Predictions of Crack Growth Rates, R-Ratio and Overload Effects Based on Smooth Specimen LCF Data and the Moving Plastic Stress Field Ahead of the Crack Tip

Previous work has attempted, often within the framework of strip yield-type models, to predict crack growth rates based on the accumulation of fatigue damage ahead of the crack tip as it moves through a structure. This study performs similar calculations using results from plastic 2D plane stress analyses run on a finite element (FE) model containing a sharp semi-circular notch representing an edge crack. Stress-distance profiles ahead of the crack tip (notch root) were extracted at the maximum and minimum points of a range of fatigue cycles with different loading amplitudes. These were used with data from smooth specimen Low Cycle Fatigue (LCF) tests to predict the build-up of fatigue damage at regularly spaced locations ahead of the crack tip and hence crack growth rates. The FE analyses were performed for a wide range of K_max values at loading R-ratios of 0, -1 and 0.5, and the growth rate predictions were compared with test data. The method was then extended to predict overload behaviour. The material studied was the nickel-based superalloy fine grain (FG) RR1000 at 20°C.