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Anodized no-brand crank, fails at pedal eye

Anodized crank fails at pedal eye. From https://www.flickr.com/photos/63373992@N07/11668277266 as of 2014/08.

No reports on rider injuries. Service history unknown. No notes on how the failure was discovered.

The crank is anodized. Anodizing is oxidized aluminum, which is harder and more brittle than the base aluminum and can contribute to the start of a failure. Bending and torsion loads often stresses the skin of a part more than the core. Because the anodizing is under high stress and is brittle, it cracks rather than stretching. The crack exposes aluminum underneath. The aluminum, once exposed to air, will oxidize, and aluminum oxide is slightly larger than non-oxidized aluminum. This oxide forms a "wedge" which stresses the aluminum next to the wedge, and under further loading, the "wedge" raises stress so aluminum next to it is more likely to crack. This leads to a vicious cycle of crack, oxide, crack, oxide, etc., until the part is sufficiently weakened and fails.

Aluminum anodizing is broadly of two types: softer "color" anodizing (which may be many colors, including black), and "hard" anodizing, which is always dark or black. The hard anodizing is much more prone to cause failures as described above. It is not clear which sort of anodizing isused here.

A note at the page above says "The cracks also show the method of failure this clearly happened on a down stroke." That is, cracks often appear perpendicular to the load that caused them, so this would indicate the load was highest at the bottom of the stroke. Some care is required, however, as it appears the crank lacks the usual "bulb" around the pedal threads, which in turn may mean the crank was simply weakest against this load direction.