Imaging the anisotropic elastic properties of paper with the INEEL laser ultrasonic camera

Abstract

An important material property in the paper industry is the anisotropic stiffness distribution due to the fibrous microstructure of paper. Ultrasonic methods offer a means of determining the stiffness of sheets of paper from the anisotropic propagation characteristics of elastic Lamb waves along the machine direction (MD) and the cross direction (CD). Currently, piezoelectric ultrasonic methods are employed in the industry to measure the elastic polar diagram of paper through multiple contacting measurements made in all directions. This paper describes a new approach utilizing the INEEL Laser Ultrasonic Camera to provide a complete image of the elastic waves traveling in all directions in the plane of the paper sheet. This approach is based on optical dynamic holographic methods that record the out of plane ultrasonic motion over the entire paper surface simultaneously without scanning. The full-field imaging technique offers great potential for increasing the speed of the measurement and it ultimately provides a great deal of information concerning local property variations and flaws in the paper. This report shows the success of the method and the manner in which it yields the elastic polar diagram for the paper from the dispersive flexural or antisymmetric Lamb wave.