Method for determination of the softening behaviour of wood and the applicability of a nonlinear fracture mechanics model

Abstract: The thesis deals with determination of the post peak behaviour of wood in pure tension and pure shear. Furhermore, the "Fictitious Crack Model" (FCM), which is a nonlinear fracture mechanics model ; originally developed for description of crack propagation in concrete, is employed to demonstrate the advantages of non linear fracture mechanics. A testing arrangement is presented with which the complete stress-displacement curve can be determined, both in pure tension and in pure shear. Experiments were made in order to show the possibility of measuring the complete stress-displacement relation for wood. The tests have been conducted on material from one piece of Scots pine timber (Pinus sylvestris). Three different grain orientations in the weakest direction, i.e. perpendicular to the grain, and four different moisture contents have been investigated in pure tension. In pure shear, only one grain orientation and one moisture content have been examined. The main difficulty encountered in such experiments is, according to this study, to find a suitable geometry for the specimens. The length of the specimen is crucial with respect to the stability conditions. Furthermore, the specimens are glued into the testing arrangement, and hence strength of the glue and ratio between glued area and ligament area are important, in order to get wood failure. In addition to the determination of the softening behaviour, two examples are presented where FCM is used. The first example is an investigation of the Compact Tension specimen. The theoretical analysis displays the influence of the material properties on the computed load-displacement curve. In addition : an experimental series was made, and a comparison is conducted between calculated and measured load-displacement curves. The second example is a very simplified analysis of the formation of cracks in wood subjected to drying. The caIculation demonstrates the possibility to theoretically simulate the formation and growth of cracks during the drying process.