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Broken crank
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This crank belonged to Fred Oswald, who weighs over 200 pounds. I thank him for the photo. The largest "beaches" started at the side of the crank parts closest to the center of the photo; the final break was at the outside edges in the photo. The U-shaped cross-section is fairly common, but it is not optimal. The inside face of the crank (with the two "wings") is in tension when the crank is near the top of the stroke. The narrowness of the "wings" constitutes a stress raiser to some extent. This is a left crank. As the cyclist presses down on the pedal outboard of the crank, the crank also is in torsion, and stress is highest at the wing where the beaches started. The smaller beaches at the outside of the crank suggest that a scratch or other defect there initiated cracking. The anodized coating of the crank also possibly contributed to failure -- see Jobst Brandt's comment on anodizing of rims. As the anodizing is harder than the underlying metal, it can create stress raisers at the surface. Scroll right as needed to see the rest of the image.
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