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Wood

Note: The information on this page is of a general nature.     For notes relating to design using timber and wood products refer to webpage Timber Design

Introduction..... Types..... Hardwoods..... Softwoods..... Plywood.....
Chipboard..... MDF..... Density..... Strength..... Machinability.....

Introduction

The Mechanical properties and availability of wood have made it a natural material for building structures, furniture, tools, vehicles, and decorative objects. Worldwide it is used more than metal or plastic.

Wood is a natural product and when used responsibly is a sustainable resource which need not result in damage to the environment. Forests can be protected by recycling and reusing the wood, using less wood and by supporting sustainable forest management

All wood is composed of cellulose,lignin, hemicelluloses, and minor amounts (5% to 10%) of extraneous materials contained in a cellular structure.

Wood comprises about 50% of cellulose which responsible for most of its mechanical properties.

Natural wood is generally composed of bundles of long fibres which are effectively water carrying tubes.   These fibres are laid in the direction of the tree trunk or branch from which the wood is removed.

The strength of wood is highly dependent on the loading direction. Wood is strongest in tension along the fibres and is weakest in the radial and tangential direction.  When loaded in its strongest direction (longitudinal along the grain - see figure below) wood can have a strength to weight ratio advantage relative to steel of 2:1.  However when wood is loaded in other directions (radial and tangential to the grain- see figure below) this advantage disappears

To use wood to its best advantage and most effectively in engineering applications, specific characteristics or physical properties must be considered.




Wood Types

It is customary to describe wood using the following terms. Of course there is some degree of overlapping in practice.

  • Carpentry- applying to structural work in buildings and ships
  • Timbering - applying to temporary work such as formwork for concrete, gantries and shoring
  • Joinery -wooden structures inside buildings, such as doors and window frames
  • Cabinetwork - cabinetry - making furniture






Hardwoods

Hardwood trees are generally broadleaved trees.   These tree species are deciduous, retaining their leaves only one growing season.   The designation Hardwood trees does not necessarily relate to the hardness of the wood.. Hardwood trees are also called broad leaf trees or deciduous trees.

Typical hardwood trees include ash, elms, oak, maple, walnut, hickory, mahogany, and walnut. Woods grown in tropical climates are generally hardwoods.   Hardwood have shorter fibers compared to softwood. some hardwoods are evergreen.





Softwoods

Softwoods are one of the botanical groups of trees that has persistent needle-like or scale-like leaves; softwoods are evergreen and have longer-length fibers than hardwoods.

Softwood trees include pines, spruces, firs , cedars. The yew is one of the few softwoods that is native to the UK.  There are a number of softwoods (yew)that are harder and tougher than many hardwoods

Larches, including tamarack, are exceptions, being deciduous "softwoods".





Plywood

This is a product made from an odd number (three, five ...) of thin layers (veneers) of wood (generally hardwoods) bonded together by an adhesive.  The alternate plies are at right angles thus ensuring that the resulting material has a high uniform strength in all directions.

Plywood does not split as easily as conventional wood and has a good dimensional stability under conditions of varying moisture conditions.  Plywood will not easily split if a nail is close to any edges.   Plywood can be considered as a high strength construction material used for internal and external load bearing panels.

Plywood is normally supplied as 1,2m x 2,4m sheets in thickness from 3 to 25mm thickness. It can however be supplied in lengths of up to 40m and in thickness of up to 50mm.





Chipboard

Chipboard is made from particles of wood bonded together with a synthetic resin and sometimes other binders.   The panels are generally 2,4m x 1,2m with thicknesses from 3mm to 40mm.   Larger sizes are available up to 5m x 1,5m.   The particles are obtained from forrest thinnings sawdust and small pieces of wood unsuitable for other uses are granulated to chips.  The resin used is generally urea formalehyde.  Chipboard is normally only suitable for interior use in the manufacture of low cost furniture, wall panels and floor panels.





MDF (Medium Density FibreBoard)

There are different types fibreboard which differentiated by
..the size and type of wood fibres used
..the method of heating
..what type of bonding agent is used
..the method by which it is pressed into shape

Medium Density Fibreboard (MDF) is a wood substitute form which is made from fine wood fibres in a resin which is bonded under heat and pressure.  It is manufactured by a dry process at a lower temperature than other fibreboards e.g hardboard.   The natural glues and resins contained within the wood are not effective.   MDF therefore uses artificial bonding agents / resins.   The resin used is usually urea formaldehyde, but some fibreboard including exterior or marine quality board will use stronger glues such as phenol formaldehyde.   MDF may be used instead of plywood or chipboard.   It is dense, flat, stiff, has no knots and is easily machined.   It is made up of fine particles and therefore does not have an easily recognisable surface grain

