Predictions of Widespread Fatigue Damage Thresholds in Aging Aircraft

Commercial transport aircraft are required to operate under the concept of damage tolerance. Because of the structural redundancy and the crack arrest capability, the current fleet of commercial aircraft was initially designed to have sufficient residual strength to sustain discrete source damage. However, fatigue damage during the life of an aircraft can significantly reduce the residual strength of an aging aircraft. It is important to predict the threshold for the onset of widespread fatigue damage, i.e., the initiation and growth of cracks at rivet holes (multiple site damage) to threshold sizes at which, in conjunction with a lead crack, the residual strength of the aircraft may fall below the limit load. A hierarchical global-intermediate-local approach is presented for the numerical predictions of the widespread fatigue damage thresholds. A detailed numerical study is presented to illustrate the importance of 1) the use of elastic-plastic fracture mechanics to assess the residual strength and 2) the importance of the local stresses due to rivet misfit, clamping, cold working, fretting, etc., in assessing the number of fatigue cycles to reach the widespread fatigue damage threshold for an aging aircraft.