## 2 Common Failure Mechanisms in Laminated Composites  
Laminated composites are engineered materials formed by stacking multiple layers (or \"plies\") of composite materials, which are typically constructed of fibers (such as carbon, glass, or acamid) embedded in a polymer matrix [29]. These layers are bonded together, with fibers oriented in various directions in each layer, giving the composite distinct combinations of high strength, stiffness, and lightweight qualities. Laminated composites are widely employed in aerospace, automotive, and other high-performance industries due to their superior mechanical qualities compared to traditional materials such as metals [30]. The arrangement of fibers in specific orientations enables customizable mechanical properties, making these materials suitable for structural applications. Common manufacturing defects include delamination, occurring when layers fail to join properly due to improper curing or contamination, and voids, represented by air pockets trapped in the composite matrix during the curing process. Misaligned fibers or uneven resin distribution can lead to fiber waviness or resin-rich patches, both reducing composite strength [31]. Over time, mechanical overloads, impacts, or fatigue can induce in-service defects such as matrix cracking, fiber breaking, and delamination [32]. Environmental variables like moisture absorption or temperature fluctuations may cause swelling, thermal tensions, and the chemical degradation of the polymer matrix, resulting in long-term damage [33]. In composites, delamination or interlayer debonding failure often results from interlaminar stress exceeding its strength under conditions such as temperature and external load. The primary mode of structural degradation in composite laminates is through the progression of delamination. Several studies on the delamination of composite materials used in aviation airfoil structures have employed non-destructive identification methods. Figure 1: Framework of the ultrasonic guided wave-based structural health monitoring process for composite materials. Researchers have experimentally investigated the causes and factors influencing the spread of delamination. Materials such as paper, Teflon film, vacuum bags, peel plies, and flash tape have been introduced into composites as artificial defects [34; 35]. Laminates with pre-set delamination exhibit relatively high modal frequencies and damping. However, limited research has been conducted on how these materials interact with delamination and how to characterize them using numerical models, although ultrasonic testing can reliably detect delamination greater than half a wavelength [36]. Specifically, mode conversion during delamination reflects acoustic waves [37], complicating the detection of signals scattered by delamination. Matrix cracking, where the resin matrix within the composite material fractures due to thermal or mechanical stresses, is challenging to detect using traditional ultrasonic testing techniques due to its small size and the potential for signal attenuation [38]. On the other hand, fiber breakage involves the rupture of reinforcing fibers within the composite. This failure mode can be identified using ultrasonic testing by noting changes in wave propagation characteristics, such as increased attenuation or shifts in phase velocity [30]. Numerous researchers have utilized numerical techniques. [PERSON] and [PERSON] [39] explored advantages in achieving mesh independence for cracked structures. In contrast, [PERSON] and [PERSON] [40] applied the extended FEM to analyze material microstructures for fractures, thereby reducing the need for mesh rebuilding in various locations. Guided ultrasonic waves (GUWs) are an effective method for detecting faults in composite structures as part of structural health monitoring (SHM). These waves, which propagate along the boundaries of a structure, are extremely sensitive to changes in material properties such as stiffness or density, making them excellent for identifying internal faults.