CHAPTER 2 LITERATURE REVIEW The literature review chapter presents the critical points regarding the current knowledge on PZT and SHM sensors. The existing approach of SHM is discussed and the analysis of PZT technology with statistical indices is also included.2.1 Structural Health MonitoringStructural health monitoring (SHM) is defined as the process of implementing implementation of a damage identification strategy for aerospace, civil and mechanical engineering infrastructures. In this case, damage can be defined as changes introduced into a system that negatively affect its current or future performance (Farrar & Worden, 2006). Damages are considered defects and these defects originate from the material level. Under certain conditions specific to the specimen alone, defects can extend to a point undesirable for the material and thus lead to rupture. Failure could result in loss of life or significant economic consequences depending on the degree of the failure. Thus, over the last decade, governments and industries have become more aware of SHM in order to prevent such adverse events. There are a growing number of approaches toward SHM over the past decade due to increased interest. One such effective approach to note would be the statistical pattern recognition paradigm (Farrar, Duffey, Doebling, & Nix, 1999). This approach consists of 4 steps: (a) Operational evaluation (b) Data acquisition and cleaning (c) Feature selection and data compression (d) Statistical model development The step of great interest would be the step data acquisition and cleaning. This component would include excitation methods, sensor types and locations, and other data collection methods. Apart from that, the temporal aspect of data collection is a central point of the paper......D serves as a good characterization of damage in SHM. Changes in the signals obtained can show changes in structure and are statistically quantified by values. The RMSD has been used effectively before, for example between the signatures of two states, as an appropriate damage index (Guirguitiu and Rogers, 1998). The RMSD is widely accepted and used to estimate the magnitude of variation between two signatures (Annamdas, 2012). The formula is given by: The RMSD index reflects changes in the structure at different stages as damage propagates along the material. There are limitations to the use of RMSD, including difficulty in locating damage and severity due to inconsistencies. However, it is capable of predicting cracks and provides statistical models in calculating the readings obtained and therefore remains a useful tool in EMI for SHM..