Evaluation of Fracture Toughness of Paper in Wet State (I) : Effect of Fiber Furnish and Refining

Fracture toughness is considered to be a good estimate of the paper breaking tendency of a paper web in a paper machine and press room. Paper breaks are caused by many factors, such as the irregular vibrations of a paper machine, impurities in the fiber furnish, and presence of shive. On a paper machine, the dryness of the paper web changes very rapidly from less than 1% to more than 95%. We tried to measure the fracture toughness of a paper web at different dryness levels in order to acquire a fundamental knowledge of paper breaks. Stretches of wet webs were also measured and compared to the changes in fracture toughness. Four different fiber furnishes were refined to different degrees, and their fracture toughness values were measured at different dryness levels (40%, 60%, 80%, and 95%). Two fracture toughness measurement methods (essential work of fracture and Tryding’s load-widening method) were compared, and found to give identical results. However, Tryding’s method was much easier to implement. The highest stretch was mostly at 80% solid content, where the highest fracture toughness occurred.