UBC Theses and Dissertations
4D imaging of paper : a novel method to characterize deformation mechanisms Golkhosh, Farzin
Recent advancements in fast synchrotron x-ray tomography has made it possible to acquire time-resolved 3D image volumes (4D Imaging) of different materials including paper that can be utilized for direct observation of their microstructure under deformation. Considering the importance of inter-fibre bonding in prediction of paper behaviour, especially when paper is formed from a pulp mixture, this thesis devises a method to evaluate the level of inter-fibre bonding using 4D imaging experiments. To do so, four samples with different levels of bonding, and six samples with different fractions of NBSK and Eucalyptus fibres were scanned by synchrotron x-ray tomography while being deformed in a tensile tester in an interrupted fashion. Qualitative observations of the sample with the weakest level of bonding revealed some significant thickness expansions in and around the regions of failure. Measurements of fibre-fibre contacts and curl index showed that inter-fibre bond breakage, in addition to fibre straightening, are the underlying mechanisms for such thickness expansions. This lead to a conclusion that out-of-plane deformations of paper samples during tensile testing can be used as a criterion to measure the level of bonding inside their networks. Qualitative observations of the samples with different levels of bonding showed that there is a strong correlation between such thickness expansions and the levels of bonding further confirming that out-of-plane deformations can be used as a measure to evaluate network efficiency. To do so, norms of the out-of-plane strain fields obtained from DVC analysis were used to quantify such thickness expansions that also showed strong correlation with the levels of bonding. Application of such analysis on mixture samples revealed some unexpected results where 100% NBSK samples showed higher thickness expansions when compared to 100% Eucalyptus samples when 100% NBSK sample is expected to have higher levels of bonding. This trend was explained by an increase of the free fibre lengths due to addition of longer fibres to the pulp mixture. This underlined the need for consideration of the contribution of free fibre lengths to such thickness expansions to evaluate the level of bonding inside samples made from mixture of fibres that have different lengths.
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