## 3 SHM Using Guided Ultrasonic Waves in Laminated Composites  
Ultrasonic wave transmission in anisotropic materials, such as composite structures, differs fundamentally from that in isotropic materials due to the directional dependence of their mechanical properties. The direction of propagation and wave polarization in ultrasonic waves determine their velocity in anisotropic materials. This phenomenon is governed by the material's stiffness tensor, which relates stress and strain across different directions [56]. [PERSON]'s equation, which links the wave vector to the stiffness tensor and material density, describes the propagation of ultrasonic waves in anisotropic materials [57]. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Defect Type** & **Description** & **Detection Method** & **Challenges** & **References** \\ \hline Drilling-Induced Damage & Damage during drilling processes can lead to delamination, fiber pullout, chipping, and fuzzing. & Ultrasonic non-destructive testing (NDT). & Surface flaws and fiber fractures are difficult to detect; they increase the risk of safety issues in aircraft parts. & [49; 50; 51; 52; 53; 54] \\ \hline Impact Damage & Damage from external impacts, such as hail, bird strikes, or dropped tools, can cause matrix cracking, fiber fractures, and delamination. & Conventional non-destructive testing (NDT). & Environmental factors such as temperature, humidity, and vibration affect detection; scanning large areas is time-consuming. & [8; 55] \\ \hline Cracking & Development of small cracks in the matrix can grow under load, leading to fiber breakage and laminate failure. & Extended Finite Element Method (XFEM) simulation. & Microcracks that are not detected early can lead to significant structural failure. & [40; 45] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 2: (a) Common failure modes of CFRP composites and (b) typical damage types of composite materials during processing and service periods. Quasi-longitudinal, quasi-shear, and pure shear waves are the three different wave modes resulting from solving this equation. Each mode's unique propagation properties, such as phase velocity and attenuation, are affected by the material's anisotropy [58; 59]. In composite materials, the presence of multiple phases and interfaces further complicates wave propagation. The interaction of ultrasonic waves with these interfaces can cause mode conversion, scattering, and reflection, all of which must be considered when interpreting ultrasonic testing results [60]. The transmission of ultrasonic waves through a carbon-fiber-reinforced polymer (CFRP) composite over three surfaces is illustrated in Figure 3. This technique is a key nondestructive evaluation method for composite materials, recognized for detecting internal flaws without damaging the structure. It operates by transmitting high-frequency sound waves (0.5-25 MHz) into the material and analyzing the reflected signals to identify anomalies such as delaminations, voids, and inclusions [61]. Composite materials, particularly carbon- and glass-fiber-reinforced polymers, present unique challenges for ultrasonic inspection due to their anisotropic and heterogeneous nature. Factors like fiber orientation, resin properties, and interfacial defects impact ultrasonic wave propagation, making signal interpretation dependent on a comprehensive understanding of wave-matter interactions specific to composites [62]. Guided wave ultrasonic testing utilizes the propagation properties of guided waves to identify and describe flaws and has become an essential method for the SHM of composite constructions [64]. This method is particularly beneficial for composite materials due to its ability to inspect large areas with minimal sensor deployment, which enhances efficiency and reduces inspection time [65; 66]. Guided waves, such as Lamb waves, can propagate over long distances with low attenuation, making them ideal for monitoring the integrity of composite structures.