Sizing of defects in thin metal plates using a frequency compound method based on nonlinear characteristics of airborne ultrasound

This study proposes a non-destructive testing method that combines air-coupled ultrasound and nonlinear harmonic techniques with frequency compounding. The frequency compounding process utilizes nonlinear phenomena observed in air-coupled ultrasound to enhance defect detection capability. The objective of this research is to achieve accurate localization and sizing of defects.To validate the proposed method, experiments were conducted using a 400 mm × 400 mm × 3 mm aluminum plate with a 5 mm square artificial thinning area to simulate wall-thinning defects. The effectiveness of the method was evaluated in terms of the accuracy of defect localization and sizing. As a result, it was confirmed that the proposed frequency compounding method, which selectively combines harmonic components up to the 10th order, maintained localization accuracy comparable to that of conventional methods while achieving an improvement in sizing accuracy of up to 50% in the wave propagation direction and 40% in the direction perpendicular to wave propagation. These findings demonstrate that the proposed method enables reliable estimation of both the position and size of defects.