CAE-assisted diagnosis and route-speed correction of resonance-sensitive fatigue in a welded torsion-beam rear axle

Fatigue cracks developed in a welded torsion-beam rear axle during an accelerated proving-ground programme, although all cracked locations met the legacy static safety-factor requirement. A crack-consistent route-equivalence workflow was developed to identify the governing road-speed condition. It combines hotspot pseudo-damage, reconstruction screening, modal interpretation, strain measurements and a targeted bench edit. The static rule missed all four sites in the observed crack family. Beijing structural generated 3.01 times the hotspot pseudo-damage of Liudong structural. Its reconstruction-qualified subset retained 66.93% of the Beijing total, and the conservative ratio against the complete Liudong route remained 1.86. Damage was concentrated on heavy corrugation at 50 km/h. The corresponding 17.49 Hz excitation was close to the 17.1 Hz axle torsional mode, whereas the 60 km/h condition was detuned. The 50/60 km/h pseudo-damage ratio was about 3800 and could not be explained by stress-range scaling alone. Crack chronology, the reconstructed life field and measured response placed the highest demand in the same welded region; the largest severe-road response reached 343.72 MPa at L3-L. Replacing only the 50 km/h condition with 60 km/h increased bench life from a five-test mean of 24.7 h to at least 165 h. The lower-bound gain was 6.68, indicating that this condition contributed at least 85.1% of the original damage. The workflow supports targeted route editing for resonance-sensitive fatigue without altering the complete durability programme.