A Failure Analysis of Full Float Axle Shaft: Bending Fatigue - Case Study

Primary function of axle shaft is to transmit the power from differential to wheel ends. Historically believed that full float axle shaft will experience only a torsional load, not the bending load as bending moment is taken care by the housing banjo. At particular heavy truck product segment, observed many field return axle shaft failed at the flange outer fillet cavity region. Failure was at different mileage. In bench validation such failure phenomenon were not observed leads to suspect overload in the field. Upon fracture analysis no metallurgy deficiency observed. Upon the fractography crack initiated from the outer side at the flange fillet center cavity region. This fracture portion is quench & tempered region. Residual stress analysis carried out to understand the cause of the failure due to improper tempering but stresses are compressive in nature. Ishikawa diagram is plotted to understand the root cause for the bending fatigue failure. Housing banjo which supposed to take bending load is analyzed & found permanently bent to some extent. This is evident that the truck experienced the overload, causing the permanent deformation of housing banjo, there by inducing the bending load in the full float axle shaft, resulting to failure of the shaft. The evidence of shaft surface rubbing on banjo spindle surface & oil baffle plate was observed. Flange geometry is constrained that case hardening is not possible. Already the superior grade material with quench & temper is used for the shaft. Hence decision taken to add the material & fill up the cavity portion at the outer portion of flange center. Design reviewed, FEA analysis shows lower stress due to increase in section modulus by even considering the overload. The failure arrested completely by filling up the cavity at the outer region of flange.