MDF can be painted to produce a smooth quality surface.   Because MDF has no grain it can be cut, drilled, machined and filed without damaging the surface.   MDF may be dowelled together and traditional woodwork joints may even be cut.   MDF may be glued together with PVA wood glue.   Oil, water-based paints and varnishes may be used on MDF.   Veneers and laminates may also be used to finish MDF

Safety: It should be noted by anyone involved with MDF that there is an accepted risk that wood dust inhaled at certain level, is a carcinogen (cancer causing) and that formaldehyde is possibly carcinogenic to humans'.   Exposure to formaldehyde by inhalation can cause irritation to the eyes, nose, throat and mucus membrane.formaldehyde can also cause dermatitis.  A number of countries have but legal limitations on the use of MDF





Wood Density

Note: Density relates to wood in seasoned condition and is really only useful for relative comparisons

Type of wood Description/UsesDensity
kg/m3
Alder - Common From Europe and UK. A soft timber similar to poplar.   Imported as plywood and used for turning.   Light reddish brown colour.   Not very durable. 530
Balsa From Euador. A low density wood used for modelmaking and insulation 40-320
Beech From UK, Europe and America. Used for furniture, flooring, musical instruments. Often used for woodstrip and woodblockflooring. 720
Birch From UK, Europe and America.General purpose timber, plywood. 660
European Boxwood, Turnery, craftwork, sports goods, musical instruments 910
American Cedar, From South and Central America. This wood is used for Cabinet making, boatbuilding, and light construction 480
Cedar,Western Red From North America. Reddish brown - non-resinous lightweight softwood with good durability under all conditions. Used for roofing shingles, greenhouse construction and for interior furniture. Can cause corrosion of contact metal. 370
Sweet Chestnut A European tree used for furniture joiner and fencing and often used for beams. Stains when in contact with damp iron 560
European Cherry European/ America tree. Used for specialised crafted furniture, decorative work, musical instruments 430
Ebony An african /Asian tree. Dark wood used for cutlery handles, musical instruments, craftwork.   This is an endangered wood1000
Elm English. Cross grained with high tendency to warp.   High temperature steaming reduces this tendency   Used for furniture, joinery, flooring, external cladding.   Durable under water and excellent for piles and dock construction 550
Elm rock North American. Tough dense and resisant to abrasion.   Durable under water and mainly used for piles and dock construction. 700
Fir, Douglas (softwood) From UK , Europe and North America. Straight grained softwood.   Higher than average strength.   Used extensively for plywood, heavy construction work, sea defences 530
Silver Fir (softwood) Cultivated in Eurpoean Plantations.  Marketed as whitewood.   Used for building, joinery, packaging 480
Hemlock UK, European and N. America. Non resinous softwood with strainght even grain.    Not very durable.   Used in construction industry for interior work generally.   Used for crates and boxes 480
Hickory North American. tough, hard smooth and straight grained. Used for spokes and rims of wheels, tool handles and machine parts 820
Horse Chestnut A European tree sometimes used for minor goods, turnery and utensils and often used beams 510
Holly Grown in UK and Europe.   Difficult to dry and will easily distort and split.   Used for Craft work, inlays 800
Larch Grown in UK softwood plantation and Europe. Good strength and durability.   A general purpose timber for outdoor use used for gates and fencing and mining timbers. 590
Lime Europe.  Fine textured soft and compact and cuts cleanly.   Not durable.  Excellent for carving and is used for musical instrument construction. Brush backs, turning, minor goods 550
Mahogany From West Africa  Courser texture than other types of Mahogany Dark wood and not as stable.   used for utility and decorative work, indoors and outdoors, from boatbuilding to furniture and joinery. 530
Spanish /Cuban Mahogany Used in joinery, high quality furniture, plywood. 720
Maple From N. America.  Even texture hard and strong. Very good timber for flooring.   Used for interior timbers and a wide range of industrial applications. 740
Oak Found UK, Europe and North America.  Best wood for construction purposes demanding strength and durability.   Wide range of applications, including joinery, furniture, fittings, flooring wood strip and woodblock flooring 720
Pine (Redwood) Found UK, Europe and North America. Wide range of applications, including, furniture, flooring and utility applcations. 510
Rubberwood Grown in plantations in Malasia. Major use in furniture production600
Rosewood Africa, South America, India. Minor use for furniture and musical Instruments850
Sapele From West Africa.   Major use in furniture, joinery, decorative applications 620
Spruce
White Wood
From UK, Europe ,North America.  Near white straight grained softwood.   Not durable in exposed locations.General used for boxes packing cases and construction work.. 420
Sycamore UK , Europe. Turned wood and veneers.. 610
Teak Mainly from S.E Asia. High quality furniture, joinery, garden furniture ..Sometimes used for beams900
Walnut, European Native to UK and Europe.  Greyish brown finely figured.   Used for furniture cabinets and gun handles and rifle stocks. 640
Willow Native to UK and Europe.  Lightweight soft perishable wood.   Used to make cricket bats, basket work, toys and other items 440
Yew Native to UK and Europe. Used for decorative veneers ornaments. -Historical use for bows 670


