Recently, aircrafts and space structures, such as space stations, are growing in size. Because weight saving is required in these structures, aluminum alloys and various composite materials, which are light weight and excellent in specific strength and specific stiffness, are widely used. When these materials are used for aircrafts and space structures, cracks or delaminations may be induced by fatigue and external impact events, etc. The possibility of causing some serious failure accidents rises even if damages are comparatively small because these materials are usually used for thin-walled components in aircrafts and space structures. In order to prevent these accidents beforehand, it is very important to detect damages by using nondestructive testing (NDT) and to secure safety and the health of the structures.
Up to date, NDT is mainly realized by using X-ray inspection, ultrasonic inspection, eddy-current test, magnetic-particle testing and liquid penetrant inspection etc. These inspection methods have high accuracy and reliability. However, they also have the following disadvantages. First, they usually take a long time to finish the inspection process because their inspection area is relatively narrow at one time point. Second, they are not suited for real-time structural health monitoring because they need to stop the structural operations for carrying out the inspections. To overcome these demerits, the structural health monitoring technique that uses Lamb wave is actively studied in recent years. There have been some methods in damage detection using Lamb wave. However, almost all of conventional methods use both experimental data of intact and damaged structures, which is not suitable for some ageing structures without the data before damage. The method in the present research uses the information of damaged structures only. The reflected wave from damage is directly employed for damage identification. Therefore, this method is simpler and more convenient than conventional methods, which can be used for various structures in a wide range.
As the conclusion of this research after theoretical and experimental investigations, this method is very effective for the health monitoring of various large-scale structures. Naturally, without the experimental data of intact structures, this method needs some additional information. One is Lamb wave velocity traveling through intact structures, the other is geometric dimensions of the structures.
In this thesis, we have verified the effectiveness of this method for its practical applications by experiments and numerical analyses.
edited by Takahito SHIMOMUKAI