A Laser Ultrasonic System for On-Line Measurement of Elastic Properties of Paper

A laser-based ultrasonic system for non-contact measurement of the elastic properties of paper was evaluated on a pilot paper coating machine operating at paper web speeds of up to 25.4m/s (5000 ft /min). Flexural rigidity and out-of-plane shear rigidity were calculated from the frequency dependence of the phase velocity of A0-mode Lamb waves. These ultrasonic waves were generated in the paper with a pulsed Nd:YAG laser. Fibre-optic delivery of the generation pulse was demonstrated. Lamb waves were detected with a Mach–Zehnder interferometer coupled with a scanning mirror/timing system to compensate for paper motion. Six paper grades ranging in basis weight from 39 to 100 g/m² and a 280 g/m² paperboard were tested. For the six paper grades, the on-line laser ultrasonic measurements of flexural rigidity agreed within experimental error with conventional laboratory contact ultrasonic measurements on stationary samples. The effects of web tension and moisture content on flexural rigidity measurements were quantified. The low-frequency signal amplitude from the paperboard was insufficient for accurate measurements.