Chapter 2: Health Monitoring of Composite Structures Using Ultrasonic Guided Waves

2.1 Introduction

The damage tolerant and fail-safe design of both new and aging aircraft, aerospace and civil structures require a substantial amount of inspection and defects monitoring at regular intervals, which add substantial amounts to their life-cycle cost. The current approach which consists of using conventional non-destructive evaluation (NDE) techniques such as C-scan, radiography, thermography, eddy current, etc. is expensive, time-consuming and often cannot be performed on structures already placed in service because of the inaccessibility or the special equipment required. According to some estimates, over 25% of the life-cycle cost of an aircraft or aerospace structure, which includes pre-production, production and post-production costs can be attributed to operation and support, involving inspection and maintenance ^{[[KUD 99]]}. The life-cycle cost of new aircraft and aerospace structures can be reduced significantly if continuous and autonomous condition-based structural health monitoring systems can be integrated into their design.

Composite materials are finding increasing use as primary structural components in many modern applications. As an example, the new Boeing 787 aircraft is expected to have the majority of its primary structure, including the fuselage and wings, made out of composite materials. However, it is well known that these materials are highly susceptible to hidden internal flaws which may occur during the manufacturing and processing of the material or while the structure is subjected to service loads. A major concern is the growth of undetected hidden damage caused by fatigue and low velocity foreign object impact that can reach a critical size during service,...