A COMPARISON OF 2 TEST METHODS FOR ESTIMATING THE FRACTURE ENERGY OF PAPER

Two tests for estimating the specific fracture energy of paper are com pared. The ''essential work of fracture'' (EWF) method and STFI's J-in tegral method -- based on one notch length -- reflect fiber length and the proportion of chemical reinforcement pulp in furnish quite simila rly. However, the two methods differ in the effect of beating. Both me asures are proportional to each other for brittle papers, but this pro portionality does not hold for tough, ductile papers, for which the J- integral gives much lower fracture energy values. The EWF decreases mo re with hard-nip calendering than the J-integral. These differences ar e partly explains by the fact that the two methods measure different q uantities. The J-integral estimates the critical value of the total en ergy release rate, whereas the EWF measures the energy release rate re aching the inner fracture process zone. Another possible explanation f or the differences is that the fracture resistance of the material cha nges with crack growth.