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Ultegra FC-6503 hollow crank, mid-section

Crank failed mid-section away from features.

Shimano Ultegra FC-6503. Hollow D-shaped crank.

Service history not-known. These cranks were manufactured roughly 2003 to 2005; crank failed circa 2013. Crank probably ridden in hilly terrain by a vigorous rider.

Note "beach marks" starting near outer face, suggesting the crack started at the dark spot and grew from there until the crank failed.

Load on pedal near the bottom of the stroke tends to put D-shaped arm under highest load on the curved outer face. In such bending, the highest load is at the "skin". The narrower the face (the rounder the D), the higher the skin stress for a given pedal load.

Further, the outer face tends to be scuffed by shoes, etc., in normal use. Scuffing can give rise to surface defects that accelerate failures.

Compare to other D-shaped arm failures HERE, HERE, HERE, and HERE.