Laser ultrasonic inspection of plates using zero-group velocity lamb modes

Abstract

A noncontact laser-based ultrasonic technique is proposed for detecting small plate thickness variations caused by corrosion and adhesive disbond between two plates. The method exploits the resonance at the minimum frequency of the S(1) Lamb mode dispersion curve. At this minimum frequency, the group velocity vanishes, whereas the phase velocity remains finite. The energy deposited by the laser pulse generates a local resonance of the plate. This vibration is detected at the same point by an optical interferometer. First experiments show the ability to image a 1.5-microm deep corroded area on the back side of a 0.5-mm-thick duralumin plate. Because of the finite wavelength of the S(1)- zero group velocity (ZGV) mode, the spatial resolution is limited to approximately twice the plate thickness. With the same technique we investigate the state of adhesive bonds between duralumin and glass plates. The S(1)-Lamb mode resonance is strongly attenuated when plates are rigidly bonded. In the case of thin adhesive layers, we observed other resonances, associated with ZGV modes of the multi-layer structure, whose frequencies and amplitudes vary with adhesive thickness. Experiments were carried out on real automotive adhesively bonded structures and the results were compared with images obtained by X-ray radiography.

S1-ZGV Modes of a Linear and Nonlinear Profile for Functionally Graded Material Using Power Series Technique

“…Additionally the numerical investigation includes the effect associated with the plate thickness variation from to 2 . As it was reported previously in literature [7,16], the ZGV frequency exhibits a linear behavior with respect to ( −1/2 ).…”

“…To exploit this phenomenon recent works evoke the idea that it may be suitable for the measurement of nanometer-scale thickness variations in homogeneous plates [6,7]. Due to the resulting differential equations of variable coefficients associated with the spatial variation of the material properties, the wave propagation in FGM remains difficult to analyze.…”

“…To our knowledge, no reports have been published on the relationship between the S 1 -ZGV frequency and material properties in an inhomogeneous free standing plate. Previous investigations on S 1 -ZGV phenomenon are limited to inspection experiments on homogenous plates and mainly focused on measuring the thickness of a coating on a relatively thin plate [3,7,17]. The present investigation includes different kinds of FGM plates with various basic materials, to extract the effects produced by mechanical parameters variation.…”

Evaluation of the Thickness and Bond Quality of Three-Layered Media using Zero-Group-Velocity Lamb Waves

“…Frequency dependency of the reflection coefficient revealed the thickness and elastic properties of the adhesive layer. Recently, some researchers have reported that zero-group-velocity (ZGV) Lamb waves have the potential to characterize properties of a layered media [3][4][5]. ZGV Lamb waves can reveal the elastic properties of anisotropic materials and thickness of a very thin surface layer with high accuracy and high spatial resolution.…”

Analysis of ultrasonic frequency response of surface attached fiber Bragg grating

“…In particular, when applied to structure health monitoring (SHM), wireless ultrasonic inspection has extensively been conducted for defect detection and thickness monitoring [1,2]. Conventional ultrasonic probes based on piezoelectricity were susceptible to technical problems such as electromagnetic interference and a lack of multiplexing capacity [3].…”