Interesting article on aerospace aluminum fatigue

[Peter Cole]

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

"Anodizing

Anodization is a mature, relatively low cost technology that is already
used by aircraft
original equipment manufacturers (OEMs) on large parts to prevent
corrosion. The aluminum
part is placed in an acidic bath for a given time under an applied
current which anodizes the
aluminum to produce a protective layer of Al2O3.
Different anodizing methods provide variations in the hardness,
thickness, and porosity of the
anodized layer as well as offering alternative levels of corrosion
performance and
environmental impact. However, it is commonly recognized that anodizing
can significantly
reduce the fatigue life due to the presence of pores and microcracks in
the anodized layer."

 

[dvt]

Peter Cole wrote:
> <http://www.lambdatechs.com/html/resources/258.pdf>
>
> "However, it is commonly recognized that anodizing can significantly
> reduce the fatigue life due to the presence of pores and microcracks
> in the anodized layer."

I seem doomed to follow you today... This is something I read a long
time ago, but never got around to posting:

<http://www.pfonline.com/articles/clinics/0503cl_alum1.html>

> Another interesting fact about dichromate sealing is that it can
> largely overcome the loss of fatigue properties caused by the anodic
> coating, especially hardcoat.

--
Dave
dvt at psu dot edu

 

[dvt]

dvt wrote:
> <http://www.pfonline.com/articles/clinics/0503cl_alum1.html>
>
>> Another interesting fact about dichromate sealing is that it can
>> largely overcome the loss of fatigue properties caused by the
>> anodic coating, especially hardcoat.

Following up to that... In a personal email from Larry Chesterfield (see
the link above), he said:

> Thanks for your question. If I were to write that article again the
> word I might leave out would be “largely” simply because it leaves a
> lot of room for interpretation. It is a demonstrated fact that
> sealing in general, and dichromate sealing in particular, does
> ameliorate somewhat the loss of fatigue strength in anodized
> aluminum.

He went on to supply references, which I have not taken the time to read:

> The Surface Treatment and Finishing of Aluminum and Its Alloys, by S.
> Wernick, R. Pinner and P.G. Sheasby, Fifth Edition, Vol 2, pg 720.

> Technology of Anodizing Aluminum, by A.W. Brace and P.G. Sheasby, pg
> 282.

> The Technology of Anodizing Aluminum, by Arthur W. Brace, Third
> Edition, pg 285. (Not the same as the second ref, but almost)

He even gave page numbers! I am truly lazy for not pulling those references.

--
Dave
dvt at psu dot edu

 

[[email protected]]

Peter Cole writes:

> "Anodizing

> Anodization is a mature, relatively low cost technology that is
> already used by aircraft original equipment manufacturers (OEMs) on
> large parts to prevent corrosion. The aluminum part is placed in an
> acidic bath for a given time under an applied current which anodizes
> the aluminum to produce a protective layer of Al2O3. Different
> anodizing methods provide variations in the hardness, thickness, and
> porosity of the anodized layer as well as offering alternative
> levels of corrosion performance and environmental impact. However,
> it is commonly recognized that anodizing can significantly reduce
> the fatigue life due to the presence of pores and microcracks in the
> anodized layer."

I posted that years ago and enraged all the wreck.bike guys who love
black rims. This is not new and I even posted pictures of crack
initiation showing an MA-2 cross section and a MA-40 that both had
significant mileage.

Jobst Brandt

 

[jim beam]

Peter Cole wrote:
> <http://www.lambdatechs.com/html/resources/258.pdf>
>
> "Anodizing
>
> Anodization is a mature, relatively low cost technology that is already
> used by aircraft
> original equipment manufacturers (OEMs) on large parts to prevent
> corrosion. The aluminum
> part is placed in an acidic bath for a given time under an applied
> current which anodizes the
> aluminum to produce a protective layer of Al2O3.
> Different anodizing methods provide variations in the hardness,
> thickness, and porosity of the
> anodized layer as well as offering alternative levels of corrosion
> performance and
> environmental impact. However, it is commonly recognized that anodizing
> can significantly
> reduce the fatigue life due to the presence of pores and microcracks in
> the anodized layer."

which is all dandy. the point regarding bike rims is whether anodizing
actually was responsible or whether there is another cause such as
extrusion flaws. it's easy enough to differentiate, /if/ you know what
you're looking at. hint: dye penetrant test won't do that for you.

