Anodising of alluminium components?

[Jon Oneill]

Hi guys,

This year we're going into much greater depth in our design activities, and have descovered some interesting information regarding aluminium anodising.

A paper "the influence of anodization on fatigue strength...." by A. CAMARGO and H. VOORWALD outlines the significatn effect of anodisation on the aluminium materials fatigue strength. In high cycle loaded components, it can have up to a 50% reduction in fatigue strength.

I wondering if any one else has come across this phenominon, and if it has influenced there choices in the past. We were planning on anodising our rim centres and stub axles, but now we'll probably be avoiding that option.

Its also interesting to note that some painting methods have been said to have a similar impacts upon the component as well.

Any thoughs or ideas? We're quite surprised here at UTS as its quite a significant bit of info, and we've never heard of it before. We'd just like to see if anyone else has, or has even seen the impacts in person?

[Jon Oneill]

Hi guys,

This year we're going into much greater depth in our design activities, and have descovered some interesting information regarding aluminium anodising.

A paper "the influence of anodization on fatigue strength...." by A. CAMARGO and H. VOORWALD outlines the significatn effect of anodisation on the aluminium materials fatigue strength. In high cycle loaded components, it can have up to a 50% reduction in fatigue strength.

I wondering if any one else has come across this phenominon, and if it has influenced there choices in the past. We were planning on anodising our rim centres and stub axles, but now we'll probably be avoiding that option.

Its also interesting to note that some painting methods have been said to have a similar impacts upon the component as well.

Any thoughs or ideas? We're quite surprised here at UTS as its quite a significant bit of info, and we've never heard of it before. We'd just like to see if anyone else has, or has even seen the impacts in person?

[Trevor]

I'd heard of this too, but only in passing in materials science textbooks. I'd be really interested to hear more about it as well, as I would expect anodizing to have no effect or a slight increase in fatigue life if anything.

My thinking is that the oxide layer is so thin it can't really effect anything, and my understanding is the process doesn't effect more than just the surface. Anodizing does seem to help surface wear, so it's nice for things like sprockets and suspension clevises. If anything this harder surface would discourage the creation of cracks (and thus fatigue) assuming the part doesn't have some crazy stress raiser in it.

I've never heard of a failure attributed to anodizing, and we've been doing it to critical parts for years.

[JDS]

I think fatigue life issues would depend on the type and thickness of anodizing. It is my understanding that fatigue life has a significant decrease only with Type I (hard) anodizing. But admittedly I have never done any research on this myself.

[exFSAE]

Maybe anodizing reduces fatigue life.

On the other hand, with a bunch of shop monkeys banging on the car, how much is the fatigue life reduced with bare aluminum parts when they get all nice and dinged up with mini stress risers all over?

[Hector]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Trevor:
My thinking is that the oxide layer is so thin it can't really effect anything, and my understanding is the process doesn't effect more than just the surface. </div></BLOCKQUOTE>
...which is where fatigue cracks form.
Once the crack is started, it's only going to grow deeper...

[flavorPacket]

I have seen a massive fatigue failure of a critical suspension component on a FSAE car because of anodizing. Yes, it happens, but normally only if your car is reliable enough to fatigue its parts...

[Jon Oneill]

From what I understand anodizing hardens the surface, making it more resilient, but it also has the effect of making the surface more brittle.

As Hector said, the material surface is where the fatigue cracks initiate, so a hardened surface, being more brittle should create more cracks and lower the components fatigue limit.

As I mentioned earlier, i'm fairly sure that some forms of painting have a similar influence upon parts. If this is true, how does anyone justify anodising rims or wheel centres. Does this justification also apply to painting, as i'm pretty sure most race wheels are powdercoated?

[vandit]

Yes, anodizing reduces fatigue strength but not 50% in every case. The % reduction in fatigue strength depends upon the thickness of your component and also on thickness of coating.

If it is a thin component like sheet, then the Fatigue strength can get reduced by great deal because anodizing always eats base material.

The thicker the anodizing layer, the greater the reduction in fatigue strength because you have a thick hard brittle layer as a surface (where fatigue cracks initiate).

Amongst Phosphoric acid, chromic acid and Sulphuric acid anodic layers, Sulphuric acid is most damaging. Phosphoric acid anodic layer is slightly better.

We anodized our uprights back in 2006 but that time we were excited to have the right color and were ignorant about reduction in fatigue strength. But so far, those uprights are still intact.

Speaking from my job experience, anodizing will reduce fatigue strength . It is a process meant for resistance against corrosion and wear and costs reduction in strength. If your component is thick enough and your stress levels are under control then anodizing shouldn't be a problem.

@flavorPacket,
Personally, i would love to see the pictures of the failed suspension component. It is my job to study failure of structures.

[sbrenaman]

There are two popular types of anodizing Type I and Type II/III. Type I is a thinner (&lt;0.001") more cosmetic layer, and Type II/III is a thicker (~0.001&lt;x&lt;~0.006") what is commonly referred to as "Hard" anodizing. There's a difference between II and III but I'm not sure what it is, I recall that there are some environmental/process considerations with one, making the other the standard.

There is a lot of good info here but I think the two types of anodizing are being used in this thread interchangeably and I think it's worth mentioning the differences.

[Richard Pare]

Anodizing does indeed reduce the fatigue strength, and it doesn't matter what Type ( I, II, or III )you are talking about.

The hard layer produced by anodizing is very brittle, not very strong, and will crack at stress levels well below the parent material. Once the crack is formed, it acts as a stress riser, concentrating the loads and ultimately causing the parent material to crack.

[Superfast Matt McCoy]

Anodize grows columnar out from the surface, which, as Hector pointed out, is where fatigue failure starts. The anodize is not strong in tension perpendicular to the columns, so the endurance limit is significantly affected. We used to post-machine areas of the anodize on retainers in high stress areas (we used hard anodize as a wear surface). also, any decent anodize shop will be able to mask off areas that shouldn't be coated.

The endurance limit on components of these cars is often not as important as stiffness or low cycle fatigue, but if you've done the FEA and you know where the critical stress areas are, you might as well give yourself another chance to impress the design judges with some carefully placed un-anodized areas.