A Sensor for Laser Ultrasonic Measurement of Elastic Properties during Manufacture

An automated sensor has been developed for use in paper manufacturing and for demonstration on a full scale paper machine during commercial operation. This laser ultrasonic sensor provides non-contact and on-line measurement of the elastic properties of paper and paperboard. It was tested on a pilot web handler at web speeds up to 12.7 m/s. Measurements of flexural rigidity and out-of-plane shear rigidity were made by fitting the frequency dependence of the phase velocity of Ao Lamb waves mode to a model equation. Ultrasonic waves were generated in the paper with a pulsed Nd:YAG laser and detected with a Mach-Zehnder interferometer coupled with a scanning mirror/timing system to compensate for paper motion. On-line measurements agreed very well with off-line laboratory measurements. Introduction In Laser Ultrasonics (LU), also known as laser-based ultrasonics, acoustic waves can be generated with a pulsed laser in a material to determine one or more of its physical properties. These acoustic waves are also often monitored with a laser-based detector, usually a form of interferometer, without physical contact to the sample (1). In this work, plate waves (also called Lamb waves) (2) are detected several millimeters from the generation point as they propagate along the sheet. A diagram of this system is shown in Figure 1. Laser ultrasonics has been applied in recent years to measurement of mechanical properties of paper in the laboratory (3,4). Further laboratory demonstrations of LU on moving paper demonstrated the possibility for routine measurement of these properties during manufacture, and for feedback control of the papermaking process based on these measurements (5,6). Further developments in signal processing and the results of a miniaturized and industrialized scanning LU sensor on moving paper are discussed in this paper. Figure 1. System for laser ultrasonic analysis of paper. Background LU signal energy in paper goes predominantly into the zero order anti-symmetric (Ao) mode plate wave (3). The Ao mode is characterized by relatively large (hundreds of nanometers) out-of plane displacements, which are easily detected with commercially available laser vibrometers. In this work, a Fourier transform, ‘phase unwrapping’ computational method was used to calculate two elastic properties from a phase velocity versus frequency dispersion curve that was constructed from two Ao wave signals (7). The properties are flexural rigidity (D) and out-of-plane Shear Rigidity, SR (for homogeneous material shear rigidity is equal to shear modulus times caliper). Flexural rigidity differs slightly (for paper it is typically about 9% larger) from Bending Stiffness (BS) through a term that depends on the in-plane Poisson’s ratios (νxy and νyx):