 

[Donald Gillies]

An interesting cause-effect issue is that non-anodized rims probably
tend to get polished more frequently by bike freaks - and this
presumably removes or diminishes surface defects. When _cleaning_ an
anodized rim, the cleaning cannot remove surface defects that have
started to grow, because the anodization layer - 7x harder than raw
aluminum - keeps the cracks from being polished away, allowing them to
grow.

So while it may be true that anodized rims crack more often, it may
have nothing to do with the anodizing itself, and everything to do
with the fact that non-anodized rims tend to have their cracks removed
or thwarted by polishing ...

it may be true that painted rims crack just as much as anodized rims.
Are we to assume that painting causes rims to crack ??

- Don Gillies
San Diego, CA

 

[Tim McNamara]

In article <[email protected]>,
[email protected] (Donald Gillies) wrote:

> An interesting cause-effect issue is that non-anodized rims probably
> tend to get polished more frequently by bike freaks - and this
> presumably removes or diminishes surface defects. When _cleaning_ an
> anodized rim, the cleaning cannot remove surface defects that have
> started to grow, because the anodization layer - 7x harder than raw
> aluminum - keeps the cracks from being polished away, allowing them
> to grow.

I never polish rims, anodized, painted or plain. I wipe them off once
or twice a year with a rag. Do you really, seriously suggest that this
prevents non-anodized rims from cracking?

> So while it may be true that anodized rims crack more often, it may
> have nothing to do with the anodizing itself, and everything to do
> with the fact that non-anodized rims tend to have their cracks
> removed or thwarted by polishing ...

You've already alluded to the mechanism for anodized rims cracking- the
existence of a hard, brittle layer. Other industries recognize that
anodizing reducing the fatigue life of aluminum components (by as much
as 40% according to some previously cited articles). The effect is well
known and in most applications it is compensated for. But not in bike
rims.

> it may be true that painted rims crack just as much as anodized rims.
> Are we to assume that painting causes rims to crack ??

Umm, you seem to suggest that painted rims *may* crack as often as
anodized rims and then go on to ask if painting rims causes them to
crack, which assumes that painted rims crack. It's a logical error,
standard issue in politics but perhaps out of place in this discussion.

 

[Michael Press]

In article <[email protected]>,
[email protected] (Donald Gillies) wrote:

> An interesting cause-effect issue is that non-anodized rims probably
> tend to get polished more frequently by bike freaks - and this
> presumably removes or diminishes surface defects. When _cleaning_ an
> anodized rim, the cleaning cannot remove surface defects that have
> started to grow, because the anodization layer - 7x harder than raw
> aluminum - keeps the cracks from being polished away, allowing them to
> grow.
>
> So while it may be true that anodized rims crack more often, it may
> have nothing to do with the anodizing itself, and everything to do
> with the fact that non-anodized rims tend to have their cracks removed
> or thwarted by polishing ...
>
> it may be true that painted rims crack just as much as anodized rims.
> Are we to assume that painting causes rims to crack ??

A paint layer is not the same as an anodized (Al2O3)
layer. An anodized rim has a thick layer of Al2O3
tightly bonded to the malleable Al beneath. This is
like a scab over a wound on skin. When flexed it cracks
and tears the flesh beneath.

Al forms a thin layer of Al2O3 in air. When it is
painted the paint is softer than the layer of Al2O3
beneath it and does not bonded very tightly, so when it
cracks the stress riser is not great enough to tear the
Al2O3 and Al beneath.

