Numerical performance analysis of a quantitative evaluation method for adhesive lengths using mode-pair of Lamb waves

Lamb waves can be utilized to estimate the adhesive properties of adhesive joints. However, an accurate and widely applicable quantitative evaluation of the adhesive length using Lamb waves remains challenging. This study proposes a quantitative evaluation method that uses the mode-pair of Lamb waves. When the A0 mode enters a bonded area from an unbonded area, it splits into two modes, called a mode-pair, under specific conditions. Given that these modes have different wavenumbers, their interference varies with the propagation distance in the bonded area, that is, the adhesive length. Hence, the adhesive length can be evaluated inversely from the Lamb wave spectrum across the bonded area where a mode-pair occurs. A finite element analysis (FEA) was conducted to validate the performance of the proposed method, considering adhesive specimens comprising two aluminum plates bonded using an epoxy with different adhesive lengths. The errors in the adhesive lengths evaluated using the proposed method were within ± 2.4% for the range in which the adhesive length exceeded 10 mm, even though noise was present. In addition, the effects of ultrasonic attenuation, ultrasonic velocity, and adhesive thickness were investigated using the FEA. The results suggest that the adhesive length can be evaluated without considering the ultrasonic attenuation of the adhesive. Moreover, the selection of a frequency range suitable for both the ultrasonic velocity and the thickness of the adhesive under measurement is a prerequisite for implementing the proposed method.