The influence of cyclic stress intensity threshold on fatigue life scatter

Understanding factors that contribute to scatter in fatigue lives of metallic structures (particularly airframes) subjected to identical spectrum is critical to maintaining safety and optimising designs. This paper first briefly discusses the sources of scatter, and then concentrates on the effect of variations in the "cyclic stress intensity threshold" (ΔKthr) on fatigue crack growth. It shows that a version of the NASGRO equation can be used to account for the crack growth scatter seen in a number of classical fatigue experiments by accounting for variations in ΔKthr. This is an important outcome for safety and is particularly useful when considering lead cracks for which ΔKthr is small (approaching zero) as these cracks appear to commence growing soon after introduction into service.