Analysis of failed motor shaft of an air cooler

A fractured motor shaft of an air cooler was sent to the laboratory to analyse the cause of failure. Investigation revealed that the shaft has failed by fatigue. Multiple fatigue cracks have originated at the fillet of the change of section and progressed over about 90% of the shaft cross section before giving way to torsional overload failure. The fatigue crack initiation appears to be due to stress concentrations arising from the sharp edges of the undercut at the fillet of the change of section. The problem appears to have further aggravated due to superimposed bending stresses on the shaft