Numerical modelling of softwood time-dependent behaviour based on microstructure

The time-dependent mechanical behaviour of softwood such as creep or relaxation can be predicted, from knowledge of the microstructural arrangement of the cell wall, by applying deformation kinetics. This has been done several times before; however, often without considering the constraints defined by the basic physical mechanism behind the time-dependent behaviour. The mechanism causing time-dependency is thought to be sliding of the microfibrils past each other as a result breaking and re-bonding of hydrogen bonds. This can be incorporated in a numerical model by only allowing time-dependency in shear modes. Thus, in the local coordinate system corresponding to the orientation of the microfibrils in the S2 layer of the cell wall, only elastic and viscous deformations are included. However, the macroscopic behaviour of the entire cell is visco-elastic in that creep, creep recovery and relaxation can be predicted with the described method of modelling. This is seen by simulating experimental results for both single fibres and tissues in creep and relaxation experiments.