The lead fatigue crack concept for aircraft structural integrity

Over many years of quantitative fractographic examination of fatigue cracking from in-service and full-scale fatigue tests of metallic airframe components, it has been consistently observed that the largest cracks formed have grown in an approximately exponential manner. These crack growth observations range from the initiation of cracks and their early growth from a few micrometers through to many millimetres in length. It appears that these lead cracks commence growing shortly after the airframe is introduced to the loading environment. Furthermore these cracks usually initiate from production-induced or, less frequently, inherent material discontinuities. Based on these two observations, an aircraft lifing methodology that is based on the results of fatigue testing programs utilising the lead crack concept has been developed and implemented as an additional tool in the determination of aircraft component fatigue lives in several Royal Australian Air Force (RAAF) fleet types. In this paper the lead crack concept is developed and its strengths and weaknesses are discussed. Examples of crack growth behaviour that are considered typical and representative of lead cracks are presented.