Failure analysis of landing gears trunnions due to service

• In both trunnions examples the failures occurred at the transition of two sections.
• Multiple crack initiation and corner cracking of the lateral surface were detected.
• Any type of initial defect was not observed in the fracture surfaces.
• Failures occurred by fatigue but in the presence of high stress levels.

This paper presents a detailed analysis of two in service failures concerning trunnions of landing gears of military aircrafts. Several tools were used in order to find out the primary cause of failure, namely, visual examination, chemical analysis and macroscopic, microscopic and microstructural examinations.The progress of the fatigue cracks were determined based on the observation of beach marks caused by the loading history and by striation marks caused by cyclic plastic deformation mechanisms related with events occurring in the loading history of the component. Additionally, the topology of the fracture surfaces provided an indication of the stress system that produced failure, which is closely dependent on the mechanical behaviour of the material.In both examples the failures occurred at the transition of two sections. The visual observation of the fractured surface immediately indicates that these failures occurred by a fatigue process. A main area of the fracture surfaces present high stress failure facies. A more detailed analysis of the fractured surfaces concludes that the surfaces that failed by fatigue is clearly smaller than the one that failed by an overload; this measuring capacity allows the investigator to consider that failures occurred by fatigue but in the presence of high stress levels at the crack initiation zone.In the first case multiple crack initiation was detected while in the second case the cracking occurred at the corner of the lateral surface of the trunnion structure.Any type of initial defect was not observed in the fracture surfaces. Therefore it can be concluded that failure probably occurred by fatigue but in the presence of higher loads in the components than those that were expected.