Network Modelling for Evaluation of Mechanical Properties of Cellulose Fibre Fluff

Sammanfattning: A network mechanics model for cellulose fibre fluff is proposed. 2D and 3D versions of the model have been implemented and the influence of various micro-level parameters on the global mechanical performance has been determined by means of computer simulations. Model networks composed of bonded fibres of arbitrary distribution in length, curvature, cross-section, stiffness and strength were investigated. The bonds were modelled as non-linear coupling elements representing stick-slip performance. The bonds were arbitrarily distributed in stiffness and strength. The fibres were arranged in a random structure according to an arbitrary orientation distribution. The network geometry was periodic, enabling a cell under observation to be regarded as one of many identical cells making up a global structure. Periodic boundary and loading conditions were used to obtain relevant results, even in the case of small network cells. The networks were analysed by means of the finite element method, and homogenized mechanical properties such as stress versus strain performance, initial anisotropic 2D and 3D stiffness properties, strength and fracture energy were calculated. Fracture localization and geometric quantities, such as the number of bonds and active part were also studied. Simulations showed that a cell 1.2 times the fibre length is sufficient for calculation of the initial stiffness values. The dependence of initial stiffness on fibre and bond stiffness, network density, fibre curl, fibre orientation and fibre length was studied. Fracture simulations showed that bond ductility is a very important parameter for 2D networks, giving a stronger and more ductile network. The influence on fracture of network density, fibre length and fibre orientation distribution was also examined. A comparison was made between 3D network simulation results and experimental results for cellulose fibre fluff. Reasonable agreement was observed up to peak stress. Analysis of the post-peak stress performance requires that size dependence due to strain localization and heterogeneity in the material, be taken into consideration. The models enable systematic analysis and design of network structures. The 2D version has the advantage of requiring less computer capacity, but in order to be able to draw quantitative conclusions regarding cellulose fibre fluff it is advisable to use the 3D version.