Evaluation of contact stiffness between solid-solid interfaces using dual-frequency ultrasound

Acoustical methods are playing an important role for nondestructive evaluation (NDE) of adhesive-bonded composites and components in industrial applications. A dual-frequency ultrasonic technique is proposed for the quantitative evaluation of contact strength between pressed solid surfaces. An ultrasonic excitation consists of two primary frequency components is applied perpendicular to the interface, and the transmitted wave is examined. Theoretical study is based on a perturbation analysis of contact acoustic nonlinearity (CAN) model, predicting the generation of difference and sum frequency waves, together with the second harmonics. Nonlinear parameters are defined to describe the nonlinearity generation efficiencies. Experiments are performed for three types of interfaces, i. e. the interfaces of two aluminum alloy blocks with and without couplant and two glass blocks. The difference frequency wave component has bigger generation efficiency than other nonlinear components, which offers an advantage of high SNR and good detection capability of contact stiffness (interfacial stiffness). For each interface, the first and second-order interfacial stiffness are measured with contact pressure increasing from near zero to about 0.8 MPa with the aid of a laser interferometer. Finally, numerical simulations are also carried out, and a consistency is found between measurements and calculations. The dual-frequency ultrasound sent to the interface generates at least four second-order nonlinear components, which enriches the CAN technique for interface quality examinations. Both measured and simulated results indicate an increase of interfacial stiffness and decrease of nonlinear parameters with growing contact pressure. Moreover, measured results show that couplant between interfaces influences the contact stiffness evaluations in an enhanced manner, while the contact pressure determined by measured interfacial stiffness values are underestimated due to the couplant. The main problem comes from the contact between the transducersample interfaces, which brings extra nonlinearity to the detected signals and affect the accuracy of measuring.