## 3 SHM Using Guided Ultrasonic Waves in Laminated Composites
### Recent Advancements in Ultrasonic Guided Waves for SHM of Composites
The use of multiple guided wave modes has shown promise in enhancing the resolution and reliability of tomographic imaging in composites, combining guided wave propagation with imaging algorithms to produce detailed maps of structural integrity [94]. This method has been successful in detecting and localizing defects in composite structures, as recent studies have demonstrated [95; 96; 97; 98]. Figure 6 illustrates the configuration for the guided wave signal collection, utilizing guided Lamb-wave testing and tomography-imaging reconstruction to pinpoint fiber breakage within a specific area. While sending or receiving sound waves, the transducer array units in the PAUT system are governed by different time delay regulations, known as the focal law. This  
Figure 5: The Lamb-wave-based air-coupled ultrasonic testing device detects delamination flaws in carbon fiber composite plates [92]. Figure 6: Experimental setup for guided wave testing, depicting the B-scan at 90\({}^{\circ}\)[99]. technology enables ultrasonic beam focusing, deflection, and movement. PAUT employs an electronically controlled array of ultrasonic transducers, allowing the beams to be steered, focused, and scanned in multiple directions without physical movements of the probe. This feature facilitates the comprehensive inspection and monitoring of complex composite structures, providing high-resolution imaging and precise defect characterization. A primary advantage of PAUT is its ability to generate and detect guided waves like Lamb waves, which are especially useful for inspecting large composite material areas. A study by [PERSON] and [PERSON] [100] revealed that PAUT could detect defects with penetration depths up to 25 mm and fault sizes as small as 0.8 mm, displaying improved signal characteristics compared to traditional ultrasonic methods. Moreover, advanced signal processing algorithms and data fusion techniques have been incorporated into PAUT systems to enhance the accuracy and reliability of SHM in composite structures [101]. The apparatus used to examine the artificial flaws (drilled holes) in the CFRP sample, producing guided waves along the fiber's direction, is depicted in Figure 7.  
Nonlinear ultrasonic techniques are increasingly used for detecting early-stage damage in composite materials. Unlike conventional ultrasonic methods, which depend on the linear propagation of sound waves, nonlinear techniques utilize the material's nonlinear acoustic behavior in the presence of damage. This nonlinearity manifests as harmonic generation, frequency mixing, or other complex wave interactions sensitive to micro-scale defects, providing critical insights into the structural integrity of composite components. [PERSON] [102] emphasized the capabilities of nonlinear ultrasonic guided waves in detecting early material deterioration through principles of nondestructive evaluation. The research demonstrated that nonlinear ultrasonic guided waves can traverse significant distances, are reactive to breathing cracks, permit access to typically unreachable areas, and reveal early signs of material deterioration. The development of the FAD approach is particularly significant. By creating high-incident wave amplitudes, the FAD method can stimulate contact vibrations in crack faces, enhancing the detectability of closed cracks [103]. A comparison of the linear and nonlinear methods, specifically of the FAD, is presented in Figure 8.  
Figure 7: Experimental setup of the PAUT guided wave technique for identifying faults in CFRP samples [100]. PWAS and FBG sensors are some of the key technologies for SHM in composites [105]. PWASs, made from lead zirconate titanate, are lightweight, cost-effective, and excel in detecting delamination and cracks through ultrasonic waves but are susceptible to environmental degradation and require maintenance.