PREDICTIONS OF WIDESPREAD FATIGUE DAMAGE THRESHOLDS IN AGING AIRCRAFT

Commercial transport aircraft are required to operate under the concep t of damage tolerance. Because of the structural redundancy and the cr ack arrest capability, the current fleet of commercial aircraft was in itially designed to have sufficient residual strength to sustain discr ete source damage. However, fatigue damage during the life of are airc raft can significantly reduce the residual strength of an aging aircra ft. It is important to predict the threshold for the onset of widespre ad fatigue damage, i.e., the initiation and growth of cracks at rivet holes (multiple site damage) to threshold sizes at which, in conjuncti on with a lead crack, the residual strength of the aircraft may fall b elow 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 illustra te the importance of 1) the use of elastic-plastic fracture mechanics to assess the residual strength and 2) the importance of the local str esses 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.