Paper and Fibre Science and Technology
Warren Batchelor, Australian Pulp and Paper Institute, Monash University
The title of the thesis was "Investigation of an alternative technique to measure fracture toughness of paper", which was accepted on the 11th of January, 2005.
The literature review discusses paper mechanical properties, runnability, the correlation of various measures of paper mechanical properties with runnability, fracture mechanics and the various methods of measuring fracture toughness.
This chapter discusses
The fracture toughness measurements in this thesis were based on the Essential work of Fracture (EWF) method. This technique gives a way to separate the fracture work and other plastic deformation. One significant problem with this method is the time and sample area required to measure a single value.
The goal of this chapter was to develop an image analysis method to measure the plastic work and use this to subtract the plastic work from the total measured work. The method was successfully developed, but it proved impossible to capture all the plastic work as most of it occurred just before fracture. The work showed that the EWF technique could be inaccurate when testing large ligament sizes, since in paper these may not yield completely before fracture. This supports thermographic imaging work published in Tanaka, A. and Yamauchi, T. (2000): Deformation and fracture of paper during the in-plane fracture toughness testing-examination of the essential work of fracture method, J. Mat. Sci. 35(7), 1827-33.
The method was published as Wanigaratne, D.M.S., Batchelor, W.J., Conn, A.B. and Parker, I.H. (2000): Image analysis of plastic deformation in the fracture of paper, Appita J. 53(6), 471-475.
The EWF technique requires too much time and sample area to be used as routine quality control measurement. This chapter looks at if it is possible to accurately predict the EWF fracture toughness from routinely measurements of tensile strength, TEA, stretch etc. A mixture of laboratory and commercially manufactured sheets were used. The best fit was obtained using a combination of tensile index and extension at fracture. However, even the best fit had an average error of 28% in predicting the EWF fracture toughness, while the worst error was over 100%. It was found that EWF fracture toughness could not be successfully predicted from standard measurements of mechanical properties. This work was published in Wanigaratne, D.M.S., Batchelor, W.J. and Parker, I.H. (2002): Comparison of fracture toughness of paper with tensile properties, Appita J. 55(5), 369-374, 385.
Fracture toughness was measured by cyclically loading a Double Edge Notched Tension (DENT) sample and increasing the maximum load each time. The work in the cycles before fracture only comes from plastic work. The work in the final cycle where the sample breaks is used to calculate the fracture toughness. Fracture toughness can then be calculated from one sample size only. The cyclic method greatly reduces the time and sample area required to measure a fracture toughness value, in comparison to the EWF method. This chapter discusses the test method and compares the cyclic fracture toughness against the EWF fracture toughness for a large number of laboratory and sheet made samples. The cyclic fracture toughness is typically 8% less than the EWF fracture toughness. The cyclic fracture toughness is used to predict paper strength. Reasonably good predictions were obtained.