# Recent Advancements in Non-Destructive Testing Techniques for Structural Health Monitoring
## 1 Introduction  
Structures and infrastructure systems, namely buildings, dams, bridges, and pipelines require proper inspection, monitoring, and maintenance to support economics, quality of life and sustainability in civil engineering. The conventional practice is based on human visual inspection, which is not able to locate any hidden damages [1, 2]. Therefore, there is a need for better inspection techniques for monitoring the deterioration of infrastructure as assessing the condition of a structure is necessary for its safety and reliability. Non-structive evaluation (NDE) provides methods for the detection of hidden damages, which provides valuable information that can be utilized to extend the lifespan of monitored structures. However, the structures' large size gives a challenge for sensor implementation, which is offering more localized inspection. In recent years, many researchers have investigated different techniques for structural health monitoring using different methods, namely electromagnetic, acoustic and others. A number of commonly monitored parameters used for Structural Health Monitoring (SHM) applications include the detection or measurement of strain, load, displacement, impact, pH-level, moisture, vibration signatures, and presence of cracks [3].  
The working principle of nondestructive testing (NDT) techniques varies depending on the measuring parameter, type of the structure, and its physical properties. For instance,compressional and shear wave velocities and electrical resistivity must be considered in terms of the building material and engineering properties of the structure. The consideration takes into account an assumption about the structure, and it can also involve calibrating the measurement, which is a requirement for most NDT surveys. There is usually a specific NDT technique that is used for the measurement of a particular parameter based on the physical properties of structures and reliability of the used method. Five major factors should be considered during the selection of the design of the NDT survey, namely (1) the penetration capability of the technique, (2) the resolution requirements for measurements, (3) the contrast in physical properties between the target and its surroundings, (4) the level of noise of the techniques, i.e., whether the provided information about the measured structure is valid and (5) historical reputation of the method in its use in the construction of the structure. The consideration of these factors should lead to the specification in the design of the NDT method or provide an alternative to NDT techniques if it is not suitable for solving a particular problem [4].  
Therefore, in this paper, the working principle of various NDT techniques is discussed. Focus is placed on the following NDT techniques for SHM: sweep frequency technique, ground-penetrating technique, infrared methods, fiber optic sensors, camera-based methods, laser scanner techniques, acoustic emission sensors, and ultrasonic methods, which are presented in Sections 2-9, respectively. The basic principles of each technique are explained, and recent research advancements are critically reviewed.