On the growth of cracks from etch pits and the scatter associated with them under a miniTWIST spectrum

This paper examines the growth of cracks from small naturally occurring discontinuities in AA7050-T7451 coupons tested under a miniTWIST load spectrum. A markerband is inserted in the spectrum to aid in Quantitative Fractography. The resulting crack growth curve is reasonably exponential in nature, which indicates that crack growth under combat aircraft spectra and civil aircraft transport spectra is similar. It is also shown that the Hartman-Schijve crack growth equation can be used to compute the crack growth histories and account for the scatter seen in the various tests by allowing for small changes in the value of ΔKthr.