## 6 Conclusions  
In summary, the paper investigated the effects of composite thickness on ultrasonic guided wave (UGW) behavior and damage detection using embedded PZT transducers. The active sensing results showed that the amplitude and group velocity of UGWs were influenced by the thickness of the composite panels, with a linear reduction observed as thickness increased. The temperature study revealed that the peak amplitude and time-of-flight (ToF) of UGWs were affected by temperature, with different behaviors observed depending on the thickness and mode. Comparisons between embedded and surface-mounted PZT transducers showed variations in the temperature responses, highlighting the advantages of using embedded PZT transducers in certain scenarios. Damage detection and localization were performed for surface-mounted artificial damage and impact damage. The results demonstrated that both the A\({}_{0}\) and S\({}_{0}\) modes of UGWs could accurately detect and locate surface-mounted artificial damage. A comparison between embedded and surface-mounted results showed that the embedded PZT transducers had lower damage index values for the A\({}_{0}\) mode but higher values for the S\({}_{0}\) mode compared to surface-mounted PZT transducers. Additionally, the embedded PZT transducers exhibited better performance in locating surface-mounted artificial damage in thick composites, particularly at 250 kHz for the 9 mm panel. For impact damage, the damage index and DAS algorithm successfully detected and located the damage using the A\({}_{0}\) mode at 50 kHz with embedded PZT transducers. The surface-mounted PZT transducers were not as effective in locating the impact damage. Overall, the results highlight the benefits of using embedded PZT transducers for UGW-based damage detection and localization, particularly in thick composite laminates. The findings provide valuable insights for the development of structural health monitoring systems and the optimization of damage detection techniques in composite structures. Experimental work, analysis and preparation of the manuscript, T.F. ; supervision of research and preparation of the manuscript, M.H.F.A. All authors have read and agreed to the published version of the manuscript. Figure 23: Localization of impact damage for the 9 mm thick panel at (**a**) 250 kHz using embedded PZT transducers and at (**b**) 50 kHz and (**c**) 250 kHz using surface-mounted PZT transducers (where the “\(\bigcirc\)” is the position for real damage and the “\(\times\)” is the position for predicted damage). **Funding:** This research received no external funding. [PERSON]'s scholarship was funded by the Aviation Industry Corporation of China, Ltd. (AVIC), AVIC General Huanan Aircraft Industry Co., Ltd., and China Scholarship Council (No. [2017] 5082). **Institutional Review Board Statement**: Not applicable. **Informed Consent Statement**: Not applicable. **Data Availability Statement**: Data sharing is not applicable. **Conflicts of Interest**: The authors declare no conflict of interest. * [PERSON] et al. (2019) [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON] Nonlinear ultrasonic inspection of smart carbon fibre reinforced plastic composites with embedded piezoelectric lead zirconate titanate transducers for space applications. _J. Intell. Mater. Syst. Struct._**2019**, _30_, 2995-3007. [CrossRef]
* [PERSON] et al. (2021) [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON] Application of PZT Ceramic Sensors for Composite Structure Monitoring Using Harmonic Excitation Signals and Bayesian Classification Approach. _Materials_**2021**, _14_, 5468. [CrossRef] [PubMed]
* [PERSON] (2001) [PERSON] Active Health Monitoring of Aerospace Composite Structures by Embedded Piezoceramic Transducers. Ph.D. Thesis, Department of Aeronautics, Royal Institute of Technology, Stockholm, Sweden, 2001. * [PERSON] (2002) [PERSON] Integrity of graphite/epoxy laminate embedded with piezoelectric sensor/actuator under monotonic and fatigue loads. _Smart Mater. Struct._**2002**, _11_, 527.