Theory of laser-generated transient Lamb waves in orthotropic plates

This paper presents a theoretical study for modelling the thermoelastic excitation of transient Lamb waves propagating along the principal directions in an orthotropic plate. The normal mode expansion method is employed to express the transient displacement field by a summation of the antisymmetric and symmetric Rayleigh-Lamb wave modes in the surface stress-free orthotropic plate. This method is particularly suitable for waveform analyses of transient Lamb waves in thin sheet materials because one needs only to calculate contributions of the lowest few antisymmetric and symmetric modes. The dispersion characteristics and the transient Lamb waveforms excited by a pulsed laser in machine-made paper are analysed numerically and discussed in detail and attention is focused on the influence of the elastic stiffness constants. This work provides a quantitative analysis for noncontact and nondestructive detection of the elastic stiffness properties of the machine-made paper by the laser-generated Lamb wave technique.