Wood Flashcards
What is wood as an engineering material?
Timber
Engineered wood products
Definition of wood
The fibrous structural part of a tree
Definition of Timber
Wood that is prepared (or intended to be
prepared) for use as a building material
What are engineered wood products?
Composites made of wood and other components (usually glues/resins):
– ‘man-made wood’
– Either use sheets, fibres or particles of wood, bonded together
Engineering uses of wood
Extremely versatile • Very easy to shape • Structural material – Lightweight – Slightly flexible (not brittle) – Low embodied energy • Non-structural or semi-structural material – Walling – Flooring – Furniture • Other uses in the built/engineered environment – Fencing, boat-building, tools/utensils
Engineering wood products from large to small…
• Glued laminated timber (glulam)
– layers of reasonable-quality timber glued together; using
better timber not quite good enough for use on its own
• Plywood
– layers (veneers) aligned with crossed grains, glued together
• Particle board/chipboard
– small chips of wood (sawdust) glued with resin and pressed
• Fibreboard (often MDF, medium density fibreboard)
– wood fibres chemically sep
Anisotropy of wood
Anisotropic: not the same in all directions
• Wood is actually orthotropic – different in every
direction (radial, longitudinal, tangential)
• Fibre direction is called grain
– Longitudinal direction is along the grain
– Ring structure due to seasonal growth patterns
– Fibres run vertically up/down
the tree trunk
– Trees need flexural strength perpendicular to grain, to
resist wind load
Engineering properties of wood
• Density (all values approximate)
– Timber, 120-1600 kg/m3 (low end: balsa, high end: box, ebony)
– MDF: 500-1000 kg/m3
– Plywood: 500-700 kg/m3
• Strength
– Compressive, parallel to grain: 20-50 MPa
– Compressive, across grain: 2-6 MPa
– Flexural: 40-120 MPa (tested as cut timber beam)
– Shear across grain: 5-10 MPa
– Tensile perpendicular to grain: 1.5-5 MPa
– Tensile parallel to grain: 50-120 MPa
• Elastic modulus
– Tangential or radial modulus ~5-20% of longitudinal
How does wetting/drying change dimensional stability of wood?
• Wetting/drying causes dimensional changes
– Shrinkage or swelling - cell alignment in fibrous structure
– Drying from green to 12% moisture gives anisotropic
shrinkage: ~4% tangential, 3% radial, <0.1% longitudinal
– Also problems with degradation (rot/mould)
• 12% moisture is ~60% relative humidity (RH) @ 20°C –
normal service conditions
– Going from 90% to 60% RH gives ~3% tangential & ~1%
radial dimensional change – important in service!
• Dimensional change (thermal expansion) from heating
is mostly counteracted by moisture-induced changes
It is a time dependent process; wood creep.
Wood will creep (change dimensions) as a
function of time when loaded
– Cells are compressed under sustained load
– Fibres can slip over each other
– Some of this is reversible upon unloading
Resins in engineered wood
Generally synthetic organic polymeric resins
– Harder than wood
– Better tensile properties
• Urea-formaldehyde
– Cheap, but not very waterproof
– Used in particleboard and MDF (lower-value
products, not designed for outdoor use)
– Be aware of formaldehyde emissions in manufacture and in service
• degrades to release formaldehyde -> carcinogenic
• must comply with indoor air quality regulations
Resin in engineered wood
• Phenol-formaldehyde or melamine-formaldehyde
types of resins also available (outdoor wood glue)
– more water-resistant
– more expensive
– Glulam mainly uses phenol-resorcinol-formaldehyde
resins (urea-formaldehyde barely used in the UK)
• Newer resins are based on polyurethane
– Used since 1980s
– High strength, fast cure, good water resistance
– More expensive, but no formaldehyde problems
Properties of resins
• Shearing strength normally ~20 MPa
– Stronger than the wood in shear
• Key property is bonding strength to wood surface
• Resin usually stronger than wood in tension also,
but not in peeling or cleavage
Properties of bamboo
Special properties make it attractive
– very lightweight and flexible -> seismic resistance
– very rapid-growing -> inexpensive
– Poor resistance to moisture, high shrinkage
• Used for housing, scaffolding etc. in many parts of the world
– Very high flexural strength, but at the cost of low rigidity
• Hollow tube structure, variable size
– Standardisation is difficult
