MICROMECHANICS OF WOOD SUBJECTED TO AXIAL TENSION

The behaviour of a small group of wood fibers of Sitka spruce during t ensile loading is investigated. The load-extension curves for both ear ly and late wood fibers consist of three distinct segments. The first segment is almost a straight line, at some stage of loading a yield po int is observed. Beyond this point the specimen becomes less stiff and undergoes a large, mainly irreversible deformation. As the load is in creased further, the curve exhibits the third segment showed by a sign ificant change in slope. These curves look different from those obtain ed on thick specimens. In this respect, the behaviour of a thin wood s pecimen subjected to cyclic type tensile loading along its longitudina l direction is also illustrated. Based on wood microstructure, a model is presented to interpret the evolution of the Young's modulus of a w ood fiber during tensile loading. The model considers wood as an assem bly of cylindrical fibers pasted together in a longitudinal direction. We have assumed the cell wall to comprise only an S-2 layer made of a composite material consisting of a lignin and hemicellulose matrix re inforced by helical microfibrils along the fiber. Furthermore, it is a ssumed that the microfibril angle alpha in the S-2 layer is not unifor m along the fiber axis and matrix degradation occurs in the zones wher e the microfibril angles are bigger. The validity of this assumption i s verified by using holographic interferometry to visualize the displa cement field of the specimen's surface under tension.