LASER ULTRASONICS IN COPY PAPER

Ultrasonic Lamb waves are generated in 90 mu m thin copy paper with a Q-switched low-power Nd:YAG laser and detected with an Ar+ laser Doppl er interferometer. This noncontact method enables one to determine two elastic constants (C-44 OF C-55) that are related to the compressive strength of the paper by matching the measured high frequency limit of the phase velocity of the ao mode with that predicted by orthotropic plate theory. The method is also used to measure the phase velocity of the nondispersive fast mode and infer the corresponding elastic const ants (C-11 and C-22), which are directly related to the tensile streng th of the paper. Although excellent agreement is obtained with orthotr opic plate theory and also with independent measurements made with con tact transducers, the results must he taken cautiously because interfe rometric measurements made simultaneously on the opposite side of the sample showed clearly the unexpected antisymmetric nature of the fast mode, thus showing the limitation of the orthotropic model when applie d to paper. (C) 1997 Society of Photo-Optical Instrumentation Engineer s.