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16 Cards in this Set
- Front
- Back
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List the three 'areas' in which timber performance is assessed. |
- Appearance (grain and colour, feature, dimensional stability) - Structural (Strength and stiffness, dimensional stability and shrinkage, straightness) - Durability (biological hazards, natural resistance) Page 1-2 |
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Briefly describe the microstructure of timber |
- The wood is made up of cells and fibres - They are bound together with rays (perpendicular to the grain direction) - The wood has a higher strength and stiffness parallel to the grain Page 3-3 |
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What is the function of the rays in the wood? |
The rays prevent the grains buckling. |
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What are the properties of wood like in the a.) grain direction and b.) perpendicular to the grain direction? |
A wood is strong and stiff parallel to the grain, but it is weak perpendicular to the grain. Page 1-5 |
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Starting from the core and moving radially outwards, what are the layers of wood called and give a brief summary of their properties.
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- Core (oldest wood, contains pith - Heartwood (cells are no longer growing, can provide protection from attack) - Sapwood (Less dense, susceptible to attack, lighter colour, transfers water and nutrients from roots to leaves) - Cambium (growth layer, laid on outside of tree) Page 1-7 |
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What are the three water percentages that characterise the water content in timber? |
100% - unseasoned timber (50% to >100% in growing tree) 25% - partially seasoned timber (fibre saturation point) 15% - seasoned timber Page 1-9, Page 1-10 |
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How is moisture content defined? |
mc = weight_water/weight_wood Note it is possible to have a moisture content over 100% Page 1-10 |
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What is the basic criteria required for wood to dry (i.e. for the mc in the wood to decrease)? |
The atmosphere needs to be drier than the wood. Page 1-11 |
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How does the size of the wood change as the mc goes below the fibre saturation point (fsp)? |
- Above the fsp, moisture is extracted from the cell cavities (little change in size) - Below the fsp, moisture is extracted from the cell walls (shrinkage perp. to grain) Page 1-9, Page 1-10 |
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What effect does reducing the moisture content have? |
Increase in: - Strength - Stiffness (reduced creep) - Durability (reduced risk of attack) - Effectiveness of coatings Decrease in: - size perpendicular to grain (from reduction in cell wall thickness) Page 1-12 |
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What effects does the wood experience over time with load applied? |
Stiffness - creep occurs at all loads, can be recoverable or irrecoverable Strength - irreversible and cumulative loss in strength, increase in duration and magnitude of load causes decrease in strength Page 1-14, Page 1-15, |
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What effects do 'features' (knots, veins, checks, pith and core wood) have on the wood |
- Knots - cause grain to diverge (reduce strength and stiffness) - Gum and resin Veins - lower shear strength and stiffness - Checks - reduced shear strength and stiffness - Pith and core wood (contains juvenile weak wood) Page 1-18 |
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Why do woods bend after being cut? |
Trees are prestressed, and cutting them relieves the stress and causes a change in shape. Page 1-19 |
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What is bow, cup, twist and spring in a wood? |
- Cup (long edges raised higher than centre) - Bow (edges raised) - Twist (look at picture) Page 1-19 |
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What is the difference between clear strength and actual strength for a timber beam? |
Clear strength refers to the strength of the wood if there were no strength-reducing natural features. Actual strength includes the features. Page 1-20 |
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Describe the tensile and compressive characteristics of commercial timber. |
- tensile strength < compression strength - Tensile failures (splintery, brittle, sudden, loud) - Compression failures ( wrinkles, ductile, slow, quiet) |
