Intelligent processing of paper based on a laser ultrasonic method

In the paper industry, the need for on-line monitoring the mechanical properties of the paper during the manufacturing process is real, because of high production rates. To this end, a contactless method is considered where both the acoustic source and the sensor are laser based. The principle of laser generated ultrasounds (LUS) relies on the sharp increase of the temperature at the surface of a solid illuminated by a high power pulsed laser beam which leads to a transient mechanical stress, and therefore to the generation of elastic waves. Because this acoustic source is broadband in nature and non-contacting, it has been widely used since the early 1980's, in the Non Destructive Evaluation field. When used in conjunction with a laser beam detection system (interferometry), the presented method provides a mean to study the propagation of elastic waves without any mechanical contact, and is therefore well suited to our goals. This study focuses on the determination of the anisotropic elastic constants of paper, using this non-contact scheme. Although various type of elastic waves are generated with LUS. only the lowest antisymmetric Lamb wave mode is monitored, because of the small thickness of the samples, and the detection system characteristics. Experimented velocity dispersion curves for that mode are obtained in various directions, and the elastic constants are recovered by fitting the theoretical and experimental dispersion curves with a minimisation procedure. This non-contacting method is intended for being implemented on-line in the papennaking machine. The measured characteristics (elastic constants, wave speed...) may allow the real-time fe ed back control of the fabrication process to optimise the use of the raw materials or the paper quality.