Deformation of wet wood under combined shear and compression

During refining of mechanical pulp, a process occurring at high speed at temperatures of 140–160°C, the flexibility and bonding ability of wood fibres are increased. To understand the mechanical behaviour of the fibres in this operation, deformations at low speed of wet wood specimens at 50°C were studied under two different combinations of shear and compression loadings. The results were compared with the behaviour of wet wood in pure compression. Some features of the deformation that occurred in earlywood were analysed using an image analysis procedure. During pure compression the cell walls bend independently of the shape of the fibre cells and their cell wall thickness. Under combined shear and compression, however, mainly the corners of the fibre cells get deformed. In a second deformation performed in compression, the fibre cells follow the same deformation pattern as given by the first deformation type whether in compression or in combined shear and compression. The interpretation was that permanent defects in the cells themselves were introduced already in the first load cycle of the wood samples. The load combination with lower shear gave the same permanent strain as the case of pure compression but using less energy.