Materials - Structure and Properties of wood

  1. Classification of trees
    • Endogenous (intertwined growth)
    • e.g Palm Trees and bamboo 
    • - very strong and lightweight
    • -not generally used in 'developed countries' 
    • Exogenous
    • e.g most other trees
    • Fibers grow from the center outward by adding concentric layers (annual rings) which give more predictable engineering properties
  2. Two types of Exogenous Trees
    Hardwood and Softwood
  3. Deciduous
    Broad leaf tree which looses foliage e.g maple
  4. Evergreen
    Tree which replaces its foliage gradually
  5. Examples of hard wood
    • Balsa
    • Birch
  6. Examples of Softwood
    • Pine 
    • Douglas Fir
  7. Radiata Pine
    • 90% sawn lumber
    • native to California
    • light in colour
    • low natural durability but easily treated
    • moderately strong
  8. Douglas Fir
    • 5% of san lumber
    • Native to Canada
    • Pinkish brown 
    • More durable than Radiata Pine
  9. Rimu
    • Commercially available in small quantaties
    • Native to New Zealand
    • Brown
    • Medium Density
    • Reasonably Strong
  10. Anatomy of a tree
    Roots anchor the tree to the ground and uptake moisture and minerals

    Crown consists of leaves and their supporting branches which produce food for the tree and seed

    Trunk provides strength and rigidity. Transport moisture and minerals up from roots and sap down from the crown
  11. New Zealand Forest Management
    Planting, Thinning and Pruning
    • Planting 
    • Typically 3m grid (1000 seedlings per hectare) 

    • Thinning
    • Allows trees to develop to max size and quality 
    • After , about 300-600 trees per hectare

    • Pruning
    • Cutting off branches on young trees to avoid knots in timber
    • Not always done
  12. Macro structure of wood
    • Composite materal
    • Outer Bark
    • Inner Bark
    • Cambium Cell Layer
    • Sapwood
    • Hardwood
  13. Outer bark
    Dense rough layer that protects interior of tree
  14. Inner bark
    Transports the sap from the leaves to growing parts of the tree
  15. Cambium
    • A layer of material between the bark and wood
    • A repeated subdivision of cambium that forms both new wood and new bark
  16. Sapwood
    • Wood near outside of the log (1/3 radius of log)
    • Takes moisture up from the roots
    • Stores food for growth
  17. Heartwood
    • Inner core
    • Non living cells
    • More resistant to decay
  18. Pith
    • Centre of the trunk
    • small cylinder of primary tissue
    • Originally formed as the new shoot of the growing tip of the tree 
  19. As the cells of the cambium grow and divide, they form a ring of new cells varying in cell diameter from
    approximately 0.02 to 5 mm depending on growth rate
  20. Earlywood or Springwood
    • Forms during periods of rapid growth
    • Cells have large diameter with thin walls
    • Light in colour
  21. Latewood or Summer wood
    • Forms later in growing season
    • Smaller in diameter but thicker wals
    • Darker in colous
  22. Wood is highly variable because
    • 1.many species
    • 2. composite material made up of components of different properties
    • 3. many flaws which are different sizes and severity
    • 4. Anisotropic material
    • 5. Highly Sensitive to moisture condition
  23. Microstructure of wood
    • -Tabular cells glued together by lignin
    • -Cellulose primary building block
    • -Cells aligned longitudinally
  24. Tracheids
    Length in softwood and hardwood
    • Longitudinally aligned cells 
    • Softwood: 3-5mm
    • Hardwood- 1mm
    • Responsible for mechanical support and transport of water and sap
  25. Two Main Zones of cell
    • Thin Primary wall that forms first
    • Followed by three layer secondary wall that provides most strength
  26. Direction of each layer and its function
    • S2 is parallel to the major axis of the cell providing strength in the longitudinal direction
    • S1 and S3 are perpendicular to the cell axis
    • acting mainly perpendicular to the grain
  27. Cell walls are framed with _____ 
    These are bound together in the cell by ________ and _____ similar to fibre glass
    Framed with Microfibrals bound by hemicellulose and lignin matrix
  28. Cellulose
    • 50% by weight 
    • Polymer formed from glucose
    • forms from ordered strands (fribrils)
    • High density
  29. Lignin
    Percentage in softwood and hardwood
    Built up of ___ 
    What type of strength?
    • 23-33% softwood 
    • 16-25% hardwood
    • Built up of phenylpropane units
    • longitudinal shear strength
  30. Hemicellulose 
    Percentage in softwood and hardwood
    Polymeric unit composed of
    • 15-20% softwood
    • 20-30% hardwood
    • Polymeric unit composed of various sugars
  31. Extractives
    Responsible for
    • 5-30% weight
    • Includes tannins, oils, resins, wax and gums
    • Partly responsible for colour, taste, odour, resistance to decay and flammability
  32. Ash
    • 0.1-3%
    • Calcium Phosphate, Potassium Silica
  33. In dry conditions, wood composition Chemicals
    • 50% Carbon
    • 44% Oxygen
    • 6% Hydrogen
    • 0.1% Nitrogen
    • 0.1-3% inorganic materials (ash)
  34. Specific Gravity
  35. Why does wood float?
    Empty cavities
  36. Anisotropic Axes of wood (3)
    • Longitudinal Axis (L)
    • Radial Axis (R)
    • Tangential Axis (T)
  37. Mechanical properties (7)
    • Tension Parallel
    • Tension Perpendicular
    • Compression Parallel
    • Compression Perpendicular
    • Hardness Perpendicur
    • Hardness Parallel
    • Bending
  38. Why is wood stronger in compression parallel to the grain? Compressive Strength?
    • Wood acts as tiny columns or tubes bonded together giving and receiving support from neighboring cells
    • 30-90MPa
  39. Compression in parallel increases how?
    • With deformation
    • Max at about a third of its thickness
  40. What reduces tensile strength?
  41. Wood is relatively weak in which direction? Tensile Strength?
    • Perpendicular
    • 2-9 MPa
    • Parallel
    • 70-150MPa
  42. Failure of wood
    Flexural Strength
    • Begins at upper part of beam (wood is stronger in tension) . Neutral Axis moves downward and final failure occurs when stress in the bottom fibres exceed tensile strength
    • 40-100MPa
  43. What is the controlling factor in beam design?
    Longitudinal or horizontal shear
  44. What is shear strength parallel to the grain?
  45. How does temperature affect the strength of wood?
    At low temperatures, strength increases slightly.
Card Set
Materials - Structure and Properties of wood
Materials - wood