Lamb wave structural health monitoring using frequency-wavenumber analysis

Lamb waves have shown great potential for structural health monitoring (SHM) in plate-like structures. Their attractive features include sensitivity to a variety of damage types and the capability of traveling relatively long distance. However, Lamb waves are dispersive and multimodal. Moreover, the propagating Lamb waves may include incident, reflected and converted waves. Various wave modes make the interpretation of Lamb wave signal very difficult. This paper presents studies on Lamb wave propagation using frequency-wavenumber analysis. By using two-dimensional Fourier transform (2-D FT), the time-space wavefield can be transformed into frequency-wavenumber domain, where various wave modes and waves propagating in different directions can be clearly discerned. By a frequency-wavenumber filtering strategy, the desired wave modes or wave propagation at certain direction can be extracted and further utilized for the purpose of SHM. The frequency-wavenumber analysis and its applications to Lamb wave SHM are illustrated through two experimental investigations. One is Lamb wave propagation in a plate half immersed in water and the other is Lamb wave mode decomposition by using two-dimensional frequency-wavenumber filtering strategy. Lamb waves are excited by piezoelectric wafer sensor and measured by scanning laser Doppler vibrometer. Various wave modes were visualized and successfully decomposed.