Hidden corrosion detection using laser ultrasonic guided waves with multi-frequency local wavenumber estimation.

Hidden corrosion damage of aircraft structures is a potential threat to the flight safety and a major maintenance factor for aircraft. In previous researches on corrosion detection, the local wavenumber estimation is obtained from a sliding wavenumber window that maximizes the wave energy at each grid point for one mode and frequency. This paper presents the methodology based on multi-frequency local wavenumber estimation for quantitative assessment of hidden corrosion in plates, which is obtained from a short-windowed Fourier transform in space domain of the wavefield at single mode and multiple frequencies. The methodology is applied to simulated wavefield to demonstrate the effect of frequency on corrosion depth image. The simulation results show that the calculated shape of the corrosion is closest to the actual value when the window size is comparable or less than half the length of the corrosion that the guided wave passes through. A fully noncontact experimental platform is developed and used to detect hidden corrosion with the methodology in aluminum plates. The methodology was verified on the aluminum plates of 1 mm thickness with the corrosion (30 mm × 30 mm × 0.5 mm and Φ30mm × 0.2 mm), and the relative errors between the estimated and actual value of the corrosion depth are not more than 6%.