A new method for predicting fatigue crack propagation rates

A method has been developed for predicting fatigue crack growth rates in Ti 6246 specimens based on a damage accumulation driving force for crack propagation. Cracks were assumed to propagate via a process of quasi-static incremental steps through the crack tip damage zone. The number of cycles to reach a critical level of damage in the process zone ahead of a crack of any given length was determined based on finite element analysis and through correlation with the measured fatigue response of smooth specimens tested under strain-control. By summing up the individual micro-increments in crack growth, predictions were made for the relationship between fatigue crack growth rate and (delta)K, the stress intensity range. Model predictions were assessed against experimental crack growth rate data obtained on beta forged Ti 6246 alloy. An approach based on a crack step micro-increment size equivalent to the cyclic plastic zone size was found to predict the general trend and magnitude for stage II crack growth rate in this alloy. The ability of the method to account for load ratio effects is assessed and discussed with reference to Kmax and (delta)K dependency of the crack growth increment.