ABSTRACT
Composite materials have a lot of advantages that make them very attractive in application to many industrial branches such as aerospace, automotive, maritime, and wind energy. Their most important advantages are low weight, high mechanical strength, high stiffness, and vibration damping ability. However, there are also many problems with the exploitation of composite materials due to their common disadvantages. Most important is their susceptibility to initiation and growth of damage in the internal structure in the form of delamination. This type of damage is located between layers of composite material and is initiated by impact. Another type of damage is matrix cracking. These two types of damage can be hidden in the internal structure and may not be visible on the surface of composites. Because of this, a need existed to develop methods to detect and localize these defects. Initially, classical nondestructive testing (NDT) methods were utilized in order to assess the state of composite structures. Later, many researchers focused their attention on the development of structural health monitoring (SHM) systems and methods that could be utilized for composite structures. In 1993, Rytter, in his PhD thesis [1], formulated four levels of SHM:
1. Damage detection 2. Damage localization
2.1 Introduction .................................................................................................... 21 2.2 Elastic Waves-Based Method .........................................................................22 2.3 Acoustic Emission Method .............................................................................26 2.4 Electromechanical Impedance Method ..........................................................28 2.5 Vibration-Based Method .................................................................................30 2.6 Other Methods ................................................................................................ 33 2.7 Summary ........................................................................................................34 References ................................................................................................................ 35
3. Quantification of damage severity 4. Prediction of remaining life of structure
In the work of Worden et al. [2], a set of fundamental axioms related to SHM was defined. First of all, each material has some internal defect and flaw. Moreover, defects cannot be detected directly by sensors because sensors do not measure damage but, rather, signals related to some physical properties that can be connected to damage initiation and growth (temperature, pressure, charge, voltage, etc.).
