###### Abstract  
Structural health monitoring (SHM) is essential for ensuring the safety and longevity of laminated composite structures. Their favorable strength-to-weight ratio renders them ideal for the automotive, marine, and aerospace industries. Among various non-destructive testing (NDT) methods, ultrasonic techniques have emerged as robust tools for detecting and characterizing internal flaws in composites, including delaminations, matrix cracks, and fiber breakages. This review concentrates on recent developments in ultrasonic NDT techniques for the SHM of laminated composite structures, with a special focus on guided wave methods. We delve into the fundamental principles of ultrasonic testing in composites and review cutting-edge techniques such as phased array ultrasonics, laser ultrasonics, and nonlinear ultrasonic methods. The review also discusses emerging trends in data analysis, particularly the integration of machine learning and artificial intelligence for enhanced defect detection and characterization through guided waves. This review outlines the current and anticipated trends in ultrasonic NDT for SHM in composites, aiming to aid researchers and practitioners in developing more effective monitoring strategies for laminated composite structures. structural health monitoring; ultrasonic techniques; laminated composite structures; non-destructive testing; machine learning +
Footnote †: journal: Applied sciences  
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M.T.T. and enables ongoing assessment throughout a structure's lifecycle. In contrast, NDE is typically performed offline, using portable or manipulable measurement systems to inspect components at specific times. It offers valuable but periodic insights, often requiring operational downtime for maintenance. Ultrasonic waves are particularly effective in detecting delamination in composite materials, a critical failure mechanism. This method precisely identifies and quantifies the extent of delamination. The Fiber/Matrix Crack Ultrasound technique is capable of detecting cracks in the fibers or polymer matrix, common damages resulting from mechanical or environmental stresses [6]. Numerous traditional testing methods have been developed for SHM of composites, such as acoustic emission, radiography, thermography, and shearography [7; 8; 9; 10; 11; 12; 13; 14; 15]. These techniques have proven to be useful throughout the lifecycle of composite products in various areas, including manufacturing inspection, inservice assessment, process design and optimization, and structural health monitoring [16]. However, their limitations, including sensitivity to environmental conditions and challenges in detecting subsurface defects, necessitated the development of more advanced methods. Various solutions have been developed to enhance the diagnosis of damaged composite structures and monitor their evolution over time [14]. Various solutions have been developed to enhance the diagnosis of damaged composite structures and monitor their evolution over time [14]. [PERSON] et al. [17] evaluated the ultrasonic-guided Lamb wave approach, highlighting its extensive detection range and high efficiency, and recognized significant advancements in engineering compared to traditional non-destructive testing technologies established over the past 50 years. Ultrasonic testing can detect interior cavities or porosity zones that might weaken or compromise the material's structural integrity. It is integrated into continuous SHM systems to provide real-time updates on the structural integrity of composite materials. Sensors can be embedded in the material or attached to the surface for ongoing monitoring. Ultrasound is highly sensitive to minor flaws, making it ideal for early damage detection, which is essential to prevent further damage in laminated composites. This technique does not require the disassembly or removal of components, allowing for in-place assessments of large or complex composite structures [18].