Dynamic Modeling of Dry-Coupled Ultrasonic Testing: Time-Varying Contact Stiffness and Experimental Validation

Ultrasonic nondestructive testing (NDT) is an essential technique for evaluating structural integrity, but its conventional reliance on liquid couplants limits applicability in harsh or field environments. Although dry coupling offers a more practical alternative, static models are insufficient to describe the fluctuating interface conditions encountered during scanning inspections. To address this limitation, we develop a dynamic dry-coupling model that incorporates time-varying contact stiffness, linking applied pressure, surface roughness, and material properties within the classical acoustic reflection framework. Finite element simulations using COMSOL Multiphysics were employed to determine the empirical scaling factor α, ensuring that the formulation remained physically consistent while avoiding arbitrary parameter fitting. Experimental validation was performed on a robotic ultrasonic testing platform equipped with a 5 MHz dual-element probe and a stepped 1018 steel block prepared with multiple surface roughness levels. By systematically varying scanning speed, contact pressure, and interface roughness, a comprehensive set of dynamic ultrasonic data was obtained. The results demonstrated clear periodic modulation of reflection amplitudes consistent with theoretical predictions, with discrepancies generally within ±10%. Collectively, these findings establish a robust and predictive framework for modeling ultrasonic reflection under dry-coupled conditions, providing a reliable basis for evaluating coupling quality in practical inspection scenarios.