Surface fatigue crack growth under variable amplitude loading

Plasticity-induced fatigue crack closure plays an important role in fatigue crack growth. Under fatigue loading with variable amplitude, however, the influence of fatigue crack closure along the crack front in three-dimensional cracked bodies is difficult to analyze. Here we develop a three-dimensional strip yield model to calculate crack opening stresses of surface fatigue cracks under variable amplitude loading. An equivalent thickness concept is incorporated to quantify the thickness effect for points on the surface crack front with a series of straight-through cracks. Using this equivalent thickness based strip yield model and fatigue crack growth rate curves obtained with standard through the thickness cracked specimen under constant amplitude loading, fatigue growth of surface cracks under variable amplitude loading are able to be predicted. We used this method to calculate fatigue crack growth of five semi-elliptical surface cracks and found that the ratios of the predicted life to the experimental life ranged from 0.86 to 1.01. The average value of the results was 0.93 and the standard deviation was 0.055. Good agreements are found between the predicted and experimental life for semi-elliptical surface cracks under variable amplitude loading. This method paves an avenue for fatigue crack growth life prediction of practical structures using materials properties obtained in laboratory according to material test standards.