Anodising of alluminium components?

[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.