1. Engineering Properties of Wood
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

5. 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

6. Modulus of Elasticity Parallel to Grain PL 60% Ult Load

8. Tensile Strength

9. Cell Wall Structure
Figure 14.3

11. Compressive Strength

13. Compressive Behavior

14. Localized Buckling

15. Flexural Strength

16. 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

19. Grain Orientation

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

21. Variations in Density

22. Moisture Content

23. Moisture Effects on Mechanical Properties

24. Shrinkage

25. Typical Shrinkage Values

26. Duration of Loading

27. Other Effects

28. 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

29. 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 times that of specimens at 6% MC.

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

31. Grading

32. Defects in Wood

33. Defects in Wood

34. Knot: Bottom

35. Knot: Top

36. Reduction in Strength

37. Design

38. Adjustment Factors: Clear Wood

39. Minimum Strength Ratios

40. Stress Modification Factors

41. Duration of Load

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

43. Temperature