An engineering model of fatigue crack growth under variable amplitude loading

Fatigue crack growth in structure components subjected to variable amplitude loading is a very complex subject. Many models have been proposed, but as yet no universal model exists. In this paper, the concept of an equivalent stress intensity factor (SIF) range corresponding to R = 0 and a modified Wheeler model are introduced. These innovations lead to a fatigue life prediction model that depends mainly on the stress ratio and the plastic zone size ahead of the crack tip. This model also describes the phenomena of retardation and arrest due to overload, and the acceleration due to a state of underload following an overload. The plastic zone size ahead of the crack tip is modeled as a continuous function of the maximum applied SIF, yield strength, and plate thickness, making its calculation precise and easy. The proposed model is validated using experimental fatigue crack growth data in 7075-T6 and 2024-T3 aluminum alloys and 350WT steel under various overload, underload, and spectrum loadings published in the literature. The predicted results are in good agreement with these test data.