Abstract:

Wood multiscale structures are usually described at three different structural levels from nanometer to millimeter,including nanoscale polymer structure,microscale multilayer structure and macroscale growth ring structure. There were differences between these multiscale hierarchical structures,including the structures and properties of wood polymers; polymer contents and microfibril angles(MFAs)of different cell wall layers; cell types,sizes and arrangement orientation in growth ring structure. Those structure differences lead to differences in mechanical properties between multiscale hierarchical structure elements. Furthermore,the processes of wood fracture include crack initiation and propagation,which are related to structure and mechanical properties differences between multiscale hierarchical structure elements and irregular evolution of "natural defects". In this paper,the structures and mechanical properties differences of multiscale hierarchical structure elements,the effects of those differences on wood fracture were reviewed. Meanwhile,it should be studied from the following aspects in the future:1) The properties differences of wood micro- and nano-scale structure. Molecular deformations of wood polymers are revealed by the differences of arrangement orientation and response to load of wood polymers. The mechanical properties differences and stress transfer mechanism between wall layers due to the difference of wood polymers distribution and orientation of microfibrils in each layer of cell wall should be analyzed. 2) The crack initiation and propagation of growth ring and cell wall structure are studied,and cell fracture types of different fracture surface also are pointed out. 3) The effects of structure differences between multiscale hierarchical structure elements on the stress distribution and stress concentration of wood growth ring and cell wall structures are analyzed that will be revealed the influence mechanism of multi-scale structure difference on wood fracture.

Key words: wood, multiscale structure, differences, fracture