Failure analysis of service fatigue cracks in aircraft structures – Going further

• Demonstrates qualitative and quantitative benefits of fractography in failure analysis. • Offers three cases studies of the application of fractography in aircraft failure analysis. • Demonstrates that fractographers can offer more insights to root cause and corrective actions beyond normal bounds. This paper explores the process of investigating and analysing unexpected fatigue cracking in aircraft primary structure. The three examples demonstrate the potential for quantitative fractography to assist fleet managers in answering key questions that arise in such cases, such as; the type of cracking, its nucleation cause, its age, its growth rate and its proximity to failure. Example 1 is cracking in a BAC 1–11 main landing gear wheel that occurred adjacent to a brake-drive block fitting. The investigation revealed that the cause of the cracking was surface damage that led to intergranular corrosion pitting, from which fatigue cracking grew. Quantitative assessment using surface marking created by overhaul actions gave a crack growth curve that suggested that the crack had not been growing very fast at the time it reached the thickness of the wheel and the wheel was not close to imminent structural failure. Example 2 investigated damage to F/A-18A/B outer wing leading edge flap attachment lugs. Fatigue cracking had nucleated from maintenance-induced damage in lug radii. The main cracks were assessed by comparison of the local fracture appearance – compared to expectation of the appearance of variable amplitude crack growth in this material gained through its close study, throughout the growth of the larger cracks to assess the growth rate, which was found to have not been growing very fast, and failure was not imminent. This was sufficient, given the short remaining life of the fleet, to support a decision not to design a fleet repair. The third example was an investigation into F/A-18A/B vertical tail stub fatigue cracking. Here measurements of similar groups of progression markings found on the fracture surface were used to infer the fatigue crack growth rates and discreet changes in the cracks path. These markings were matched to downtime events during service life. These major markings, when correlated to the ‘similar groups of markings’ growth assessment showed good agreement, and the crack growth curve developed was used to support extending the life of the stubs with known cracking. These studies underscore the importance of understanding the service life and event history of a structural component being examined, and the role of fatigue research in improving investigators’ practical knowledge. The authors emphasize that quantitative fractographic methods, when used in failure analysis of service aircraft structure can significantly contribute to understanding the mechanisms of fatigue crack growth and can greatly aid in fleet management decisions.