Prediction of the combined high- and low-cycle fatigue performance of gas turbine blades after foreign object damage

This paper addresses the problem of predicting residual fatigue life on aircraft engine blades after foreign object damage. An elastic predictive approach is described based on arrest of short cracks propagating from a notch, which is an idealisation of the damage. This approach is extended for the case of combined cycle loading, where the blade experiences a combination of centrifugal and vibrational loading. Experiments were performed using ballistic impact to damage Ti6Al4V ‘blade-like’ specimens which are then tested in fatigue using a novel combined cycle test. The predictions are compared to the experimental results and shown to give satisfactory agreement. A further development of the approach is then described which enables the incorporation of shaken-down residual stresses, caused by the impact and subsequent fatigue loading. Some sample results are shown for a constant residual stress distribution and compared to the experimental results.