Micromechanical modeling of wood fiber composites

An analytical concentric cylinder model for an N-phase composite with orthotropic properties of constituents was previously presented by the authors. The model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. With only minor modifications the model allows for including also free hygroexpansion terms in the elastic stress-strain relationship to deal with orthotropic phase swelling. Thus the effect of wood fiber ultrastructure and cell wall hygroelastic properties on wood fiber composite hygroexpansion can be analyzed. Using properties available from literature on the three main wood polymers, cellulose, hemicellulose and lignin multiscale modeling was performed to calculate the hygroexpansion coefficients of the fiber cell wall and an aligned wood fiber composite. The fiber cell wall was modeled regarding each individual layer S1, S2 and S3 as a balanced and symmetric laminate since it was assumed that the fiber will be restricted from bending and rotation within the composite. Regarding the fiber cell wall as a balanced and symmetric laminate enables us to calculate "apparent" fiber properties when rotation is not allowed. In reality the fiber's helical structure leads to an extension-twist coupling and thus a free fiber will deform axially and also rotate upon loading in longitudinal fiber direction making the response more compliant. Within the composite the fiber rotation will be restricted however. Therefore, the decision was also to compare the two extreme cases (i) free rotation and (ii) no rotation of the fiber in the composite.Godkänd; 2008; Bibliografisk uppgift: USB; 20081208 (ysko