Abstract: Different from crack growth models based on classical Linear-Elastic Fracture Mechanics (LEFM), the average growth rate based model describes the average crack propagation per flight hour under a random load spectrum using a simple empirical formula. The model treats the random spectrum as an equivalent constant amplitude spectrum with only two characteristic parameters reflecting all the complex load-sequence effects and other effects. A growth rate scaling method is adopted to adjust the average growth rate obtained under one test load spectrum to predict the crack growth rate under another untested load spectrum. First, the definition and parameter derivation procedures of average growth rate models based on the Frost & Dugdale formula and Paris formula are introduced. Then, representative coupon fatigue test data of a typical bulkhead critical location under different random spectra is reviewed and the average growth rate model and growth rate scaling method have been comprehensively validated. After comparative analysis, the Paris equation based average growth rate model is recommended. It has been demonstrated that significant prediction accuracy gains of the crack growth rate and life span under variable amplitude loading spectra are achieved when the Paris equation based model is combined with the growth rate scaling method. Meanwhile, the accuracy of the growth rate scaling method is related to the classical LEFM model selected in the derivation of theoretical parameters. Thus, the average growth rate based crack growth model in conjunction with the growth rate scaling method is anticipated to provide a reliable option for the individual aircraft tracking program to better assess the aircraft structural service life.

Key words: crack growth model, average growth rate, growth rate scaling method, Frost & Dugdale equation, Paris equation