Measurement of wavenumber of wall-thinned plate using spatial local wavenumber filtering

Abstract

The surface of a shell-type structure can generate cracks or wall-thinning due to corrosion, etc. Those can eventually lead to the fracture of the structure, which can trigger enormous fatality and property loss. Thereby, a laser imaging technology on such structures as thin plate structure
or piping whose thickness is relatively thin in comparison to the area, has been steadily studied for the past 10 years. The most typical among the laser imaging technology is the pulse laser imaging. By using the same, a new technology for inspecting and imaging a desired area within a relatively short period of time was developed, so as to scan various structures including the thin-plate structure and piping. However, this method builds image by measuring waves reflected from defects, and has a time delay of a few milliseconds at each scanning point. Moreover, complexity
of the systems is so high due to additional components such as laser focusing parts. This paper proposes laser imaging method with increased scanning speed based on excitation and measurement of standing waves in structures. It is shown that defects in a structure can be visualized by
generating standing waves with single frequency and scanning the waves at each point by the laser scanning system suggested in this work. To quantitatively evaluate thickness of a plate, wavenumber is calculated by acoustic wavenumber spectroscopy, and relationship between wavenumber and thickness of the plate is established by Rayleigh-Lamb frequency equation. The proposed technique is validated by wall-thinned plates that have constant wall loss and a linear thickness variation wall loss.