Bibliography: p. 150-160. / Wood is a popular constructional material; annually the world uses approximately 10⁹ tons of it, which is more than twice that of steel. It is a little surprising, therefore, that in comparison to steel, little progress has been made in research on the mechanical properties of wood. This is particularly so in the field of fracture, which has relevance not only to the structural use of timber, but also to the important processes of machining and cutting. One of the reasons for this relative lack of knowledge concerning the fracture behaviour of wood, is that wood has such good specific toughness properties that the designer has little recourse to rigorous brittle fracture criteria. In addition, attempts to relate gross mechanical properties to the micromechanical behaviour have been hampered by insufficient knowledge of the microstructure itself. However, with the advent of electron microscopy, much of the nature of this complicated cellular composite has been established, and the Materials Science approach to the assessment of the mechanical and fracture properties of wood is now possible. Wood contains many types of strength reducing defects. Checks for instance, are cracks, usually in the radial-longitudinal plane, and are caused by preferential shrinkage during seasoning. Knots, on the other hand, are far from being crack-like and involve much grain disturbance and also a loss in load bearing area. There is some attraction, therefore, in attempting to use a quantitative fracture mechanics approach to assess the strength of flawed timber, since this is a method of analysis which has been developed to deal with fracture which initiates from inherent flaws. Although, as will be reviewed in Chapter 2, this approach has been the subject of various recent studies, it is shown in subsequent chapters to have serious limitations as a practical fracture criterion for wood. This is particularly so in the most common structural situations where wood is stressed parallel to the grain, in which case failure from inherent crack-like defects, such as checks, is only of secondary importance. Nevertheless the general fracture mechanics methodology is relevant to an investigation such as this one, combining as it does a consideration of the microstructural behaviour, and its effect on the gross mechanical properties.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/18186 |
| Date | January 1977 |
| Creators | Boatright, Stephen William James |
| Contributors | Garrett, G G |
| Publisher | University of Cape Town, Faculty of Engineering and the Built Environment, Centre for Materials Engineering |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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