A Re-Examination of Failure Analysis and Root Cause Determination

29Metallographic ExaminationA transverse cross section through the fracture initiation site was prepared formetallographic examination. The fracture surface profile is relatively flat. No crackbranching was seen. The microstructure consists of dark-etching tempered martensite.No gross plastic deformation was observed at the fracture initiation site.ConclusionThe pin failed as a result of fatigue which initiated at the outside cylindrical surface.This surface exhibited wear and corrosion pitting. The fracture initiated at a shallowcircumferential groove. The corrosion of the fracture surface appears to have occurredsubsequent to the fracture. No evidence of stress corrosion cracking was observedIMPORTANT FAILURE ANALYSIS PRINCIPLES REPRESENTED IN THIS CASEHISTORY1. In this case, examination of the fracture surface and the microstructure weresufficient to reveal the mode of failure. Determination of the cause of failurewould require a knowledge of the use conditions.2. Fatigue failures are sometimes quite subtle. The materials used in the parts thatfail may be completely as-specified, but the service conditions exceed their longtermability to resist the stresses applied during use.Case Hstory #5. Cap Screw Assembly FailureCap screws in a robot designed for the removal of partsafter they are molded in an injection-molding machinefailed as a result of cyclic loading and possible vibrationsin the system. Redesigning the system to minimizefatigue sensitivity was recommended. Some of the failedscrews are shown at left, above.