Mechanical Characterization of Paper by Laser Speckle Interferometry
A laser speckle interferometry method applied to the determination of strains in paper is described. We are able to characterize the two-dimensional strain field of the viewed surface for different test configurations (tension, compression). The method is shown to provide detailed strain information all along the stress-strain curve until failure.
Paper materials have certain unique characteristics related to their fibrous network composition. Internal light reflections and refractions on numerous surfaces in the translucent structure create a decorrelation phenomenon. This problem is avoided by using a thin coating technique.
The strain field is shown to be inhomogeneous. A technique for strain determinations over a square grid (7 columns, 22 rows) with a 3 mm spacing is described. Our method allows a determination of differences in properties for machine-made papers (which are anisotropic) between the machine direction (MD) and the cross-machine direction (CD). Additionally, a method ta compute the in-plane Poisson ratios is proposed.
Details concerning the optical filtering technique used for information processing are given. This semi-automatic system provides a high level of confidence in the measurements and permits the detection of very fine effects.
KeywordsHolographic Interferometry Machine Direction Laser Speckle Paper Material Fiber Orientation Distribution
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