Swelling of cellulose fibres in composite materials: Constraint effects of the surrounding matrix

Wood fibres have several highly desirable properties as reinforcement in composite materials for structural applications, e.g. high specific stiffness and strength, renewability and low cost. However, one of the main drawbacks is the swelling of these hydrophilic fibres due to moisture uptake. Since the fibres in the composite are generally embedded in a relatively hydrophobic matrix, the surrounding matrix should restrain the swelling of the fibres. The present study investigates this constraint effect and establishes a micromechanical model to predict the swelling of embedded fibres based on experimentally characterised microstructural parameters and hygroelastic properties of the constituents. The predicted swelling is in concert with direct measurement of various wood–pulp fibre composites by means of three-dimensional X-ray microtomographic images.