Fundamental study of laser generation of narrowband Lamb waves using superimposed line sources technique

In this paper, a laser generated ultrasound technique called superimposed line sources is presented. The technique generates narrowband Lamb waves with a dominant wavelength by superimposing signals of line sources at the pitch corresponding to the desired wavelength. The superposition is performed in software after data are collected so that it permits flexibility of selecting the desired wavelength afterwards. By selecting the dominant wavelength in signals, signal complexity is reduced and the speeds and frequencies of wave modes with the selected wavelength can be determined through dispersion curves. Wavenumber–frequency domain filtering and continuous wavelet transform are used in this research to further reduce the complexity of the signals. Finally, simulations and experiments are conducted to investigate the defect detection ability of the technique. Reflection coefficients are calculated for fundamental A0 and S0 wave modes. Good agreement is found between the simulations and experiments and the result of coefficients with respect to defect and sample dimensions will be shown.     

Advanced techniques for nanocharacterization of polymeric coating surfaces

Surface properties of a polymeric coating system have a strong influence on its performance and service life. However, the surface of a polymer coating may have different chemical, physical, and mechanical properties from the bulk. In order to monitor the coating property changes with environmental exposures from the early stages of degradation, nondestructive techniques with the ability to characterize surface properties with micro- to nanoscale spatial resolution are required. In this article, atomic force microscopy has been applied to study surface microstructure and morphological changes during degradation in polymer coatings. Additionally, the use of AFM with a controlled tip-sample environment to study nanochemical heterogeneity and the application of nanoindentation to characterize mechanical properties of coatings surfaces are demonstrated. The results obtained from these nanometer characterization techniques will provide a better understanding of the degradation mechanisms and a fundamental basis for predicting the service life of polymer coatings. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology on November 12–14, 2003, in Philadelphia, PA.