Failure Analysis of Automotive Power-Shift Transmission Shaft

This article addresses a root cause of transmission shaft failure that has potential to stall the transmission function at field applications. The broken shaft from an automotive power-shift transmission is collected from the field and examined. This investigation is carried out through visual inspection, material characterization, metrology inspection and finite element analysis. The broken shaft is observed with various ratchet marks that are normal to shaft axis. The fracture surface is observed with fatigue striations that indicates the shaft experiences cyclic load that propagated the cracks and caused fatigue damage. The failure initiation point on the broken surface is identified as the grinding relief groove that is manufactured at unpremeditated region. Numerical models are developed using Nx NASTRAN software to get better insights on the failure mechanism. The result shows the groove increased the local stress concentration during loading and reduced the safety factor of shaft design. The numerical model predictions are reasonably correlated with the broken shaft that provide greater insights about the failure mechanism. Based on this investigation, a proposal for the shaft design is suggested to avoid further failure in the component.