## 3 SHM Using Guided Ultrasonic Waves in Laminated Composites
### Recent Advancements in Ultrasonic Guided Waves for SHM of Composites  
The anisotropic properties of composite materials significantly affect guided ultrasonic wave propagation, causing directional variations in phase velocities and scattering behaviors. This complexity arises from the directional dependency of stiffness and density, leading to challenges like mode overlap and conversion. Numerical simulations using Christoffel's equation predict wave behavior, while experimental methods such as selective mode excitation and wave-mode isolation enhance defect detection [78]. Advanced \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sensor Type** & **Principle** & **Advantages** & **Limitations** & **Applications** \\ \hline \multirow{3}{*}{PZT} & Utilizes the piezoelectric effect to generate and receive waves. & High sensitivity, compact size, low power consumption. & Limited temperature range, susceptible to mechanical damage. & Damage detection in aerospace and automotive composites [21]. \\ \hline \multirow{3}{*}{Fiber Optic Sensors} & Detect strain and temperature changes using light signals. & High accuracy, immune to electromagnetic interference. & High cost, require specialized equipment for installation. & Long-range monitoring of large-scale structures, such as wind turbine blades [71] \\ \hline \multirow{3}{*}{Laser-Based Sensors} & Non-contact laser vibrometry measures surface displacements. & Non-contact measurement, high precision. & Affected by environmental conditions, complex setup. & Non-destructive evaluation of aerospace composite panels [72]. \\ \hline \multirow{3}{*}{Electromagnetic Acoustic Transducers (EMATs)} & Generate and detect ultrasonic waves using magnetic fields. & Non-contact, effective in harsh environments. & Limited range, require strong magnetic fields. & Inspection of thick composite structures [73]. \\ \hline \multirow{3}{*}{Capacitive Sensors} & Detect changes in capacitance due to material deformation. & Low cost, simple design. & Susceptible to low sensitivity for small deformations, temperature-dependent. & Monitoring strain in composite repairs [74]. \\ \hline \multirow{3}{*}{Ultrasonic Phased Array} & Utilizes multiple sensors to focus ultrasonic beams. & High-resolution imaging, wide area coverage. & Complex data interpretation, expensive hardware. & High-resolution imaging detects delamination [75]. \\ \hline \multirow{3}{*}{Smart Material Sensors} & Integrate sensors within composites for real-time data. & Continuous monitoring, embedded in the structure. & Challenging to replace or repair, potential for signal interference. & Real-time SHM in aerospace and marine applications [76]. \\ \hline \multirow{3}{*}{Magnetostrictive Sensors} & Employ the magnetostrictive effect to detect strain changes. & Robust, no direct contact needed. & Complex calibration, limitations due to magnetic material properties. & Monitor corrosion in pipeline systems [77]. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sensor technologies for guided ultrasonic waves for SHM. Recognizing the inherent uncertainties in defect detection and localization, some researchers have incorporated probabilistic approaches into guided wave tomography (GWT). For instance, [PERSON] et al. (2022) proposed a Bayesian imaging framework for guided wave-based damage detection in composites [93]. This technique employs Sparse Bayesian learning across numerous measurement vectors to enhance the processing efficiency of array signals in Lamb-wave SHM and NDT, allowing for the effective imaging of deterioration in composite laminates. Moreover, by poorly aligning interference waves with dictionary atoms that represent effective waves, the M-SBL method significantly reduces their impact, thus providing crucial information for risk assessment and decision-making in SHM applications.