--
Michael Press

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> <http://www.lambdatechs.com/html/resources/258.pdf>
>>
>> "Anodizing
>>
>> Anodization is a mature, relatively low cost technology that is
>> already used by aircraft
>> original equipment manufacturers (OEMs) on large parts to prevent
>> corrosion. The aluminum
>> part is placed in an acidic bath for a given time under an applied
>> current which anodizes the
>> aluminum to produce a protective layer of Al2O3.
>> Different anodizing methods provide variations in the hardness,
>> thickness, and porosity of the
>> anodized layer as well as offering alternative levels of corrosion
>> performance and
>> environmental impact. However, it is commonly recognized that
>> anodizing can significantly
>> reduce the fatigue life due to the presence of pores and microcracks
>> in the anodized layer."
>
> which is all dandy. the point regarding bike rims is whether anodizing
> actually was responsible or whether there is another cause such as
> extrusion flaws. it's easy enough to differentiate, /if/ you know what
> you're looking at. hint: dye penetrant test won't do that for you.

It's very likely that anodizing, besides being a source of defects for
crack initiation, will also amplify the effect of other material
defects. Anodize film growth is strongly affected by surface
irregularities, and discontinuities in film thickness create a lot of
residual stress.

The bottom line is that virtually any kind of anodizing, even over
prefect substrate, will lower aluminum fatigue by 15-50%. A really bad
process for bike rims.

 

[Peter Cole]

Another interesting paper:

http://rclsgi.eng.ohio-state.edu/~goetz/Goetz_Thesis_SP05.pdf

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> Peter Cole wrote:
>>> <http://www.lambdatechs.com/html/resources/258.pdf>
>>>
>>> "Anodizing
>>>
>>> Anodization is a mature, relatively low cost technology that is
>>> already used by aircraft
>>> original equipment manufacturers (OEMs) on large parts to prevent
>>> corrosion. The aluminum
>>> part is placed in an acidic bath for a given time under an applied
>>> current which anodizes the
>>> aluminum to produce a protective layer of Al2O3.
>>> Different anodizing methods provide variations in the hardness,
>>> thickness, and porosity of the
>>> anodized layer as well as offering alternative levels of corrosion
>>> performance and
>>> environmental impact. However, it is commonly recognized that
>>> anodizing can significantly
>>> reduce the fatigue life due to the presence of pores and microcracks
>>> in the anodized layer."
>>
>> which is all dandy. the point regarding bike rims is whether
>> anodizing actually was responsible or whether there is another cause
>> such as extrusion flaws. it's easy enough to differentiate, /if/ you
>> know what you're looking at. hint: dye penetrant test won't do that
>> for you.
>
> It's very likely that anodizing, besides being a source of defects for
> crack initiation, will also amplify the effect of other material
> defects. Anodize film growth is strongly affected by surface
> irregularities, and discontinuities in film thickness create a lot of
> residual stress.
>
> The bottom line is that virtually any kind of anodizing, even over
> prefect substrate, will lower aluminum fatigue by 15-50%. A really bad
> process for bike rims.

but not in the context of the real world where pitting and corrosion can
initiate cracking much more quickly. it's a trade. aluminum rims
ridden outside of the stanford area are subject to all kinds of
environmental conditions and also wear. if the anodizing-affected
fatigue life of an anodized rim exceeds the rim's wear life, bleating
about anodizing's alleged evil is an exercise in bovine excrement
deployment.

 

[[email protected]]

David Rees <[email protected]> writes:

> I once read that crack propagation due to anodizing was not a
> problem with Al because the oxide layer (it helps me to think of it
> as ceramic) does not adhere that tightly to the substrate, whereas
> it's a big problem with Ti.

The ceramic oxide is par of the base metal and is as well connected as
can be. The main effect can be seen when bending a hard anodized
piece of aluminum and one that is cosmetically anodized. The hard
coat makes visible large cracks while the other crazes with such
closely spaced fine cracks that they are not readily visible. Neither
coating flakes off as would be the case if they were not well
attached.

Today, anodized rims have only a cosmetic thin layer because
manufacturers became aware of the effect.

Jobst Brandt

 

[bfd]

<[email protected]> wrote in message
news:[email protected]...
> David Rees <[email protected]> writes:
>
> Today, anodized rims have only a cosmetic thin layer because
> manufacturers became aware of the effect.
>
Does that mean *todays* anodized rims, like the Mavic Open Pro, are better
and not as prone to cracking as anodized rims like the old Mavic MA40?