M.S. Thesis Presentation by David Allen Griggs
Monday, April 16, 2001

(Dr. Yves Berthelot, advisor)

"A Laser-Based Ultrasonic System to Measure the Mechanical Properties of Paper Products in a Controlled Environment"


The current development of an on-line laser-based ultrasonic system to monitor paper strength during production necessitates a study of strength properties in terms of paper temperature and moisture content.  Indeed, for a given paper quality, the mechanical properties of paper will change as the temperature and moisture content change during various stages of production.  A robust quality control system should be able to compensate for these variations in product condition.

The primary goal of this thesis was to design and construct an off-line laser ultrasonic system to determine the dependence of bending stiffness and shear rigidity on temperature and moisture content.  A fiber-optic interferometer which detects laser generated ultrasound in paper samples placed inside an environmental chamber has been built.  The zeroth-order anti-symmetric, A0, Lamb waves were processed using the analytic wavelet transform to extract group velocity dispersion curves.  The experimental curves were compared to predictions based on orthotropic plate theory to calculate the associated bending stiffness and shear rigidity.  The system was calibrated by measuring the dispersion in a sample with known properties (silicon wafer).  In this study, copy paper with energy travelling perpendicular to the preferential fiber direction was tested at three temperatures (25o, 50o, 75o) for a range of moisture contents (3.5% to about 9%).

The combined measurement error and sample variability was estimated to be 14% for the bending stiffness and 42% for the shear rigidity.  It was found that the bending stiffness decreases with temperature at a rate of 1.1x10-6 Nm /oC and at a rate of 2.5x10-5 Nm per 1% moisture content.