Effect of Protective Coating Thickness on Ultrasonic Guided Wave Propagation in Plate‐Like Structures

ABSTRACT Ultrasonic Guided Waves (UGWs) are widely employed for the inspection and monitoring of plate‐like structures due to their long‐range propagation capability and sensitivity to structural and material changes. In real aerospace components, however, structural surfaces are commonly covered by protective layers, such as coatings or metallic films, whose influence on guided‐wave propagation must be understood to ensure reliable data interpretation. This work investigates the effect of a protective surface layer on UGW propagation using a plate‐like portion of a real full‐scale thermoplastic composite demonstrator. A dedicated experimental campaign was conducted on coated and uncoated regions of the same component, enabling direct comparison under identical material and geometric conditions. Guided waves were excited and sensed using a network of piezoelectric transducers, focusing on the fundamental symmetric Lamb mode (S0). Results show that the protective layer causes a systematic increase in time‐of‐flight due to reduced group velocity associated with increased effective thickness. Conversely, signal amplitude remains essentially unchanged within the investigated frequency range, indicating negligible additional attenuation. These findings demonstrate that thin protective layers primarily affect dispersion characteristics rather than energy transmission, confirming the viability of UGW‐based monitoring on coated composite structures when coating‐induced velocity shifts are properly accounted for.