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4.3.1 Describe the structure of natural timber.
Natural timber is a natural composite material comprising cellulose fibres in a lignin matrix. The tensile strength of timber is greater along the grain (fibre) than across the grain (matrix).
Wood is a fibrous material. The structure of wood similar to a bunch of parallel straws (the cellulose fibres), which are bonded together with a glue (lignin matrix). The fibres are long and slender and are aligned with the long axis of the trunk which gives it an interesting property behaviour.
When load is applied parallel to the axis of the fibres, they are very strong in tension and have reasonably good compressive strength until they start to buckle.
When the the load is applied perpendicular to the axis of the fibres, they will tend to crush under compression and are weakest in tension, where the “glue” bond fails and the straws literally tear apart.
4.3.2 Outline that timber can be classified according to the conditions needed for tree growth.
Consider temperate and tropical conditions. A general knowledge of the geographical distribution of world timber resources is required.
Study the distribution of forest map of the world below. Temperate forests tend top be in cooler regions and tropical tends to be between the Tropic of Cancer and Capricorn.
4.3.3 Outline that conifer trees are referred to as softwoods and that these grow only in temperate regions.
Recognize the characteristics of softwood trees.
Characteristics of softwood trees are ...
4.3.4 Outline that deciduous trees are referred to as hardwoods and that these grow in both temperate and tropical regions.
Recognize the characteristics of hardwood trees.
Characteristics of hardwood trees are ...
Some examples of hardwood trees includes eucalyptus, elm, maple, oak, and beech.
4.3.5 Discuss the issues relating to the consideration of timber as a renewable resource.
Consider time to reach maturity, soil erosion, greenhouse effect and extinction of species. The issues should be placed in local, national and international contexts.
4.3.6 List two examples of composite timbers.
Consider particle board (chipboard) and plywood.
4.3.7 Compare the characteristics of particle board, laminated woods (for example, plywood), pine wood (a softwood) and mahogany (a hardwood).
Consider composition, hardness, tensile strength, resistance to damp environments, longevity and the aesthetic properties of grain, colour and texture. The ability to produce sketches showing cross-sectional views of the structure of the materials is expected.
4.3.8 Outline criteria for the selection of timber for different structural and aesthetic design contexts.
Consider timber for buildings, furniture and children’s toys.
Buildings - often the timber rafters, studs, beams are hidden from view so there is no need for wonderful looking hardwoods but rough sawn pine is fine. If within a house there is flooring then aesthetics plays a role and a hardwood would be selected.
Furniture and children's toys need to withstand some amount of wear and therefore need to be durable. In furniture aesthetics plays an important role as well. In toys the wood may be painted or stained.
4.3.9 Describe the reasons for treating or finishing wood.
Consider reducing attack by organisms and chemicals, enhancing aesthetic properties and modifying other properties.
4.3.10 Explain three differences in the selection of timbers for flooring if it were made of a hardwood, a softwood or a composite material.
Consider durability, ease of maintenance and aesthetics.
Bulleted list and italicised paragraphs are excerpted from Design Technology: guide. Cardiff Wales, UK: International Baccalaureate Organization, 2007.
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