Calculating crack growth from small discontinuities in 7050-T7451 under combat aircraft spectra

• A significant portion of an aircraft’s fatigue life is spent in the physically short crack regime.
• A variant of the Hartman–Schjive equation was shown to collapse both short and long crack rate data.
• This infers that short crack data can be derived from long crack da/dN versus ΔK data.
• The Hartman–Schjive variant was used to predict the lives of both low and high Kt coupons tests under aircraft loading.

This paper supplements previous work which showed that the crack growth rate in a large range of metallic materials fitted a variant of the Hartman–Schijve equation where da/dN is a function of (ΔK − ΔKthr)α, where ΔKthr is a parameter that reflects the apparent fatigue stress intensity threshold of the material, and α is approximately 2. For the case of 7050-T7451 aluminium alloy the same equation is shown to fit both long and short crack growth data once the appropriate ΔKthr is chosen for each specific data set. This equation is used here to produce accurate predictions of the fatigue crack growth in 7050-T7451 aluminium alloy specimens with both a low and high stress concentration subjected to two combat aircraft loading spectra. Thus, it is postulated that if long crack data are fitted to the variant of the Hartman–Schijve equation then accurate predictions can be made in the short crack regime for a wide range of materials.