Torsional Failures in Hollow-Axle Half-Shafts from an F1000 Race Car

Independent failures of two hollow-axle half-shafts from an F1000 race car were investigated. This race car is powered by a 2008 generation Suzuki GSXR1000 motorcycle engine that uses a chain drive to spin the rear differential to which the axle half-shafts connect. The first axle half-shaft failed during testing at Circuit of the Americas in October 2020 and the other while racing in the SCCA Sprints at Road America in January 2021. Notably, both axles failed in a similar manner and the direct cause of the failures were fatigue cracks that propagated from torsional loading. The metallurgical investigation revealed that pre-existing flaws on the inner surface along the length of hollow-axle shaft initiated the fatigue cracks. Metallography and etching indicated the flaws were likely to be quench cracks. Prior to these failures, similar hollow-axle shafts from the same vendor were used without issue. This led to an investigation into possible changes in the manufacturing process responsible for the failures. Recommendations were made to mitigate the stress concentrations and thermal stresses which initiated fatigue cracks in these axle shafts.