| Laser ultrasonics has been used to investigate the propagation of Lamb waves in copy paper. Paper can be considered as an orthotropic medium (9 elastic constants), and it was found that some of these elastic constants {dollar}rm(Csb{lcub}11{rcub}, Csb{lcub}22{rcub}, Csb{lcub}44{rcub}, Csb{lcub}55{rcub},{dollar} and possibly {dollar}rm Csb{lcub}33{rcub}){dollar} can be obtained from the phase velocity dispersion relations of the two lowest order Lamb wave modes, {dollar}rm ssb{lcub}o{rcub}{dollar} and {dollar}rm asb{lcub}o{rcub}.{dollar} Generation was accomplished using a pulsed Nd:YAG source laser. The waves were detected using both out-of-plane and in-plane heterodyne interferometers.; Experiments were performed to investigate the effect of generation laser spot size, energy density, and total energy on the transfer of energy from the generation laser to the {dollar}rm ssb{lcub}o{rcub}{dollar} and {dollar}rm asb{lcub}o{rcub}{dollar} modes. The shift in frequency content of the Lamb waves was examined as a function of the generation spot size. A cut-off energy density of roughly 12.6 {dollar}rm J/cmsp2{dollar} was observed for each mode as well as an optimum spot radius of about {dollar}90pm20 murm m.{dollar} Damage at the generation point was also assessed.; Efforts were then made to characterize Lamb wave propagation in copy paper. After assessing the advantages and disadvantages of out-of-plane vs. in-plane detection systems, the pros and cons of using different source/receiver configurations (one source and two detectors vs. two sources and one detector) were discussed. The process by which the {dollar}rm asb{lcub}o{rcub}{dollar} mode dispersion curve and the phase velocity of the {dollar}rm ssb{lcub}o{rcub}{dollar} mode were obtained is then described, as well as the elastic constants that can be obtained from each. The {dollar}rm ssb{lcub}o{rcub}{dollar} mode velocities measured using laser ultrasonics were found to be in good agreement with measurements obtained using contact transducers. The nature of the {dollar}rm ssb{lcub}o{rcub}{dollar} mode was also investigated, which was seen (surprisingly) to be antisymmetric. |
