Degradation of aircraft structures

Maintenance and reliability of aircraft is a major safety concern and economical factor. Many civilian and military aircraft have been in service for 35 years or more. Aircrafts that were originally designed for a service life of 20 years are currently considered for life extensions of up to 80 years. The cost of corrosion and fatigue related maintenance on these aging structures has increased dramatically. A study conducted in 1998 showed that the direct costs of corrosion maintenance to the United States Air Force were $775 million/year. These costs continue to rise in spite of Air Force structure changes resulting in a 20% reduction in the overall fleet. In an effort to reduce these maintenance costs, several programs have been initiated for the development of methods to manage and control corrosion and fatigue damage in aging aircraft. Nondestructive evaluation (NDE) of materials will undoubtedly play a major role in future maintenance programs. These NDE methods must be able to detect and quantify material defects to prevent catastrophic failure or to provide input data for life prediction models. The NDE methods must have sufficient sensitivity to detect microstructural changes and early stages of damage so that component replacement plans can be implemented prior to failure. As a result of this new maintenance approach, NDE methods will be required to monitor the changes in the material properties that are used to define maintenance cycles. Typically, NDE is applied to production quality control, periodic systems inspection, and condition monitoring. A new focus that has been developed in the last years is NDE for materials research and development. Such laboratory techniques can help to understand degradation processes, such as the quantification of corrosion kinetics or the degradation of corrosion protective coatings. An understanding of the generation and propagation of defects on a microstructural level will help improve the materail properties and develop models for lifetime prediction. The NDE methods discussed in this book fall into four main categories: - Detection and characterization of corrosion damage. - Characterization of polymer-based corrosion protective coatings. - Characterization of low cycle and high cycle fatigue damage. - Characterization of fretting and fretting fatigue damage. Since the earlist stages of damage occur in the material microstructure, NDE techniques for early detection of damage must be sensitive to changes in microstructure. For this reason the book will consider: - NDE inspection methods that are sensetive to microstructure and intrinsic materials property variations, and - High-resolution microscopic and imaging techniques. Detection and quantification of early stages of damage require high resolution, high sensitivity NDE techniques. However, the inspection results must be correlated with the damage states in the material, crack initiation criteria, and relevant mechanical properties in order to predict component life. NDE results have to be compared to benchmark techniques such as destructive fatigue and corrosion tests, as well as scanning and transmission electron microscopy that quantify microstructure states and microstructure variations during materials degradation