The lead crack concept history 30 years on

icaf2023 Tracking Number 7

Since the early 1990s extensive quantitative fractography (QF) of early nucleation and growth of airframe fatigue cracks that have resulted in accidents, or led to threats to structural safety, have shown that the largest – or “lead” ‒ cracks often show approximately exponential growth. These observations have been formalised in the lead crack concept and development of the Lead Crack Fatigue Lifing Framework (LCFLF) [1-3]. This has now progressed to a stage where it provides a robust and appropriately conservative method of assessing crack growth, enabling the setting of inspection and maintenance periods for a variety of in-service fatigue cracking problems in aircraft. The framework has been used extensively in airframe life predictions and or life extensions by numerous airworthiness authorities. The LCFLF relies on determining the equivalent crack-like sizes of the fatigue-nucleating discontinuities and the crack depths at known points in the fatigue lives. This method is flexible in that it may be pragmatically combined with fracture mechanics models of crack growth, provided they have been verified by actual measurements [3]. This paper summarises the current knowledge state for the LCFLF and provides some new examples of the framework directed at determining the crack growth history from limited quantitative fractography or in-service crack length measurements. It has increasingly become an important tool for aircraft sustainment and fatigue failure analyses. [1] Molent L, Barter SA, Wanhill RJH (2010) The lead crack fatigue lifing framework, DSTO Research Report DSTO-RR-0353, DSTO Defence Science and Technology Organisation, Fishermans Bend, Victoria 3207, Australia. [2] Molent L, Barter SA, Wanhill RJH (2011) The lead crack fatigue lifing framework, International Journal of Fatigue, 33: 323-331. [3] Wanhill RJH, Molent, L. Barter, SA (2019) Fatigue Crack Growth Failure and Lifing Analyses for Metallic Aircraft Structures and Components, SpringerBriefs in Applies Sciences and Technology, Springer Nature B.V., Dordrecht, The Netherlands.