## 1 Introduction  
Composite structures are increasingly utilized in various armor applications due to their superior characteristics. However, the performance and behavior of these structures may be compromised by damage or impacts incurred during manufacturing, service, or maintenance [1]. Cracking, impact damage, and interlayer delamination commonly manifest during the long-term utilization of armor composites [2; 3; 4; 5]. Failure to promptly detect and repair such damage can lead to structural degradation, reducing the residual strength and fatigue performance of the materials, thus posing significant safety risks. Consequently, real-time impact monitoring for armor composite structures is imperative. Nowadays, the methods of SHM for composite structures include terahertz radiation [6], X-rays [7], piezoelectric sensors, etc. Lightweight, highly functional, and intelligent armor protection technology utilizing single pure metals has been under development since the 1960s [8]. Presently, in order to withstand projectile attacks and corrosion, armor composite sandwich structural plating is typically composed of bulletproof ceramic material as the panel, metal material as the back plate, and an interlayer laminated with composite materials, offering high hardness, superior bulletproof performance, low density, and robust elastic-plastic characteristics. Additionally, new materials have been incorporated into armor composites [9]. During shooting tests, alumina ceramics have demonstrated the ability to enhance the bulletproof capabilities of armor composites, attributed to their high elasticity performance and stability in oxidation and reduction atmospheres at temperatures up to 1950 \({}^{\circ}\)C. Another material, carbon fiber, can dissipate the kinetic energy of projectiles through tensile failure, thus decelerating the velocity of warheads owing to its low fracture elongation [10]. Furthermore, armor composite is segmented into a backplane and panel. The back plate must absorb residual energy through deformation, necessitating high stiffness and bending strength. Consequently, metal materials such as armor steel, aluminum alloy, and titanium alloy are employed as armor backplating materials, owing to their low density, high specific strength, strong corrosion resistance, structural bearing capacity, and bullet resistance. The discontinuity of transverse stress between multi-layer materials results in directional changes in wave velocity, and beam mixture contributes to the phenomenon of wave velocity propagation bending and angular changes in waveform distortion. Greater thicknesses can induce more complex signal multi-modality and dispersion phenomena, thereby amplifying the attenuation of monitoring signals and presenting challenges in damage monitoring. Indeed, the thickness of multi-layer armor composite materials can exceed 5 mm [2; 3; 4; 5]. In the case of sandwich armor composite materials, the presence of multiple layers and substantial thicknesses, along with the discontinuity of transverse stress between the layers, can induce directional changes in wave velocity. Greater thickness results in more pronounced multi-modal and dispersion phenomena of Lamb wave signals, exacerbating the attenuation of monitoring signals and complicating the accurate determination of Lamb wave propagation speed. As a promising technology for quantitatively identifying damage in composite structures, guided Lamb wave monitoring is widely employed in structural health monitoring (SHM) using piezoelectric sensors. [PERSON] [11] detected damage to a T800H/3631 carbon fiber composite structure with a thickness of 1 mm based on the variation in arrival time in the 50 mode along the 0\({}^{\circ}\) direction and monitored the amplitude change in the earliest arriving wave packet to determine the size of the damaged area. [PERSON] [12] impacted a 4.65 mm thick carbon fiber composite material consisting of 12 layers at different positions and trained a neural network to monitor the positions of impacts with varying energy levels.