CHARACTERIZATION OF THE TRANSVERSECOEFFICIENT OF HYGROEXPANSION OF WOOD FIBRESIN COMPOSITE PREFORMS

摘要

Wood and other cellulosic fibres have many advantages as reinforcement incomposite materials compared with conventional manmade fibres. The low price, recyclability andhigh specific mechanical properties are some of the benefits. A disadvantage is the high degree ofhygroexpansion of wood fibres. An experimental procedure has been developed to determine theeffective hygroexpansion coefficient of wood fibres from the curvature change of fibre mats atdifferent relative humidity. Fibre mats, which are preforms for composites, generally have a gradientin fibre orientation distribution through the thickness. This gradient in fibre orientation has beencharacterized through the fibre mat by an image analysis method. A micromechanical laminate modelwas used to relate the fibre stiffness parameters and hygroexpansion coefficient of each layer based onthe fibre orientation distribution. The measured curvature was used as input in a laminate modelwhere the thickness change was taken into consideration. The effective transverse hygroexpansioncoefficient was then determined from a relatively simple macroscopic model. For unbleached kraftpulp fibres, the value became 0.11 % strain / % change in moisture content, which is not far fromvalues arduously determined from single wood fibres. The presented method could be used to rankdifferent fibre qualities according to their potential as reinforcement in composites with improvedhygroscopic dimensional stability.