I. Introduction
Structural Health Monitoring (SHM) refers to the process of nondestructive autonomous in situ damage detection and characterization of engineering structures. for critical aerospace, civil and petrochemical structures, technologies for earlier damage identification both during manufacturing and in service are needed to maintain product quality and safety during the life of the structure. SHM offers the potential for long-term in situ damage detection and the cost effectiveness resulting from continuous monitoring with minimal human involvement.