Reliability Analysis of Aircraft Structures under Random Loading and Periodic Inspection

Abstract

A reliability analysis of fatigue-sensi tive aircraft structures, based on the application of the approach developed in the vibration theory, is presented. Operational service loads, composed of ground loads, ground-air-ground loads, and gust loads, are all random in nature. The fatigue process involved here consists of crack initiation, crack propagation, and strength degradation. The time to crack initiation and the ultimate strength are random variables. After a fatigue crack is initiated, fracture mechanics is applied to predict crack propagation under random loading. While the fatigue crack is propagating, the residual strength of the structure decreases progressively, thus increasing the failure rate with time. The aircraft structure is subjected to periodic inspection in service. When a fatigue crack is detected during inspection, the implicated component is either repaired or replaced, so that both the static and the fatigue strength are renewed. Such a renewal process is taken into account in the present analysis. The detection of an existing fatigue crack during inspection is also a random variable which depends on the resolution capability of the particular technique employed and the size of the existing crack. Taking into account all the random variables as well as all the random loadings, the solution for the probability of first failure in a fleet of aircraft is derived. Finally, numerical examples are given to demonstrate the effect of inspection and fleet size on the fleet reliability.