Validation of the USAF characteristic K approach for long cracks under variable amplitude loading and naturally occurring small cracks under a combat aircraft load spectrum

Damage tolerance analysis of structures requires reliable predictions of fatigue crack growth rates for both microscopically small and long cracks. Due to the largely empirical nature of crack growth under cyclic loading, several models have been developed to capture the growth rate of cracks. The development of a model that can be used to predict the growth of both long and short cracks under a random loading spectrum, however, has continued to challenge researchers. The present paper seeks to address this issue by validating the USAF characteristic K approach using the Hartman-Schijve variant of the NASGRO equation. In order to achieve this, two sets of data were analysed; the first set uses the long crack growth data for Al 7075-T6, Al 2024-T3 and Mil-Annealed Ti-6Al-4V under variable amplitude loading and the second set uses the test results for small cracks growing from naturally occurring material discontinuities under a representative combat aircraft load spectrum. In each case it was shown that by defining a value of the characteristic stress intensity factor, K that best captures the load history, the Hartman-Schijve variant of the NASGRO equation was able to predict the growth of cracks reasonably well.