Hygro-Mechanics of Wood Fibre Composite Materials

Abstract: The work presented in this thesis deals with modelling and testing of hygro-mechanical properties of wood fibre composite materials. The material studied experimentally is high pressure laminate, HPL, which is a wood fibre network composite, made up of craft paper impregnated with phenolic or melamine resin. An analytical model for the hygro-mechanical properties has been developed. The model calculates the stiffness, hygroexpansion, creep and mechanosorption behaviour of the composite material from the given corresponding properties of the constituents. The analytical model is based on the assumption of homogeneous strain. This assumption is validated by comparison of the stiffness and hygroexpansion predictions with the predictions of another analytical model developed as a part of the research. The model is validated also by the results of two finite element homogenisation methods, one 2-dimensional and one 3-dimensional. The comparisons show that the homogeneous strain assumption overestimates the composites stiffness by approximately 15 % and underestimates the composites hygroexpansion by approximately 15 %. Experimental tests of two HPL materials with melamine resin as matrix material and with different fibre volume fraction were carried out. The tests comprised strength, stiffness, Poisson’s ratio, free hygroexpansion, creep, mechanosorptive strains, moisture sorption isotherms and moisture equilibrium curves. Strain was recorded in the direction of the inplane tensile load and in the transverse direction. The results show that creep, unlike the elastic stiffness, is significantly influenced by the moisture content. The creep was of very different character in transverse direction as compared to the creep in the direction of the load, and furthermore linear with respect to the stress up to at least 40 % of the failure stress. By development of an empirical hygro-mechanics model was mechanosorption identified and found to be of significant magnitude. The free hygroexpansion of the HPL composite was found to much larger than first expected. The analytical model is compared with the test results. Material data for hygroexpansion, creep and mechanosorption of the matrix material is however lacking. This made it difficult to validate the analytical model by the test results. By assuming matrix material data it was found that the analytical model can be capable to capture the hygro-mechanical behaviour of composite materials. Tests of the matrix material are proposed. It is believed that the proposed model can be successfully used for predicting the hygromechanical properties of new wood fibre composite materials. Such a model can be a useful tool in the process of optimal design of new composites and thus extending the good use and the area of application of wood fibre composites.