4D synchrotron tomographic imaging of network and fibre level micromechanics in softwood paper

A 4D imaging study (3D + time) combining synchrotron tomography with in situ tensile testing has been carried out to observe the fibre and network level micromechanics of paper made from northern bleached softwood kraft (NBSK). Quantitative image analysis and digital volume correlation is used to characterize local deformation, the evolution of fibre-fibre contacts, and fibre straightening in a ”freeze-dried” handsheet as well as standard handsheets low consistency refined at different refining energies. In the freeze-dried handsheet having low fibre conformability, the results show that deformation at the network level occurs because of fibre straightening and possible inter-fibre bond breakage. Further, significant out-of-plane deformation near the failure regions was observed, which led to auxetic behaviour. In the refined handsheets, a strong inverse correlation is seen between refining energy, thickness expansion, and the number of broken fibres. The use of out-of-plane strain norms is proposed as a method to determine network efficiency (i.e. the ratio of the network’s elastic modulus to that of the constituent fibres) as well as the relative contribution of fibre pull-out to the overall failure of the handsheet.