**2. Vibration-Based Structural Health Monitoring Techniques**

Generally, in terms of vibration parameters used, vibration-based structural health monitoring techniques can be classified into three categories: frequency domain, time domain and time-frequency domain approaches.

### *2.1. Frequency Domain Methods for Vibration-Based SHM*

Modal parameters including frequency, mode shape, and damping are usually used in frequency domain methods for vibration-based SHM [11–15]. In addition, several Frequency Response Function (FRF) related parameters have also been widely adopted. Compared to time domain methods and time-frequency domain methods, the frequency domain methods can be used in more situations because the frequency domain properties of structure are more stable. Even if the structure is subjected to greatly different loading conditions, the extracted dynamic properties are almost the same, and they only depend on the structure itself. However, it should be noted that the algorithms to extract these properties are not easy to implement and sometimes time consuming.
