A Re-Examination of Failure Analysis and Root Cause Determination

22A set of U notch impact test specimens were machined from the shaft and tested at+73ºF. The results are given in the table below.Impact PropertiesSpecimen No. %Shear %Cleavage Lateral Expansion Impact Strength (Ft. lbs.)(mils)1 10 90 12 252 15 85 13 243 15 85 12 24.• These results show that the material is not brittle and• that it conforms to the specifications.FigureFigureMetallographic ExaminationTransverse cross sections through each corner of the keyway were prepared formetallographic examination. Secondary cracks were found in each corner of thekeyway in the as-polished condition as shown in Figure 3 (approx. 50X).Secondary cracks were also found at the exterior surface adjacent to the intersection ofthe keyway and the exterior surface as shown in Figure 4 (approx. 50X). The surfacesof the shaft were rough in texture because of corrosion. In addition many of thesecondary fatigue cracks were found to be wide and wedge shaped which ischaracteristic of corrosion fatigue. The microstructure consists of dark-etchingtempered martensite. No surface decarburization was found.ConclusionThe conveyor drive shaft failed as a result of corrosion fatigue in bending. Failure wasinitiated at both corners of the keyway. Numerous secondary fatigue cracks wereobserved and indicate the presence of a large number of stress concentration sitescaused by corrosion pitting of the surface. Fatigue is the progressive failure of acomponent subjected to repeated or fluctuating strains at stress levels below the yieldstrength of the material. Fatigue cracks initiate at locations of maximum local stressand/or minimum local strength. The shaft conformed to the specified chemical and