FRACTURE-RESISTANCE - THEORY AND EXPERIMENT

The fracture resistance of chemical and mechanical pulps has been measured under a variety of conditions. Some of the results are interpreted in terms of a theoretical model of paper failure developed in an earlier publication. It is found that the fracture resistance of unbeaten kraft pulp increases with bonding, implying that fibre pull-out rather than fibre breakage is the dominant mechanism of energy dissipation for this pulp during fracture. The increase of fracture resistance with bonding is related directly to the effective fibre length which can be calculated by applying the theory of micromechanics of crack extension in short-fibre composites to paper. Calendering of kraft pulp decreases fracture resistance significantly by apparently decreasing both fibre length and bonding. Increasing moisture in a newsprint sheet increases the sheet fracture resistance. The fracture resistance of newsprint with loading direction was found to be relatively constant compared to other strength properties. The fracture resistance of mixtures of chemical and mechanical pulps follows the rule of mixtures for hardwood kraft or sulphite chemical pulp but does not for softwood kraft. This behaviour is interpreted from theoretical considerations as a result of the extent of bonding between the chemical and mechanical fibres.