###### Abstract  
This paper describes a study that focuses on assessing the influence of composites with different thicknesses (2 mm, 4 mm, and 9 mm) on embedded ultrasonic guided waves (UGWs) under varying temperatures. The study also demonstrates the effectiveness of these embedded sensors in identifying damage. A novel cut-out method that included an embedded diagnostic layer and phased-array lead zirconate titanate (PZT) transducers, created using the ink-jet printing technique in the manufacturing process was employed. The research then focused on studying the behavior of UGWs under varying temperatures for each composite panel. This analysis aimed to understand how temperature variations affected the propagation of guided waves in thick composites. Finally, artificial damage on the surface and impact damage were introduced, both embedded and surface-mounted PZT transducers were used to detect and locate these damages in different thickness composite panels. The results of damage localization indicated that the embedded PZT transducers were more sensitive than the surface-mounted transducers in locating the damage in thick composites. structural health monitoring (SHM); embedded lead zirconate titanate (PZT) transducers; thickness influences; thick composites; damage detection and localization +
Footnote †: journal: Applied Sciences  
transducers are isolated from exposed environmental conditions, they experience reduced aging and maintain excellent stability, durability, and repeatability [18; 22]. Therefore, it is necessary to embed PZT transducers into composites, especially for thick CFRP composites, to enhance the durability of the transducers and improve the sensitivity of UGWs for damage localization. Embedded PZT transducers not only address the limitations of surface-mounted transducers but also provide enhanced monitoring capabilities for structural health assessment in CFRP composites. The study conducted by [PERSON] et al. [18] focused on comparing the A\({}_{0}\) mode of UGWs at 100 kHz, generated by both surface-mounted and embedded PZT transducers, for glass fiber-reinforced plastic (GFRP) composites. The researchers also investigated the electro-mechanical impedance (EMI) properties of the transducers and compared them. The EMI results revealed that the embedded PZT transducers exhibited a 50% higher coupling factor compared to the surface-mounted transducers in GFRP composites. This indicates that the embedded transducers had a more effective energy transfer between the transducer and the composite structure. The higher coupling factor suggests improved sensing capabilities for damage detection. In terms of signal-to-noise ratio (SNR), the study found that the embedded PZT transducers provided a better SNR compared to the surface-mounted transducers. A higher SNR indicates a stronger and clearer signal relative to the background noise, which enhances the detection capability of barely visible impact damage (BVID). BVID refers to damage that is not easily visible but can significantly affect the structural integrity of the composite. Additionally, the researchers utilized the damage localization RAPID algorithm to locate the BVID for different impact energies. The algorithm, combined with the embedded PZT transducers, allowed for accurate and reliable identification of the damage location, even for varying impact energies. This demonstrates the effectiveness of embedded PZT transducers in localizing and assessing BVID. The study by [PERSON] et al. [23] proposed a novel embedding technique for PZT transducers in composite laminates. They utilized woven E-glass fiber fabric to provide electrical insulation for the top of the embedded transducers. The researchers found that artificial delamination and BVID could be detected using second harmonic generation and nonlinear modulation of the ultrasonic spectrum, employing a laser Doppler vibrometer (LDV).