Andrew R Stewart 

The crank arm failure looks to have been right at the point that the end mill would center on (the tool that cut the groove) as it was at the end of the groove. I have seen this type of failure before. The cutting of the groove leaves a stress risor at the end of the groove, or actually at the center of the radius of the end. It's hard to see completely but there seems to be a shadow that radiates across the arm's cross section and is centered on the groove. This shadow could be the crack collecting grime over the years. At some point the cross section that was not yet cracked couldn't handle the stress of normal pedalling.

This is why the drillium/relief groove is so questionable if not forged into the part. A surface stress relief like media blasting prior to anodizing migh have reduced the chance of a crack starting. BITD we would lightly file the sharp webbing between the crank arm and the spider arm of the Campy NR cranks to reduce the stress concentrations. As a young guy i found it interesting that a very little bit less material could improve the lifespan of Aluminum parts. Andy.