Lightweight materials for aircraft applications

https://courses.washington.edu/mengr541/ramulu/541/notes/fatiguethreshold%20review.pdf

Near-threshold fatigue: a review

The current status of the effects of impurities and rare earth elements on the microstructure and the mechanical properties of Al−Li alloys has been surveyed in the present paper. Particular attention has been paid to the mechanical properties in respect of different conditions and the mechanisms responsible for both impurity-induced embrittlement and rare earth element microalloying. Al−Li alloys usually show a high sensitivity to impurity-induced embrittlement. High hydrogen concentration in Al−Li alloys results in severe hydrogen embrittlement. Impurities Fe and Si could form some brittle compound particles to produce the negative effects on the microstructure. Alkali metal impurities such as Na and K could result in the formation of grain boundary liquid, recrystallized grains, coarse particles distributed along grain boundaries and δ′ (Al 3 Li) precipitation-free zones (δ′-PFZ). By reducing the tendency for brittle intergranular fracture, the effects of rare earth elements, such as cerium and scandium, as alloying additions, on the microstructure and the mechanical properties are usually beneficial. Rare earth modification for Al−Li alloys improves ductility, fracture toughness and possibly reduces their anisotropy to a certain degree. These microalloying effects of rare earth additions might suppress the impurity impairment. Adding an amount of rare earth elements somewhat reduces the stress concentration sensitivity in high strength Al−Li alloys while the notch strength is hardly affected. Though the microalloying of rare earth elements improves the mechanical properties, in particular, the ductility of smooth specimens, high strength Al−Li alloys still have high sensitivity to stress concentrations. This might be a possible obstruction to the widespread application of high strength Al−Li alloys in primary aerospace structures containing geometry notch design. In order to achieve an essential improvement in the ductility and fracture toughness of Al−Li alloys, it is necessary to find further efficient alloying approaches associated with rare earth constitutents.

Overview of the effects of impurities and rare earth elements in Al−Li alloys

Fatigue crack growth of UDIMET 720 Li superalloy at elevated temperature

The material-dependent variability of fatigue crack growth rates of aluminium alloys in the Paris regime

A series of fatigue-crack growth rate (FCGR) tests was carried out on 8090 Al-Li plate to examine the effects of specimen orientation on fatigue-crack growth. The directionality of fatigue fracture behavior is found to be related to the strong {110}〈112〉 texture in this alloy. Based on a previously developed transgranular FCGR model using restricted slip reversibility (RSR) concepts, [1] a mechanistic model is developed for transgranular fatigue-crack growth in highly textured materials. The model takes the form of the Paris relationship with a power law exponent of 3, and the material texture is shown to strongly influence the proportional factor. The effect of texture on FCGR is related through a geometric factor cos2
 ϕ, where ϕ defines the angle between the load axis and the normal of the favorable slip plane. The effect of specimen orientation on FCGR in 8090 Al-Li alloy is shown to be related to a combination of its anisotropic mechanical properties and the variation of angleϕ with specimen orientation. The model further predicts that fatigue-crack growth rates will be slower in many textured materials than texturefree materials becauseϕ > 0 and cos2
 ϕ < 1.

The orientation dependence of fatigue-crack growth in 8090 Al-Li plate

Near-threshold fatigue crack growth was studied in 8090-T8771 Al-Li alloy tested in moist laboratory air. The testing was conducted using (1) the ASTM E-647 load-shedding procedure, (2) a power-law load-shedding procedure, and (3) a constant-amplitude (CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs) with different crack closure levels under identical testing parameters, the conventional {Delta}K{sub eff} (=K{sub max} {minus} K{sub op}) fails to correlate the test data and the modified {Delta}K{sub eff} (=K{sub max} {minus} {chi}K{sub op}, where {chi} is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this ally. The model explains the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF) in terms of microstructure-environment synergy. The transition is related to the material`s short-transverse grain size.

Near-threshold fatigue crack growth in 8090 Al-Li alloy

Experience pratique concernant une augmentation de la duree de vie basee sur le concept de tolerance aux dommages dans le cas de composants de moteurs a turbine

M. D. Raizenne

Papers

The orientation dependence of fatigue-crack growth in 8090 Al-Li plate

Near-threshold fatigue crack growth in 8090 Al-Li alloy

Practical experience with damage tolerance based life extension of turbine engine components