Composite materials, with their high stiffness, high strength, and low weight properties, are attractive materials for industries with advanced needs such as the space, defense, naval systems, and transportation industries. However, due to the complex mechanism of fiber-matrix bonding, the mechanical properties of composite materials may be severely degraded by the presence of a defect. This limitation restricts engineering design from taking full advantage of the benefits of composite materials. While recent studies are continuing to develop composite materials having damage tolerant qualities (less brittle fibers and higher fracture toughness), this study aims to identify methods to measure the health condition of composite structures, fatigue, and its remaining life by identifying defects. The changing physical parameters, also called characteristic damage parameters, are listed as stiffness reduction, strength reduction, changes in dynamic properties (such as natural frequencies and damping properties), ultrasonic and acoustic emission properties, thermal and electrical properties. Various non-destructive testing (NDT) methods have been developed to measure characteristic damage parameters. In this study, up to date identified NDT methods are reviewed and their feasibility in health monitoring and damage detection of composite materials are evaluated. Integration of intelligent sensing and signal analysis processes for health monitoring of composite structures is emphasized.
Composite materials are formed by composition of two or more materials with different forms. Fiber-reinforced composites, most commonly used in structural applications, are formed by embedding high-strength, high-stiffness fiber materials into ductile matrix materials. Although the fibers have high strength, they are not able to withstand environmental conditions due to their brittle nature. On the other hand, matrix materials do not show high strength values, however, they are ductile, possess toughness, keep fibers in desired locations, and act as the transfer medium for the load on the fibers.