Implications of the lead crack philosophy and the role of short cracks in combat aircraft

The Australian Defence Science and Technology (DSTO)/Royal Australian Air Force (RAAF) approach to the management of fatigue cracking in combat and trainer aircraft makes use of the “lead crack” concept. In this approach crack growth is assumed to initiate from small naturally occurring defects/discontinuities with dimensions of approximately 10 μm and growth is assumed to commence from day one. As a result, for certification purposes, we need to address the so called short crack anomaly, whereby for a given ΔK the crack growth rate (da/dN) is significantly greater for short cracks than it is for long cracks. In this paper we reveal that there are several instances where this anomaly vanishes if da/dN is represented as a function of (ΔK − ΔKthr), where ΔKthr can be thought of as an apparent threshold. We then show that for operational aircraft the “true” da/dN versus ΔK curve is an amalgam of the short and long crack growth curves. We next show that existing test procedures used to establish the effect of composite repairs to cracks in fleet aircraft overestimate their effect. We also show that the growth of cracks from small naturally occurring defects exhibits little, if any, R ratio effects.