(Dr. Yves Berthelot, advisor)
"Determination of the Bending Stiffness of Copy Paper and its Dependence on Temperature and Moisture Using Laser Ultrasonic Lamb Waves"
There is a need for a non-contact, non-invasive system for monitoring paper quality during the manufacturing process. Laser ultrasonics is one such method studied in this work. The A0 Lamb mode is excited in copy paper using a pulsed Nd:Yag laser. The resulting out-of-plane motion is detected using a Argon ion laser interferometer. The frequency dependence of the group velocity of the A0 mode is found from the wavelet transform of the measured signal. A best fit of the persion curve against predictions based on orthotropic plate theory yields an estimate of the bending stiffness of the sample.
The overall objective of the project was to measure the dependence of the bending stiffness of copy paper on both temperature and moisture using laser ultrasonics. This information would be useful in calibrating any system used in a paper mill during manufacturing. A significant amount of work was done to compare three signal processing methods: the Short-Time Fourier Transform, the Wavelet Transform, and the Phase Unwrapping method. The Wavelet Transform was used for reporting results. Data was taken at temperatures ranging from 25C to 95C, and at relative humidity ranging from 1% to 70%. The results indicate that bending stiffness varies linearly with both parameters. Bending stiffness decreases with temperature at a rate of 4x10-6 Nm/C and it decreases at a rate of 2x10-5 (N/m)/PMC with moisture content. Data was taken in both the machine direction and the cross direction.