging of 2XXX alloys is accompanied by loss in toughness with resultant decrease in fatigue crack growth resistance at intermediate and high stress intensities Ref 45, Source: Ref 33 Fig. In air, the cladding appreciably lowers fatigue resistance. In seawater, benefits of the cladding are readily apparent.
Reduced Porosity Materials. The size of microporosity in commercial products is affected by the forming processes used in their production. A recent program was undertaken to determine whether the fatigue strength could be improved by the control of microporosity.
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Five variants of plate were produced to provide a range of microstructures to quantify the effects of intrinsic microstructural features on fatigue durability Table The material is characterized by extensive amounts of centerline microporosity. Despite the centerline microporosity, this material still meets all existing mechanical property specifications for thick plate.
The new-quality material represents the current benchmark for commercially available material. The processing methods used in the production of the new-quality material are a result of a statistical quality control effort to improve alloy thick plate Ref Material taken from two plant-scale production lots of each quality level provided the material for this program.
Both materials are 5. Static mechanical property characterization of the two plate pedigrees showed no significant differences in properties other than an increase in short transverse elongation for the new-quality material Ref 49 , and both materials meet the AMS material specification minimums. The fact that both materials meet the property requirements of the AMS specification underscores the limitation of existing specifications in that they do not differentiate intrinsic metal quality.
Smooth axial stress fatigue tests were performed for both the old-quality and the new-quality plate materials.
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The tests were done on round bars with a gage diameter of Gage sections were sanded longitudinally to remove circumferential machining marks. The specimen orientation was long-transverse L-T relative to the parent plate. The lifetimes of the specimens are plotted in Fig. The data show that the cumulative distribution of fatigue lifetimes for the new-quality material is substantially longer than for the old-quality material.
The holes were deburred by polishing with diamond compound only on the corners and not in the bore of the hole; this resulted in slight rounding of the corners. The fatigue lifetime data are plotted in Fig. Report This.
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