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 Δ 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, viz.: cycle-by-cycle analysis; the USAF characteristic K approach, etc. Specific attention is paid to the growth of lead cracks in operational aircraft which are shown to exhibit near exponential crack growth and to essentially have a cubic dependency on stress. It is shown that cracks growing in composite repairs exhibit the same crack length and stress dependency. This finding is then linked to current approaches which use the cubic rule to assess repairs to RAAF aircraft
