FATIGUE FAILURE ANALYSIS FOR AUTOMOTIVE TRASNMISSION

A fatigue failure in the transmission input shaft was identified during a bench-level endurance test under 2nd gear loading conditions. The test transmission’s input shaft comprises fixed 1st, reverse, and 2nd gears, with the remaining gears mounted as floating. The shaft was subjected to cyclic torsional loads, and failure occurred after a defined number of cycles. Metallurgical analysis revealed a brittle fracture surface with crack initiation at the outer surface, propagating to core in a helical pattern, ultimately resulting in complete shaft fracture. To monitor and replicate the failure, the test setup was instrumented with a Reilhofer Delta Analyzer for early fault detection. TTL signals from accelerometers mounted on the transmission and a bench speed sensor were fed into the system, which generated FFT spectra and trend indices. A warning alarm triggered upon deviation in the trend index, indicating premature damage initiation. The test was subsequently halted for component inspection, revealing tool serration marks and initial hairline cracks near the 2nd gear location. Order analysis confirmed the dominance of the 2nd gear order and its harmonics. Additionally, the trend index energy was six times higher than baseline levels, indicating abnormal mechanical behavior. Finite element simulations were conducted on models with and without tool serration marks on the input shaft. The results showed significantly elevated stress concentration and shear stress in the failure zone for shafts with tool marks, whereas smooth shafts exhibited no severe stress concentrations. The input shaft was re-machined to eliminate tooling marks, and subsequent testing showed no abnormal trend index variation, confirming alignment with simulation predictions. This paper outlines the root cause analysis, simulation correlation, and mitigation strategy to prevent fatigue failure in rotating transmission components under cyclic loads. The methodology is scalable to similar components operating under dynamic loading environments.