Aiming at characterizing interfacial roughness of thin coatings with unknown sound velocity and thickness, we derive a full time-domain ultrasonic reflection coefficient phase spectrum (URCPS) as a function of interfacial roughness based on the phase screen approximation theory. The constructed URCPS is used to determine the velocity, thickness, and interfacial roughness of specimens through the cross-correlation algorithm. The effect of detection frequency on the roughness measurement is investigated through the finite element method. A series of simulations were implemented on Ni-coating specimens with a thickness of 400 μm and interfacial roughness of 1.9–39.8 μm. Simulation results indicated that the measurement errors of interfacial roughness were less than 10% when the roughness satisfies the relationship of Rq = 1.6–10.0%λ. The measured velocity and thicknesses were in good agreement with those imported in simulation models with less than 9.3% error. Ultrasonic experiments were carried out on two Ni-coating specimens through a flat transducer with an optimized frequency of 15 MHz. Compared with the velocities measured by time-of-flight (TOF) method, the relative errors of inversed velocities were all less than 10%. The inversed thicknesses were in good agreement with those observed by optical microscopy with less than 10.9% and 7.6% error. The averaged interfacial roughness determined by the ultrasonic inversion method was 16.9 μm and 30.7 μm, respectively. The relative errors were 5.1% and 2.0% between ultrasonic and confocal laser scanning microscope (CLSM) method, respectively.
Identification of the Velocity, Thickness, and Interfacial Roughness of Coating Using Full Time-Domain URCPS: Cross-Correlation-Based Inverse Problem
Manuscript received August 28, 2018; final manuscript received December 2, 2018; published online January 22, 2019. Assoc. Editor: Ed Habtour.
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Ma, Z., Lin, L., Jin, S., and Lei, M. (January 22, 2019). "Identification of the Velocity, Thickness, and Interfacial Roughness of Coating Using Full Time-Domain URCPS: Cross-Correlation-Based Inverse Problem." ASME. ASME J Nondestructive Evaluation. February 2019; 2(1): 011008. https://doi.org/10.1115/1.4042177
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