For the cutting simulations, materials are composed of material points (nodes) connected by material lines (fibres). Material lines are like spring beams, which have an associated elastic modulus and stiffness. A material point (or node) has a meshwork of material lines that form the contact area. Planes of nodes are typically called 'fibre layers'. Material lines in each fibre layer have finite stiffness and are not independent of nodes in other fibre layers. Each material line in a fibre layer is called a 'fibre' that is equivalent to a cohesive zone in the explicit crack growth model. Fibres are either elastic or plastic. Fibres in the wood samples were assumed to be plastic. Wood is assumed to be isotropic, so all fibres in each fibre layer have the same properties. This assumption can be relaxed, but at a significant cost in the model complexity.
The physical mechanism of wood fracture is the initiation and growth of specific types of cracks. Wood has been classified into three zones based on density of growth rings. In the lower zone, the wood has the same density as the surrounding bark. In the middle zone, the density is increased and is the growth zone for sapwood; the higher density is in the heartwood. The lower density, sapwood layer has many fewer pores and water inclusions than the heartwood. The crack will have to grow the length of the lower zone, because the critical stress for the crack initiation is in the lower zone. Once the crack enters the heartwood, it must go through the heartwood and through several ring layers from the middle zone in order to reach the lower zone and continue into the lower zone. As the crack continues, it becomes more difficult to move the crack any distance through the middle zone, because the heartwood grows relatively quickly compared to the lower zone. 3d9ccd7d82