Thomas Joffre

Modelling of the hygroelastic behaviour of normal and compression wood tracheids

Compression wood conifer tracheids show different swelling and stiffness properties than those of usual normal wood, which has a practical function in the living plant: when a conifer shoot is moved from its vertical position, compression wood is formed in the under part of the shoot. The growth rate of the compression wood is faster than in the upper part resulting in a renewed horizontal growth. The actuating and load-carrying function of the compression wood is addressed, on the basis of its special ultrastructure and shape of the tracheids. As a first step, a quantitative model is developed to predict the difference of moisture-induced expansion and axial stiffness between normal wood and compression wood. The model is based on a state space approach using concentric cylinders with anisotropic helical structure for each cell-wall layer, whose hygroelastic properties are in turn determined by a self-consistent concentric cylinder assemblage of the constituent wood polymers. The predicted properties compare well with experimental results found in the literature. Significant differences in both stiffness and hygroexpansion are found for normal and compression wood, primarily due to the large difference in microfibril angle and lignin content. On the basis of these numerical results, some functional arguments for the reason of high microfibril angle, high lignin content and cylindrical structure of compression wood tracheids are supported.

Trabecular deformations during screw pull-out: a micro-CT study of lapine bone

The mechanical fixation of endosseous implants, such as screws, in trabecular bone is challenging because of the complex porous microstructure. Development of new screw designs to improve fracture fixation, especially in high-porosity osteoporotic bone, requires a profound understanding of how the structural system implant/trabeculae interacts when it is subjected to mechanical load. In this study, pull-out tests of screw implants were performed. Screws were first inserted into the trabecular bone of rabbit femurs and then pulled out from the bone inside a computational tomography scanner. The tests were interrupted at certain load steps to acquire 3D images. The images were then analysed with a digital volume correlation technique to estimate deformation and strain fields inside the bone during the tests. The results indicate that the highest shear strains are concentrated between the inner and outer thread diameter, whereas compressive strains are found at larger distances from the screw. Tensile strains were somewhat smaller. Strain concentrations and the location of trabecular failures provide experimental information that could be used in the development of new screw designs and/or to validate numerical simulations.