More info on the effect of anodizing on the fatigue of Al aerospace alloys. Most articles have only the abstract available; you can purchase the article or perhaps visit a university library.
IMPROVING THE FATIGUE RESPONSE OF AEROSPACE STRUCTURAL JOINTS
Cindie Giummarra and Harry R. Zonker
Alcoa Inc., Alcoa Technical Center, Pittsburgh, Pennsylvania, USA
Key Words: fretting, joints, fatigue, surface treatments, anodizing, peening
“
Abstract. The effect of various surface treatments on the fretting fatigue and joint fatigue performance of a 7xxx series aluminum alloy was investigated with the objective to reduce the nucleation and growth of fretting cracks and enhance the fatigue life of aerospace joints.
The results indicate that anodizing does not influence the fretting fatigue performance and the type of anodizing does not affect the joint fatigue life. UltraCem coating inhibited fretting crack nucleation in the fretting specimen, increasing the fatigue life. Shot peening increased the fretting fatigue life significantly due to the compressive residual stresses it imparts; however, the stresses were not deep enough to influence the fretting cracks which nucleated in the hole bore of the joint specimens. Laser peening and low plasticity burnishing induce deeper compressive residual stresses than shot peening, which appear to inhibit the growth of fretting cracks in both the fretting and joint specimens, resulting in a significant fatigue life improvement.”
A 12-page article (free). Very informative.
ICAF 2005 Proceedings, Hamburg Germany
Lots of info at the AERADE site, but only abstracts are available to non-subscribers. See
Aluminium alloys - endurance data - various alloys, mean stress, anodising, fretting. at
Mechanical properties of amorphous anodic alumina and tantala films using nanoindentation
G Alcalá et al. (2002)
Nanotechnology 13 451-455.
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Abstract. The hardness and Young's modulus of barrier-type, amorphous anodic oxides have been determined by nanoindentation. The procedure used shallow indents, of 55 nm depth, with alumina, tantala and alumina/tantala `mixed oxide' films of about 500 nm thickness. The results revealed respective hardnesses of approximately 7.0, 5.3 and 6.5 GPa, and respective Young's moduli of approximately 122, 140 and 130 GPa. Thus, the hardness and Young's modulus followed opposite trends, with alumina having the highest hardness and lowest modulus, and the `mixed oxide' having intermediate properties.
The hardness and Young's modulus of amorphous alumina are factors of about 3.1-3.7 times lower than those of crystalline aluminas.”
I included this only to show that anodic oxide properties differ from those of crystalline bulk alumina.
Don't know the anodize process used --
anyone with article please describe -- Ken.
Abstract, the article costs $30.
Humidity effects on the fracture mode transition in anodized 2024-T351 aluminium J. H. Wilson1, T. S. Sudarshan1 and H. H. Mabie,1
Journal of Materials Science Letters, Volume 3, Number 9, Pages: 773 – 775, September 1984.
“that while a thin anodized coating has little effect. on fatigue life, a thick coating will cause a decrease. in fatigue life. This decrease in life has ...”
That's all the free info. Available for purchase at
It's well-known that humidity has an effect on the properties of the anodic coating (partially hydrated oxide). The effect is relatively large for unsealed coatings cf. sealed coatings. I have more info in books if interested.
Effect of anodizing on the fatigue and corrosion-fatigue strengths of sheet duralumin with stress concentrators
A. V. Karlashov and R. G. Gainutdinov,
Kiev Institute of Civil Aviation Engineers, USSR
Materials Science
Volume 7, Number 5, Pages: 597 – 598, September 1974
Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 7, No. 5, pp. 87–89, September–October, 1971.
“The effect of anodizing on the fatigue strength of the alloy in air and in a corrosive medium is explained by both the residual stresses in the film and ...”
Note: The original Duralumin has evolved into the alloys known as 2017, 2117, and 2024. Don't know what was used in the USSR.
Article for sale at
Shear lips on fatigue fracture surfaces of aluminum alloys
J. ZUIDEMA, F. VEER, C. VAN KRANENBURG
Fatigue Fracture of Engineering Materials and Structures
Vol. 28 Issue 1-2 Page 159 January 2005
ABSTRACT
“A fatigue crack normally grows in so-called mode I, with a flat fracture surface perpendicular to the loading direction. Sometimes the crack front becomes slanted, at about 45° with the loading direction. In that case it is possible that the original crack growth direction is maintained, but also a deviated growth direction can be found. The paper describes various effects related to the occurrence of slant growth due to shear lips on fatigue fracture surfaces in (thin) sheets. After a general introduction the attention is focused on the relations between shear lips and fatigue crack growth. Questions about why shear lips develop and about other aspects of shear lip behaviour will be answered.”
Article costs $26. This one cited Wilson et al. (1984).