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4.12 Investigation of fibre-fibre joint properties on two scales: adhesion simulations of model surfaces and evaluation  of mechanical properties for high-density sheets

Today, cellulose materials are gathering interest in new application fields, and the demand  on strength and durability of fibre network materials is increasing. Two entities determine the strength of the network: the fibre joints and the individual fibres. This work focuses on the role of fibre joints. First, cellulose model surfaces, developed by researchers within fibre technology at KTH, are analysed numerically during drying. Second, sheets in the high-density range are mechanically tested. The role of the fibre joints is evaluated in a configuration where the validity of network mechanics is not yet known.  Adhesive properties in fibre networks develop during drying, why their moisture dependence is interesting. The model surfaces are homogenous cellulose beads, made from resolved pulp, and placed on an adhesive surface to dry. During drying, the bead shrinks, resulting in a change in contact area. The process is replicated in a finite element model (FEM); using cohesive interactions and tailoring the cohesive parameters, the geometry is fit to the experimental findings.   High-density materials made of pressed fibres are believed to behave more like a continuum, compared to classical paper network materials. To investigate this transition, tensile and biaxial testing is carried out on randomly oriented sheets at several different densities.   In the future, the results are to be implemented in a network model. The results from model surface simulations can give a better estimation of fibre-interaction properties, while the mechanical testing can be used to investigate the validity of network models for high-density materials.