## 1 Introduction  
Laminated composite structures that are currently used for structural applications, such as windmill propeller blades, rotorcraft blades, marine propeller blades, and turbine blades, are manufactured by stacking fiber-reinforced lamina in a sequence of interest to provide high strength, stiffness, and stability. These structures are more vulnerable to damage such as the de-bonding of laminate or delamination that originates from dynamic loads, impact loads, excessive deformations during the service, or poor manufacturing methods [1]. Delamination is the most common and severe form of damage that reduces the stiffness, stability, and load-carrying capacity of the structure. Delamination takes place inside the composite structure without being evident from external surfaces, resulting in matrix breakage and leading to deteriorating mechanical properties. Especially under dynamic loads, delamination can pose a severe threat to the reliability, integrity, and usability of laminate structures without any visible indications. Furthermore, the delamination in composites is usually difficult to identify in rea time, even with cost-intensive and time-consuming non-destructive testing (NDT) techniques [2]. Henceforth, efficient inspection methods and early detection are necessary to prevent catastrophic failures of structures. Structural health monitoring (SHM) is a state-of-the-art technology to monitor the condition and diagnose the damage and faults in machine and structural components. SHM uses advanced techniques, such as the vibration-based method [3; 4; 5], fiber Bragg grating (FBG) [5], acoustic emissions (AE) [6], impedance-based method [7], Lamb wave methods [8], etc. The use of the FBG technique is limited in the real-time health monitoring of the composite structures, because of the difficulty in embedding the optical fibers during fabrication. Furthermore, very fine grids of FBGs are necessary to capture changes in a structure [9]. The AE method cannot be used to detect damage in a structure unless the damage generates and propagates acoustic signals. Moreover, skilled personnel are required to differentiate the AE signal from the surrounding noise. On the other hand, impedance-based and Lamb wave techniques require a large number of transducers to precisely detect the location and size of the damage [10]. Compared with these approaches, the vibration-based method is generally proposed to diagnose the structural faults or damage, as the damage that is present in a laminated composite structures causes variations in dynamic behavior and the modal parameters, by which the incidence of faults can be detected [11]. Vibration signatures contain vital information regarding the structural health condition, such as the source of failure and its severity [12]. By analyzing the vibration signatures and the damage present in the laminated composite structures that cause variations in the dynamic behavior and modal parameters, the incidence of faults can be detected. [PERSON] et al. reviewed the damage diagnostic techniques in composite structures using vibration-based methods. This is achieved through a real-time monitoring system by adopting well-developed signal processing techniques [13]. Hence, the vibration signal analysis was chosen to monitor the health status of the composite plates. Vibration signals are generally captured using the accelerometer sensor in time, frequency, and time-frequency domains [14]. The frequency and time-frequency domains require essential mathematical functions to process the signal. Due to this time and computational complexity, the time-domain format is generally used for fault diagnosis studies. The captured time-domain data consists of cumulative information about the nature of the structure. This information has to be properly analyzed to decide on the structure. Hence, vibration signal analysis is essential for extracting meaningful information. Vibration signals can be analyzed using spectral analysis, wavelet analysis, waveform analysis, etc.