NONCONTACT LASER GENERATION AND DETECTION OF LAMB WAVES IN PAPER

A laser ultrasonics method has been used to investigate the propagatio n of Lamb waves in paper, and thus determine paper stiffness in a nonc ontact manner. Ultrasonic pulses were generated on nonmoving paper usi ng a Nd:YAG pulsed laser, An Ar:ion CW laser was used in combination w ith two optical heterodyne interferometric systems to probe out-of-pla ne and in-plane surface motions of Lamb waves. Two simultaneous propag ation modes were observed with substantially different velocities and frequency contents. They were identified as the fundamental dilatation al and bending modes for Lamb waves, respectively Velocity measurement s obtained for the dilatational mode were found to be in very good agr eement with velocities obtained using contact bimorph transducers. Res ults collected along machine and cross-machine directions for selected fine papers and paperboards are reported.