# Weakening Detection of Composite Structure Adhesive Layer Based on Nonlinear Guided Waves
## 1 Introduction  
Composite structures are used extensively in various industries owing to their exceptional fatigue resistance and high process flexibility, including construction engineering, healthcare, and more. However, the properties of the adhesive layer in composite materials may diminish as a result of cumulative wear and tear from use, aging naturally, or other circumstances. This is called adhesive layer weakening. The requirement to identify adhesive layer degradation in multilayer composite structures has grown more pressing as engineering materials and structural complexity have increased [1, 2, 3, 4]. Traditional linear ultrasonic detection techniques encounter several obstacles and challenges due to the intricate interfacial coupling effects and scattering problems in multilayer composite systems. [PERSON] et al. [5] used traditional ultrasonic guided wave methods to evaluate the adhesive quality of specimens. The results indicated that, although delamination was detected atrelatively long distances, the method showed lower accuracy in terms of defect quantification. [PERSON] et al. [PERSON] et al. (2018) investigated the effect of composite material thickness at different temperatures on damage detection using linear guided waves, and the results suggested that damage localization was inaccurate in a 9 mm thick composite plate. Nonlinear ultrasonic guided wave techniques have many benefits over conventional linear ultrasonic detection techniques [PERSON] et al. (2018); [PERSON] et al. (2018); [PERSON] et al. (2018), including high sensitivity, a broad detection frequency range, multimodal features, non-contact nature, and high resolution [PERSON] et al. (2018); [PERSON] et al. (2018). Therefore, it is crucial to investigate nonlinear ultrasonic guided wave detection for adhesive layer weakening in multilayer composite systems [PERSON] et al. (2018); [PERSON] et al. (2018). [PERSON] et al. [PERSON] et al. (2018) examined the generation characteristics of second-order harmonics of nonlinear Lamb waves in the context of single-layer plates. Three prerequisites for fundamental-frequency guided waves to produce second-order harmonics with cumulative effects are group velocity matching, phase matching, and non-zero energy flux. [PERSON] (2018) conducted numerical simulations and experiments to investigate the interaction between mixed non-collinear Lamb wave pulses and non-planar wave fronts. [PERSON] (2018) calculated the acoustic field of accumulated second harmonic Lamb waves in solid plates, and their results indicated that regardless of the symmetry of the fundamental frequency Lamb wave field, the acoustic field of the accumulated second harmonic is always symmetrically distributed. Furthermore, regardless of whether the fundamental wave is symmetric or antisymmetric, its second harmonic exhibits an accumulation effect. To identify fatigue cracks in steel structural joints, [PERSON] et al. [PERSON] et al. (2018) used nonlinear guided waves to assess and quantify the fatigue cracks by creating second-order harmonics. As cracks grew, so did the percentage decrease in nonlinearity under vibration. [PERSON] et al. [PERSON] et al. (2018) investigated the phenomenon of second-order harmonics in guided waves propagating through isotropic, unstressed elastic pipes. The nonlinear coefficient increased with the propagation distance. This confirmed that the second-order harmonics of guided waves propagating in pipes reveal a cumulative effect as the distance increases. [PERSON] et al. [PERSON] et al. (2018) conducted computational modeling and research to investigate the feasibility of utilizing nonlinear ultrasonic guided waves for detecting early-stage material degradation in metal plates. They analyzed the second harmonic of Lamb waves in aluminum plates with free boundaries.