Crack growth from naturally occurring material discontinuities under constant amplitude and operational loads

This paper discusses how cracks that grow from small naturally occurring material discontinuities under operational load spectra behave. The growth of small cracks under a representative maritime aircraft flight load spectrum is discussed first. The results of this study, when taken in conjunction with the authors previous studies into cracks growing under combat aircraft load spectra, illustrate how for cracks that grow from small naturally occurring material discontinuities under operational load spectra crack growth can often be easily and accurately computed. It is also shown that the Hartman–Schijve variant of the NASGRO crack growth equation is able to accurately represent the growth of small cracks in two different rail steels. It is further shown that the growth of both small and long cracks can be described by a family of da/dN versus ΔK curves and that, for 7050-T7451, the experimental procedures commonly used to determine a closure free da/dN versus ΔK curve produce curves that are consistent with those obtained using the Hartman–Schijve equation and allowing for small variations in the term ΔKthr.