Unexplained 2024 Fatigue Failures 1

[vibeman1336]

Hello everyone,

We have been using 7000 series aluminum extensively for years in our products and have developed FEA-based fatigue analysis methodologies that are effective in predicting test success. Recently, we have used some 2024-T351 (AMS 4120)in an application similar to one where we had used 7075-T73 (AMS 4141 or alternately AMS 4124) in the past. Type 2 anodize (thin hard) was specified for either material. In two instances to date, 2024 components have failed with no errors discovered in the analysis performed or in the material properties or part geometry. Per MIL-HDBK, the two alloys are roughly equivalent in strength, with the only significant-appearing difference in elongation (7075 has much more capability in elongation, but that should only come into play upon yielding). Is anyone aware of fatigue issues with anodized 2024? Can anyone shed light on the 'mystery'?

Thanks,

 

[kenvlach]

Anodizing lowers the fatigue curves, and the thicker the anodize, the greater the reduction. The anodic layer is primarily Al₂O₃, so it can crack when the substrate flexes, serving as a source of surface flaws.
Critical fatigue parts are shotpeened before anodizing to minimize this crack progation into the substrate.

This is a major reason aircraft parts have traditionally been chromic acid anodized (Type I per MIL-A-8625F). Chromic acid anodize is thinner and (if I remember correctly) more flexible than sulfuric acid anodize.

 

[TVP]

You need to have a thorough metallurgical analysis performed. Potential issues that can affect fatigue (other than the anodizing issue that kenvlach already identified) include surface defects, excessive non-metallic inclusions, corrosion, and unfavorable residual stresses. Have you already investigated these items? What about basics like grain size, mechanical properties of the actual parts, etc.?

 

[kenvlach]

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

http://www.lambdatechs.com/html/resources/258.pdf

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

http://aerade.cranfield.ac.uk/subject-listing/esdu/ES104.html

Mechanical properties of amorphous anodic alumina and tantala films using nanoindentation
G Alcalá et al. (2002) Nanotechnology 13 451-455.  
“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.

http://www.iop.org/EJ/abstract/0957-4484/13/4/302

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

http://www.springerlink.com/(zkg0s2...,217,249;linkingpublicationresults,1:400129,1

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

http://www.springerlink.com/index/U878Q41682R55525.pdf

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).

http://www.blackwell-synergy.com/links/doi/10.1111/j.1460-2695.2004.00837.x

 

[kenvlach]

Besides examining for non-metallic inclusions as mentioned by TVP, look for large intermetallics. I once used SEM to pinpoint the origin of anodize defects on some expensive Al 7075-T6 forgings. Expected to find only the tiny heat-treat precipitates (10-100 nm) at max resolution. Instead, found primary solidification intermetallics 25 microns long that had survived forging & solutionizing heat treatment.
This problem occurred from time to time with every source, including Alcoa, Norsk, Pechiney, Kobe, etc. (customer tried every big-name producer in the world!). You can filter the pour and call it 'clean,' but the problem arises afterwards. 2024 is as bad or worse than 7075 w.r.t. non-solutionizing intermetallics IMHO.

Al 2024 is the most corrosion-prone of all the common Al alloys. It is worse than any non-2xxx alloys under any surface treatment condition -- anodize, Alodine, paint...
The intermetallics have a different galvanic potential than the aluminum matrix (bad enough) and interfere with the formation of protective anodize or chromate coatings (worse). Also, during anodizing of 2024, the metal surface just below the anodize becomes enriched in copper, so more galvanic effects.

An interesting alloy.
Ken

 

[MikeHalloran]

2024 is also interesting because it machines better than butter and has a high yield point.



Mike Halloran
Pembroke Pines, FL, USA

 

[kenvlach]

What's the temperature of the butter???

Mike is correct, there seems to be an inverse relationship between machinability and corrosion resistance.

 

[thundair]

For what it is worth no one mention that 2024 hates heat

Regards

 

[metman]

vibeman1336 -- Besides all the possible contributing factors posted, you said, "with the only significant-appearing difference in elongation (7075 has much more capability in elongation, but that should only come into play upon yielding. Precisely. Localized yielding tends to blunt the crack tip and mitigate the effect given by kenvlach as “A fatigue crack normally grows in so-called mode I, with a flat fracture surface perpendicular to the loading direction.... more elongaion promotes localized yielding enhancing fatigue life.

Is 7075 more corrosion restistant than 2024? If so SCC could be a factor depending upon environment.

 

[sreid]

You might find answers at this website;

http://fatiguecalculator.com/index.htm

 

[vibeman1336]

Hey, thanks for responding.

We are very experienced in the use of anodize of all types and thicknesses. A thorough metallurgical analysis was done, revealing no anommilies. And our 'tried and true' FEA-based analysis results were reviewed with no findings. The question I really want to address with materials experts is 'Are there any peculiarities associated with the 2024 alloy in the anodized state that might help explain this fatigue failure?'. Why do our 'tried and true' methods work adequately with 7075, but result in unpredicted failure with 2024?

Thanks for taking the time to consider my question.
Happy Holidays!

 

[vibeman1336]

Oops,

I am new to this sight and did not see some of the responses. I will ask our metallurgist to consider kenvlach response concerning intermetallics. In fact, I am refering this entire string to our metallugist.

Thanks

 

[swall]

vibeman1336--were the product forms for the 7075 and 2024 components the same? I.E. were they both plate or both extrusions so that we have an apples to apples comparision?

 

[kenvlach]

A search of the International Journal of Fatigue turned up 104 articles for 2024, many with 2024-T3 or -T351 in the title. Abstracts are free, articles are $10.

http://www.sciencedirect.com/scienc...serid=10&md5=87dccd9b42bd064f632032d1c8a08e99

E.g.,
"A study of nucleation and fatigue behavior of an aerospace aluminum alloy 2024-T3," Ali Merati, Institute for Aerospace Research, Ottawa, Canada. Jan. 2005, Pages 33-44.
Abstract
"The fatigue failure process exploits the weakest links (discontinuities) within the test material, which act as nucleation sites for crack origins. This paper summarizes the results of a study on the 2024-T3 aluminum alloys in different forms (clad and unclad), loading directions, thickness and environment. Microstructural features such as particles, grain size, and clad layer, which dominate fatigue performance have been identified, classified, and statistically characterized. Two distinct mechanisms were found to be responsible for crack nucleation. Constituent particles were found to be the crack origins in unclad sheets...."

Good that you have a metallurgist available.