Estimation of aircraft structural fatigue life using the crack severity index methodology

In this work, fatigue test results under different random spectra have been usedto validate the crack severity index methodology in which only the normal load factor and aircraft gross weight were consideredasthe load. The original crack severity index methodology was developed to quantify the relative severity in terms of crack growth potential using the stress spectra recorded at the wing root. The crack severity index values have been used to determine the flight mission severity through interpolation between known flaw growth curves of reference usages. This interpolated flaw growth curve enables aircraft users to predict the cumulative fatigue crack growth damage based on individual aircraft activity and provides a useful guide for scheduling future inspection and maintenance activities. To validate the crack severity index methodology based on normal load factor and aircraft gross weight data instead of stress spectra, experimental fatigue lives were obtained for the stresses represented by normal load factor and aircraft gross weight spectra of different random loads. Then, the differences in fatigue life as severity measurements were compared with the crack severity index values. A good correlation was obtained, which validated the crack severity index methodology. The test results were also used to tune a simulation equation for crack growth. The good correlation between the crack severity index values and fatigue test or crack growth simulation results suggests a simple way of predicting the structural fatigue life from crack severity index values.