Orthotropic Behavior of Aged Paper*

Tensile testing was used to measure the mechanical properties of paper at various orientations relative to the machine direction of paper. New, artificially aged, and naturally aged paper were shown to behave as orthotropic materials, as shown by the orientation dependence of the Young’s modulus. The Young’s modulus data could be fit to a simplified equation, indicating the shear modulus is relatively insensitive to the specific orientation. The shear modulus as measured by ultrasonic measurements was similar to that measured by fitting the modulus vs orientation data obtained by tensile testing to the orthotropic equation. The inelastic tensile strength could also be described by a simple non-linear equation, and there was more deviation of the data from this prediction for brittle papers than for non-brittle papers. The extent of fiber orientation (E_x/E_y) could be inferred by either the Young’s modulus or the tensile strength, i.e. E_x/E_y ≈ TS_x/TS_y. The measured strains were lowest for brittle paper. All brittle papers gave a strain to failure < 1% whereas the non-brittle papers gave a strain to failure > 1%.