FORE327 Wood Propertys

Define basic density and explain how it differs from oven-dry density.

Basic density = oven-dry mass / green volume

oven-dry density = oven-dry mass / oven-dry volume.

Basic density includes cell cavities, oven-dry density does not.

How does basic density relate to wood strength and stiffness?

Higher density generally means higher strength and MOE due to more cell wall per unit volume.

What factors (species, age, site) influence basic density?

Genetics, growth rate, juvenile vs mature wood, latewood proportion, environmental stress.

Why does latewood have higher density than earlywood?

Latewood has thicker walls, smaller lumens, less void space.

How is density measured (direct vs indirect methods)?

Direct: water displacement; indirect: X-ray, NIR, pilodyn.

Why is density often used as a proxy for other wood properties?

Because many properties (strength, hardness, MOE) scale strongly with density.

Define moisture content (MC) and explain how it is calculated.

MC (%) = (wet mass – oven dry mass) / oven dry mass × 100.

What is the fibre saturation point (FSP) and why is it important?

MC (~30%) where cell walls are saturated but no free water; below FSP, properties change with MC.

How does bound water differ from free water?

Bound water in cell wall, affects shrinkage; free water in lumens, little effect on dimensions.

Explain hygroscopicity of wood.

Wood attracts/holds water vapour due to polar hydroxyl groups in cell walls.

How does MC affect mechanical properties?

Below FSP, increased MC decreases stiffness and strength.

What is sorption hysteresis and why does it occur?

Equilibrium MC higher in adsorption than desorption due to cell wall relaxation and bound water history.

Describe how MC is measured (oven-dry, electrical, etc.).

Oven-dry gravimetric, electrical resistance, capacitance, NIR, microwave.

What causes shrinkage in wood?

Loss of bound water below FSP causes cell wall contraction.

Distinguish between longitudinal, radial, and tangential shrinkage.

Longitudinal very small (<0.3%), radial moderate, tangential about twice radial.

Why is tangential shrinkage usually about twice radial?

Orientation of rays and microfibrils causes differential wall contraction.

How is shrinkage related to MC?

Below FSP shrinkage is proportional to water loss; none above FSP.

Define dimensional stability and discuss factors influencing it.

Ability to maintain size/shape; affected by density, MFA, extractives, juvenile wood, MC changes.

What is anisotropy and why is it important in timber drying?

Properties differ by direction; shrinkage differs → causes warp and checking.

How does MFA and reaction wood affect shrinkage?

High MFA and reaction wood increase longitudinal shrinkage.

Define warp and list the main types (bow, cup, twist, crook).

Warp = distortion during drying

bow (length)

cup (width)

twist (spiral)

crook (edge).

Explain why tangential shrinkage > radial causes cupping.

Unequal contraction pulls board toward bark side.

What role does spiral grain play in twist?

Spiral cell alignment causes torsional distortion as wood dries.

How does juvenile wood contribute to warp?

High MFA, abnormal shrinkage, uneven stress.

How can drying practices reduce warp?

Uniform schedules, restraint, stress relief conditioning, pre-sorting.

Why is end-matched or quarter-sawn timber less prone to distortion?

Quarter-sawn aligns rings vertically, balancing shrinkage; end-matching equalises restraint.

Define stress, strain, modulus of elasticity (MOE).

Stress = force/area, strain = deformation/length, MOE = stress/strain (stiffness).

Explain anisotropy in MOE (longitudinal vs radial vs tangential).

Longitudinal MOE >> radial/tangential due to cell orientation.

How does density relate to stiffness?

More cell wall material per volume increases MOE.

How does MFA influence elastic behaviour?

Low MFA = stiff; high MFA = more flexible.

Distinguish between elastic and viscoelastic response.

Elastic = immediate reversible; viscoelastic = time-dependent, partial recovery.

How does MC affect MOE?

Below FSP, higher MC lowers stiffness.

Why is longitudinal MOE much greater than radial/tangential?

Cell walls aligned with load, continuous fibres resist bending.

Define modulus of rupture (MOR) and how it is tested.

MOR = max bending stress at failure; determined by static bending tests.

How do defects (knots, slope of grain) influence strength?

Interrupt fibres, concentrate stress, reduce MOR.

How does MC affect bending strength?

Below FSP, increasing MC reduces MOR markedly.

Explain compression vs tension failure in wood.

Compression = cell wall buckling, kinking

tension = fibre rupture.

Why is longitudinal strength >> transverse?

Fibres aligned with load resist much more than perpendicular orientation.

What is the effect of loading rate on strength?

Faster loading → higher apparent strength.

How do juvenile wood and reaction wood affect strength?

High MFA, lower density, abnormal shrinkage → reduced MOR.

How does load duration influence strength?

Strength decreases as load duration increases (creep-rupture).

Define impact strength and its practical relevance.

Resistance to sudden loading; important for handles, sports equipment.

What is toughness and how is it measured?

Energy absorbed to failure; pendulum or work under stress–strain curve.

How do MC and grain direction affect toughness?

Higher MC increases ductility; off-axis grain lowers toughness.

Describe creep and distinguish normal vs mechano-sorptive creep.

Creep = time-dependent strain

mechano-sorptive = accelerated by MC cycling under load.

What factors increase creep in service?

High stress, high MC, temperature, cyclic humidity.

Define hardness and how it is tested.

Resistance to indentation; Janka or Brinell methods.

How does MC affect hardness?

Hardness decreases with increasing MC below FSP.

Outline wood’s thermal properties and why timber can survive fire longer than steel.

Low conductivity, charring insulates core, maintains load capacity vs steel softening.

What is meant by machinability and why does it matter?

Quality of surface and ease of cutting; affects manufacturing cost and finish.