CHAPTER 2 LITERATURE REVIEW The literature review chapter presents the critical points regarding the current knowledge of PZT and SHM sensors. The existing approach to SHM is discussed and the analysis of PZT technology with statistical indices is also included. 2.1 Structural Health Monitoring Structural Health Monitoring (SHM) is defined as the process of implementing a damage identification strategy for aerospace, civil and mechanical engineering infrastructure. In this case, harm 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 on a material level. Under certain conditions specific to the sample alone, defects can grow to a point that is undesirable for the material and thus lead to failure. Failure could lead to loss of life or large economic consequences depending on the severity of the failure. Therefore, in the last decade there has been increased awareness towards SHM by governments and industries to prevent such unwanted events. Over the past decade, there has been an increase in the number of approaches to SHM due to growing interest. One such effective approach to consider would be the statistical pattern recognition paradigm (Farrar, Duffey, Doebling, & Nix, 1999). This approach consists of 4 phases: (a) Operational evaluation (b) Data acquisition and cleaning (c) Feature selection and data compression (d) Statistical model development The phase of great interest would be the acquisition and cleaning phase of the data. This component would include excitation methods, sensor types and locations, as well as other data collection methods. Aside from that, the temporal aspect of data collection is a great… half of the paper… D serves as a good characterization of damage in SHM. Changes in the obtained signals can show changes in structure and are statistically quantified by values. The RMSD has indeed been used before, for example between two-state signatures, as an appropriate damage index (Guirguitiu and Rogers, 1998). RMSD is widely accepted and used for estimating the amount of variation between two signatures (Annamdas, 2012). The formula is given by: The RMSD index reflects the changes in the structure at different stages as the damage propagates along the material. There are limitations to the use of RMSD that include difficulty in localizing damage and severity due to inconsistencies. However, it is able to predict cracks and provides statistical models in the calculation of the obtained readings and therefore is still a useful tool in EMI for SHM.