Application of modern aluminum alloys to aircraft

Crack tip shielding phenomena, whereby the “effective crack-driving force” actually experienced at the crack tip is locally reduced, are examined with reference to fatigue crack propagation behavior in metals, composites and ceramics. Sources of shielding are briefly described in terms of mechanisms relying on the production of elastically constrained zones which envelop the crack (zone shielding), on the generation of wedging, bridging or sliding forces between the crack surfaces (contact shielding) and on crack path deflection and meandering. Examples are taken from the fatigue behavior of high strength lithium-containing aluminum alloys, aluminum alloy-aramid fiber-epoxy laminate composites, and zirconia ceramics. It is shown that, whereas crack tip shielding can provide a potent means of enhancing “resistance” to crack growth, such extrinsic toughening mechanisms can result in the apparently anomalous behavior of “small cracks” and to the susceptibility of brittle materials to fatigue failure.

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Mechanisms of fatigue crack propagation in metals, ceramics and composites: Role of crack tip shielding☆

The variation of cyclic crack propagation rates, under thecombined influence of crack kinking (deflection) and fracture surface contact (closure), is estimated from simple linear elastic analyses of tilted cracks. The predictions of the models are consistent with the experimental results of linear and kinked crack advance in high strength aluminum alloys testedin vacuo. Examples of crack deflection in various engineering alloy systems and some generalizations ofaverage deflection parameters based on microstructural and mechanical factors are discussed. The individual contributions to overall growth rates from deflection and closure processes are evaluated for different mechanical and metallurgical conditions. The significance, implications, and limitations of the models are outlined.

Fatigue crack deflection and fracture surface contact: Micromechanical models

https://nanomechanics.mit.edu/sites/default/files/documents/Hanlon-et-al-nc-fatigue-IJF-2005.pdf

Fatigue behavior of nanocrystalline metals and alloys

The design of an aluminum alloy having good strength while maintaining a high resistance to fracture is discussed. Theory suggests that the desired microstructure consists of a small volume fraction of an ultra-fine dispersion of hard particles. In addition to conventional heat treatments, dispersion hardened aluminum alloys have been recently produced by rapid solidification or mechanically alloying and powder metallurgy consolidation. Alloys which can serve as models for mechanistic studies of nucleation of non-coherent phases as well as the basis for a new class of engineering aluminum alloys are identified.

Overview no. 102 Theory assisted design of high strength low alloy aluminum

The effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack growth resistance have been studied for a 7475 aluminum alloy. The alloy was tested in the underaged and overaged conditions with grain sizes of 18 μm and 80 μm. The fracture surface exhibited increased irregularity and planar facet formation with increased grain size, underaging, and tests in vacuum. These changes were accompanied by an increased resistance to fatigue crack growth. In air the 18 μm grain size overaged material exhibited relatively poor resistance to fatigue crack growth compared with other microstructural variants, and this was associated with a lower stress intensity for closure. All materials exhibited a marked improvement in fatigue crack growth resistance when tested in vacuum, with the most significant difference being ˜1000× at a ΔK of 10 MPa m1/2 for the 80 μm grain size underaged alloy. This improvement could not be accounted for by either an increase in closure or increased crack deflection and is most likely due to increased slip reversibility in the vacuum environment. The intrinsic resistance of the alloy to fatigue crack growth was microstructurally dependent in vacuum, with large grains and planar slip providing the better fatigue performance.

The effect of microstructure and environment on fatigue crack closure of 7475 aluminum alloy

The high cycle fatigue crack growth characteristics of coarse grainedα-titanium have been studied in vacuum, air, water and brine. Tests were carried out on single-edge-notch tension test-pieces at anR ratio of 0.35, a frequency of 130 Hz, and a ΔK range of 5 to 25 MPa-√m. The use of channelling patterns in the SEM permitted detailed crystallographic information to be correlated with fatigue fracture morphologies. Three distinct modes of fatigue crack growth were identified. Cleavage-like facets on the basal planes (0002); the formation of which was encouraged by increasing severity of environment and increasing stress normal to (0002), striations on planes normal to (0002) consistent with a mechanism involving intersecting prism slip systems, and furrows in the [0001] direction associated with fine lines parallel to the 1123 direction. It is shown how the relationships between grain orientation, stress geometry and testing environment may be presented in the form of Grain Orientation Control Maps. The rate of fatigue crack growth in individual grains was dominated by the mode of growth; below a ΔK of 10 MPa√m the cleavage-like mode was up to 10 times more rapid than the other modes of growth.

C. J. Beevers

Papers

The effect of microstructure and environment on fatigue crack closure of 7475 aluminum alloy

The influence of grain orientation on the mode and rate of fatigue crack growth in α -titanium