Engineering analysis of notch root fatigue crack growth under spectrum loading

Abstract

An engineering procedure is described to predict notch root fatigue crack growth under spectrum loading. Long-crack growth rates and closure data, notch root stress-strain analysis and Rainflow cycle counting constitute major inputs to the prediction process. The procedure models non-uniform extension of the part-through crack front, crack growth in compression fatigue and the effect of tensile and compressive overloads. Crack growth predictions under constant-amplitude and combat aircraft spectrum loadings were compared with data from the literature for 2.3 mm and 6.25 mm thick 2024-T3 alloy sheet material. Good predictions were obtained for the thicker material using the linear elastic fracture mechanics approach. The thinner material required inclusion of strain-based intensity and residual stress effects for more realistic predictions. Neglecting these factors as a rule provided life estimates that are not conservative.