Modelling small fatigue cracks interacting with grain boundaries

A theoretical model which describes the microstructural aspects of short fatigue cracks is further developed to quantify the strength of microstructural barriers. The effectiveness of a barrier to decelerate or arrest short fatigue cracks in single phase materials depends on the relative orientation of the grains ahead of the barrier. A method of calculating the progression of orientation factors of the grains traversed by a fatigue crack is presented. Introducing the orientation factor into the crack growth equation allows the model to be used in the prediction of crack growth rate and of fatigue life. Comparison with experimental results, obtained with an Al-Li alloy, establishes the accuracy of the predictions. The sequence of grain orientation factors ahead of the crack is also used to predict the stress boundary conditions for short fatigue crack propagation in the form of a Kitagawa-Takahashi diagram which is compared with the experimental results due to Lukas.