Laser ultrasonics in copy paper
Abstract
Ultrasonic Lamb waves are generated in 90 micrometers thin copy paper with a Qswitched lowpower Nd:YAG laser and detected with an Ar^{+} laser Doppler interferometer. This noncontact method enables one to determine two elastic constants (C_{44} or 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 a_{0} 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 constants (C_{11} and C_{22}), which are directly related to the tensile strength of the paper. Although excellent agreement is obtained with orthotropic plate theory and also with independent measurements made with contact transducers, the results must be taken cautiously because interferometric 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 applied to paper.
 Publication:

Optical Engineering
 Pub Date:
 February 1997
 DOI:
 10.1117/1.601423
 Bibcode:
 1997OptEn..36..408B