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This paper summarises the findings of an international collaborative programme that focuses on the problem of the growth of cracks that arise from natural corrosion in bridge steels. The experimental data presented in this paper confirm that the bridge steel da/dN versus ΔK relationship is similar to that seen by the high‐strength aerospace steels D6ac and 4340. It is then shown that the methodology developed to predict the growth of small naturally occurring cracks in aerospace materials can also be used to compute the growth of cracks that arise because of natural corrosion in bridge steels.

All repairs to airframes now need to be assessed as to their effect on the damage tolerance the aircraft. To this end this Chapter first discusses difference between the analysis tools needed for ab initio design and aircraft sustainment. It is shown that using small or physically short crack da/dN versus ∆K data results in reduced through life costs and increased aircraft availability. The tests procedures needed to validate composite, or supersonic particle deposition (SPD), repairs to operational aircraft are also discussed as is their relationship to the ASTM fatigue test standard E647-13a. This leads to an examination of the problem of crack growth from small naturally occurring material discontinuities under operational load spectra. A range of tools are available to account for crack growth in operational aircraft and several such tools are discussed, 2 Reference [2] also provides an historical perspective as to how the military, the Federal Aviation Administration (FAA) and industry have dealt with these threats. 3 Widespread fatigue damage (WFD) is defined [11] as the simultaneous presence of cracks at multiple structural locations that are of sufficient size and density such that the structure will no longer meet the residual strength requirements of FAR section §25.571(b). 4 Damage tolerance requirements were not adopted by the FAA until 1978, see [12]. 5 Limit of validity-The period of time (in flight cycles, flight hours, or both) up to which widespread fatigue damage will not occur in the airplane structure.

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The USAF requirements for the durability and damage tolerance certification for additively manufactured (AM) aircraft structural parts, which are detailed in Structures Bulletin EZ-19-01, raise a number of new and, as yet, unanswered questions. The present paper attempts to address three questions: How to perform a fracture mechanics-based analysis of crack growth in an AM part so as to account for the residual stresses, how to perform a fracture mechanics-based durability analysis of a cold spray repair so as to account for both the induced residual stresses and the presence of multiple co-located cracks, and how to perform a fracture mechanics-based durability analysis of an AM part so as to account for the presence of multiple collocated surface braking cracks. In this context, the present paper reveals the potential of the Hartman–Schijve variant of the NASGRO crack growth equation to accurately predict the growth of each of the individual (collocated) cracks that arose in a cold spray-repaired specimen and in a specimen from a crack that nucleated and grew from a rough surface.