Probabilistic Fretting Fatigue Assessment of Aircraft Engine Disks

Fretting fatigue is a random process that continues to be a major source of damageassociated with the failure of aircraft gas turbine engine components. Fretting fatigue isdominated by the fatigue crack growth phase and is strongly dependent on the magnitudeof the stress values in the contact region. These stress values often have the most influ-ence on small cracks where traditional long-crack fracture mechanics may not apply. Anumber of random variables can be used to model the uncertainty associated with thefatigue crack growth process. However, these variables can often be reduced to a fewprimary random variables related to the size and location of the initial crack, variabilityassociated with applied stress and crack growth life models, and uncertainty in thequality and frequency of nondeterministic inspections. In this paper, an approach ispresented for estimating the risk reduction associated with the nondestructive inspectionof aircraft engine components subjected to fretting fatigue. Contact stress values in theblade attachment region are estimated using a fine mesh finite element model coupledwith a singular integral equation solver and combined with bulk stress values to obtainthe total stress gradient at the edge of contact. This stress gradient is applied to the crackgrowth life prediction of a mode I fretting fatigue crack. A probabilistic model of thefretting process is formulated and calibrated using failure data from an existing enginefleet. The resulting calibrated model is used to quantify the influence of inspection on theprobability of fracture of an actual military engine disk under real life loading conditions.The results can be applied to quantitative risk predictions of gas turbine engine compo-nents subjected to fretting fatigue.