A Method for Reliability Assessment of Aircraft Structures Subject to Accidental Damage

Abstract

A method has been proposed to assess the reliability of aircraft structures subject to accidental damage using in-service damage data. The development of this method is motivated by the increasing use of composite materials in aircraft structures, which are particularly sensitive to impact damage. In addition, damage to composite structures may not be visible or may be difficult to detect. Based on the equivalent Level of Safety (LOS) concept, this method consists of two essential elements: a probabilistic assessment of detected damages and the probability of non-destructive inspection methods to detect such damage. Accidental damage threats are first characterized by identifying damage type, size, cause, and frequency for different aircraft models and structural components. The damage threats are then modeled by constructing histograms and detected damage probability distributions. Probability of detection models for various non-destructive inspection techniques are then used to calculate the equivalent Level of Safety. This approach can be used to design a new aircraft structure that is equivalently or more reliable as current designs. Furthermore, maintenance service guidelines and optimum inspection intervals may be estimated once design limit load is established. Engineers can use this methodology in the future as a guide to establish design and inspection criteria while considering safety risk and maintenance cost at the same time. The current reliability method relies on the completeness of the reported data on detected damage. Fleet service data currently available may not contain accurate information on damage size and the associated inspection method. In addition, damage reporting procedures vary from one operator to another, and damage data available for composite structures are limited. Therefore, sample representation of the extracted damage sizes and the probability of reporting are issues that must be addressed in order to apply the method to practical aircraft design. In this paper, 100 % reporting and 100 % sample representation is assumed. The results of the current reliability analysis method may give insight on general damage threats and future methods for designing and inspecting composite aircraft structures.