A comparison of two test methods for estimating the fracture energy of paper

Two tests for estimating the specific fracture energy of paper are compared. The essential work of fracture (EWF) method and STFI's J-integral method-based on one notch lenght-reflect fiber lenght and the proportion of chemical reinforcement pulp in the furnish quite similarly. However, the two methods differ in the effect of beating. Both measures are proportional to each other for brittle papers, but this proportionally does not hold for tough, ductile papers, for which the J-integral gives much lower fracture energy values. The EWF decreases more with hard-nip calendering than the J-integral. These differences are partly explained by the fact that the two methods measure different quantities. The J-integral estimates the critical value of the total energy release rate, whereas the EWF measures the energy release rate reaching the inner fracture process zone. Another possible explanation for the differences is that the fracture resistance of the material changes with crack growth. Application : a theoretical and practical review of two methods for characterizing crack propagation in sheets under stress. Differences in the results for both methods are explained, and the authors discuss the relevance of both measures as estimates of web durability.