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Lecture # 20: Engineering Properties of Wood

Lecture # 20: Engineering Properties of Wood. Engineering Properties of Wood. Grain Orientation Density Moisture Temperature Duration, Rate of loading Defects Other Variability MECHANICAL PROPERTIES. Anisotropic Axes of Wood. Terms: Slope of grain Cross grain Diagonal grain

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Lecture # 20: Engineering Properties of Wood

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  1. Lecture # 20: Engineering Properties of Wood

  2. Engineering Properties of Wood • Grain Orientation • Density • Moisture • Temperature • Duration, Rate of loading • Defects • Other • Variability • MECHANICAL PROPERTIES

  3. Anisotropic Axes of Wood • Terms: • Slope of grain • Cross grain • Diagonal grain • Spiral Grain

  4. Mechanical Properties of Wood • Parallel to the grain • Perpendicular to the grain Modulus of Elasticity Tensile Strength Compressive Strength Flexural Strength Shear Strength • Large Variability

  5. Modulus of Elasticity Parallel to Grain PL 60% Ult Load PL 20 - 50% Ult Load

  6. Tensile Strength

  7. Cell Wall Structure Figure 14.3

  8. Compressive Strength

  9. Compressive Behavior

  10. Localized Buckling

  11. Flexural Strength

  12. Shear Strength Greatest Shear Strength- primary bond Relative sliding of microfibrils - secondary bond 5 to 15 MPa Bundle of straws; 1/4th of the strength  to grain

  13. Grain Orientation

  14. Density (G) G0 = Oven-dry specific gravity MC = Moisture content

  15. Variations in Density

  16. Moisture Content

  17. Moisture Effects on Mechanical Properties

  18. Shrinkage

  19. Typical Shrinkage Values

  20. Duration of Loading

  21. Other Effects

  22. Creep • Creep: The increase in strain or deformation with time under constant stress. Stress Strain Time Time Strain Strain Creep Relaxation Elastic Elastic Time Time

  23. Creep • Creep strain is permanent and: • is much more severe when loaded perpendicular to the grain. • about 50% of elastic strain for seasoned wood that remains dry or unseasoned wood that remains green. • about 3.5 times the elastic component for green wood that is allowed to dry in service. • of plywood at 18% MC is about 3 - 4 times that of specimens at 6% MC.

  24. Creep • creep strain increases in a cyclic moisture environment • an increase in relative humidity from 65 to 80% increases the creep 50%.

  25. Grading

  26. Defects in Wood

  27. Defects in Wood

  28. Knot: Bottom

  29. Knot: Top

  30. Reduction in Strength

  31. Design

  32. Adjustment Factors: Clear Wood

  33. Minimum Strength Ratios

  34. Stress Modification Factors

  35. Duration of Load

  36. Service Condition Note: a moisture content < 15% is dry

  37. Temperature

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