Identification interface stiffness of coating/substrate structure based on ultrasonic pressure reflection coefficient amplitude spectrum

Abstract

Due to the variation of interface stiffness, interface morphology, and testing frequency, coating/substrate interface stiffness identified through ultrasonic methods are usually non-unique and inaccuracy. This paper, a method for quantitative inversion interface stiffness of coating/substrate structure is proposed based on a constructed ultrasonic pressure reflection coefficient amplitude spectrum (URCAS). The URCAS of coupling medium/non-metallic coating/metal substrate structure is derived based on interfacial spring model. The high-sensitivity frequency and interface stiffness ranges are determined by analyzing the sensitivity of URCAS to testing frequency and interface stiffness in advance. The accurate inversion of interface stiffness is achieved through a least square objective function under the high-sensitivity frequency and interface stiffness ranges. The feasibility of proposed ultrasonic method was verified by numerical simulations and ultrasonic experiments implemented on epoxy resin coating/aluminum substrate specimens. Simulation results show that the inverted interface stiffnesses are in good agreement with the preset values, and the maximum relative error is less than 6% in the high-sensitivity interface stiffness range, while the inversion accuracy of interface stiffness significantly reduces, and the relative error exceeds 21% in the low-sensitivity range. Experimental results show that the tested tensile strength and interface stiffness have similar trends. The proposed ultrasonic method can quantitatively identify the interface stiffness and effectively characterize the bonding quality of coating/substrate interface.