## 3 Results
### Sensors' Utilization  
The use of a particular sensor type is a critical decision during the development and implementation of the SHM process. Similarly, the performance attributes/factors/parameters for which the data are collected is another important decision. The entire network design depends upon the selection of sensor type and performance attributes. Based on these \begin{table}
\begin{tabular}{l l l l} \hline \hline
**S. No** & **Category** & **Number of Research** & **Reference** \\ \hline
1 & Electromagnetic vibration-based & 6 & [39; 42; 45; 47; 67; 70] \\ \hline
2 & PZT vibration-based & 5 & [73; 74; 75; 76; 77] \\ \hline
3 & Solar & 6 & [38; 48; 53; 68; 69; 78] \\ \hline
4 & Wind & 3 & [38; 69; 70] \\ \hline
5 & RF & 1 & [72] \\ \hline
6 & Thermal & 1 & [73] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Statistics for energy harvesting techniques. \begin{table}
\begin{tabular}{c l l} \hline \hline
**S. No.** & **Description** & **Details** \\ \hline
1 & Bibliographic information & Title of the research, list of authors, year of publication, and information about the scientific database \\ \hline \multicolumn{3}{l}{**Extraction of data**} \\ \hline
2 & Overview & The fundamental idea and goals of the selected research \\ \hline
3 & Results & Results achieved by the selected research \\ \hline
4 & Validation & The employed testing method for the verification \\ \hline \multicolumn{3}{l}{**Synthesis of data**} \\ \hline
5 & WSN platforms & First predefined category \\ \hline
6 & Energy harvesting techniques & Second predefined category \\ \hline
7 & Sensor types & The type of sensor deployed in the SHM system \\ \hline
8 & Inspection scale & The scale of the SHM system (local or global) \\ \hline
9 & Response type & The type of the system response (static or dynamic) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Details of data extraction and synthesis. decisions, the routing protocol is targeted. Similarly, the algorithms for damage detection and localization are entirely based on the type of sensors and the corresponding performance attributes. It can be observed from Table 5 that the most important sensors in the selected research articles are accelerometers (27 studies), temperature sensors (14 studies), and strain gauges (11 studies). The accelerometer is used to indirectly measure the stiffness, pylons, and hanger cables in SHM applications using vibration measurement. It is frequently utilized due to its low cost and ease of use. The temperature is commonly used in SHM applications for compensation and estimation purposes of environmental factors. Table 5 shows that some types of sensors have been utilized only in one selected article. This utilization is for a specific purpose. It includes acoustic emission (AE) sensors, wind gauge sensors, RFID sensors, infrared sensors, light sensors, sound sensors, and polyvinylidene fluoride (PVDF) sensors. The AE sensors utilize stress waves. These stress waves are generated by the immediate adjustment of internal stress. Some possible causes for this immediate adjustment are the growth of fatigue cracks and dislocation movements. In other words, the primary reasons for the stress waves are concerned with the damage. Therefore, the assessment of acoustic emissions can be linked with the failure of the material and can be sensed by using an AE sensor [79]. For example, [PERSON] et al. [39] proposed a real-time system for monitoring the active fatigue cracks in bridges using the acoustic emission sensor. The system is capable of the detection and accurate localization of fatigue cracks with high accuracy. The PVDF piezoelectric-film sensors (abbreviated as PVDF sensors) are based on the piezoelectric effect. The piezoelectric effect is the capacity of different materials to produce an electric charge across its boundaries. The electric charge is produced when mechanical stress is produced [80]. For example, [PERSON] et al. [64] proposed an effective strategy using accelerometers and PVDF sensors.