The Effect of Wet-Pressure on the Acoustic Emissions and Fracture Toughness Characteristics of Paper

A study of the acoustic emissions characteristics of handsheet made under diverse wet-pressures is presented. It is believed that the acoustic emissions technique describes the inelastic deformation, damage, and fracture characteristics of paper well. It is concluded that there exists an optimum wet-pressure at which a paper with better mechanical properties such as high strength and optimum ductility and best fracture toughness may be manufactured. It is suggested here that beyond this “optimum” wet-pressure it is possible to over-press the handsheets during the manufacturing stage. This causes internal damage in the microstructure of the fibrils and instigates the evolution of inherent flaws or defects in the paper. These defects or flaws become active during the tensile loading of the paper and extend or propagate under loading, thus reducing the overall macroscopic deformability, compliance, and ductility of paper. Alternatively, below this optimum wet-pressure the paper does not develop its full potential strength. The results of the acoustic emissions testing have consistently suggested that fracture resistance toughness, and therefore, failure in paper is a strain-related phenomenon. As acoustic emission is a deformation (or strain) sensitive event, a new definition for toughness of paper is introduced, which is equal to the measured area under the cumulative hit distribution curve. The exact, asymptotic, as well as simplified approximate expressions have been derived, which may be used evaluate the acoustic emissions toughness of paper. The results of the acoustic emissions testing indicated that based upon this definition of AE toughness, the toughness of paper was highest for papers made under the optimum wet-pressure condition. It was also suggested that Tensile Energy Absorption (TEA), a measure of the area under the stress-strain curve, may not be a suitable measure for the fracture toughness of paper. This conclusion is more substantiated by the fact that fracture of paper is a strain[1]related phenomenon, and therefore, the fracture toughness should not be biased by stress