The purpose of mechanical pulping is to separate fibres from the wood matrix and develop them into a form suitable for paper and board production. This fibre separation and development requires a large energy input, where much of this energy is dissipated into heat. Therefore, energy can be saved by designing a process with as little heat dissipation as possible. Numerical simulations would be an effective tool for solving such process design problems, but there is currently no suitable model for the behaviour of the wood. To enable rational wood compression modelling, an image-based stress and strain analysis method for the split-Hopkinson pressure bar compression tests was developed. The results of this image-based stress analysis differed by only approximately 5 % from the strain gauge-based method. The image-based stress analysis is applicable to all split-Hopkins pressure bar testing. An image-based local strain measurement method was developed for earlywood and latewood analysis. The strain analysis method can also be used for strain analysis of other layered materials during high or low strain rate compression. In this study, two models have been developed for the radial compression of Norway spruce: a simple compression model and a dynamic (strain rate dependent) compression model. Both models are based on high strain rate and quasi-static compression tests of Norway spruce at three different temperatures: room temperature, 100°C and 135°C. Native wood, chemically treated wood and mechanically fatigued wood were tested. The local compression behaviour is important for mechanical pulping. The compression models developed here have separate layers for earlywood and latewood compression. The dynamic wood compression model can be used for numerical simulations of the wood deformations, which occur during mechanical pulping. These models are the first high strain rate and high temperature compression models for moist wood which have separate layers for earlywood and latewood. Initial quasi-static simulations were also conducted. One important practical implication for mechanical pulping is that identically repeated compressions does not develop the wood fibres. The fibre should preferably be slightly rotated between compressions in order to get flexible in all directions. The significant difference in EW and LW stiffness suggests that the fibres need different kind of treatment. The wood also needs to be treated at the right temperature since the stiffness is strongly dependent on temperature
