GUSTAFSSON, Johan
Lund University
Lund - Sweden

NAVI, Parviz
Laboratory for Buildings Materials
Lausanne - Switzerland
parviz.navi@epfl.ch

Abstract
The interest in wood fracture analysis has two major reasons: the need to know the load bearing capacity of wooden structures and the desire to optimise wood machining processes, i.e. to minimise the costs and energy consumption and achieving the best results with regard to the properties of the wood and the wood based materials and products. Although from an engineering point of view the first and second cases represent two opposite aspects of fracture but both cases implies that good methods of fracture analysis are highly important. Wood is an orthotropic material, it has several hierarchical levels of material structure, and its load bearing capacity and fracture properties varies with load direction. The orthotropic nature of wood gives six possible orientations of plane cracks in the three principle planes of the material. Wood is an organic and hydrogen bond dominated material for which the fracture behaviour is highly affected by temperature and humidity. Experimental results have moreover shown that failure of wood is affected by creep and other duration of loads (DOL) effects. The DOL effect is influenced both by the moisture content, the moisture gradient and the rate of moisture variation.
In wood the crack propagation along the grain direction is very different from cracking across the grain. The tensile strength across the grain is only a few percent of that parallel to the grain, and there is difference in fracture toughness of about the same order of magnitude. The poor perpendicular to grain fracture properties in combination with the lack of use of sufficiently good methods of fracture modelling and strength analysis in timber engineering strength design have resulted in several cases of severe and very expensive cases of structural damage.
Many researchers have studied the relationship between fracture morphology and the fracture toughness and the influence of moisture content variation to formulate the relation between wood fracture properties and wood density or find out the influence of cell size on the wood micro-failure mode. Different experimental results indicate the multiplicity of cell interfaces and natural defects in wood play predominant role in wood fracturing. In spite of wonderful progress in computing power, application of fracture mechanics to wood cracking still remains a difficult problem. It has been reported that the orthotropic and heterogeneous nature of wood as well as the hydrogen bonds at wood molecular level are key factors when trying to establish a valid relation between wood fracture behaviour and wood micro-structural features. One should note that the application of fracture mechanics in the design of wooden structural elements and for analysis of wood machining is certainly not a new idea, but still in a fairly early stage of development. Many interesting and promising models have been developed by eminent researches in this challenging field and many interesting works are in progress.