MODELLING FOR FATIGUE CRACK GROWTH PREDICTION IN AIRCRAFT

Fatigue crack growth models, having application to Mirage III0 aircraft, have been calibrated with test data. Of the four crack growth retardation models examined—Wheeler, Willenborg, modified Willenborg and Crack Closure—the Wheeler and modified Willenborg models are the most satisfactory but both require calibration by test. Even so, crack growth is not accurately predicted when the specimen geometry and the test sequence are varied from those used in calibrating the models. Apart from the crack growth models, the main sources of inaccuracy in predicting crack growth are the inadequacy of the growth rate basic data, incorrect assumptions of crack shape and uncertainty in stress history. Thus, crack growth life may not be confidently predicted to better than a factor of two on actual life and, in some cases, the factor may be as high as ten.