The density of wood varies widely for different woods, and for the same wood.    The density is significantly affected by the moisture content which varies through its life from initial cutting to final use.  The normal range of wood densities varies form about 320 kg/m3 to 720 kg/m3. Imported woods can be obtained with densities as low as 160 kg/m3 and as high as 1020 kg/m3.






Strength of wood

Wood is a natural product and its properties vary continuously throughout processing from when the tree is cut down.  Green wood has high moisture content (generally) and this results in reduced strength.    As it is dried it loses the moisture content and becomes stronger.

Wood may be described as an orthotropic material; that is, it has unique and independent mechanical properties in the directions of three mutually perpendicular axes: longitudinal, radial, and tangential.   The longitudinal axis is assumed parallel to the fiber (grain); the radial axis is normal to the growth rings (perpendicular to the grain in the radial direction); and the tangential axis is perpendicular to the grain but tangent to the growth rings.  (Think of the grain as the tree rings running up the trunk /branch - planks of wood are simply sections of the tree trunk (or branch)

There are a massive number of values for mechanical properties of woods available mostly form american sources (see links below).   This page will simply identify a range for Modulus of rupture for two woods Oak and white wood.  The rupture modulus is the longitudinal stress in the extreme fibre at failure due to bending.

Wood Strength Properties Table

Please note that these are maximum strength values..  If the wood is subject to bending in any other direction the wood will fail at much lower values.  The stress at failure of wood is shear or compression (in any direction) is lower

Note: BS 5268: Part 2 2002 includes guidance on the permissible stresses in timber.    The standard relates permissible stressess to the strength grade of wood.    An example extracted from the standard is a wood with a strength class of D30 ( a low strength oak) has a permissible bending stress of 9 N/mm2.     Please note that this is a permissible stress as opposed to actual rupture (ultimate values) in the table below.

Notes Relating to timber strength grades are provided on webpage Timber design

Notes indicating fastener spacings and loads in timber joints are provided on webpage Timber joints.

Wood Name Moisture % Modulus of
Rupture
Modulus
of Elasticity
Softwoods
    MPa MPa
Fir Douglas 41 54 10400
  12 93 12700
Hemlock 51 49 8700
  13 83 10400
Larch 66 53 7900
  13 92 9900
Pine (Scots) 89 46 7300
  12 89 10000
Redwood (Baltic) 51 44 7700
  13 83 10000
Spruce (European) 53 39 7400
  14 72 10200
Spruce(Canadian 43 39 8800
  12 74 10500
Hardwoods
Ash (European) 48 66 9500
  12 116 11900
Balsa 11 23 3200
       
Beech (European) 88 65 9800
  12 118 12600
Birch (European) 76 63 9900
  12 123 13300
Elm (English) 140 40 5200
  12 68 7000
Elm Wych 75 68 9400
  12 105 10600
Greenheart 34 140 15900
  14 181 21000
Kari 64 77 13400
  12 139 17900
Mahogany 64 54 7400
  12 78 9000
Oak Europe 89 59 8300
  12 97 10100
Obeche 76 37 4600
  12 54 5500
Polpar Black 154 41 6800
  12 12 8600
Saraya White 55 60 9100
  12 80 10100
Teak Burma 48 84 8800
  10.6 106 10000




Machinability

Wood is generally machined by Sawing, Planing, Turning, Sanding etc..Below is a crude table identifying the machinability of various wood types.

From a number of samples machined the proportion of good quality results.   * Very low.   ** Low   *** Medium,   **** High ,   *****Over 90%

Wood Planing Shaping Turning Boring Mortising Sanding
Alder *** * **** *** ***  
Ash **** *** **** ***** *** ****
Beech **** * ***** ***** ***** **
Birch *** *** **** ***** ***** **
Cherry **** **** **** ***** *****  
Chestnut **** * **** ***** **** ***
Elm * * *** ***** **** ***
Hickory **** * **** ***** ***** ****
Maple *** *** **** ***** ***** **
Maple(soft) ** * *** **** ** **
Oak ***** * **** ***** ***** ****
Sycamore * * **** ****** ****** *
Walnut *** ** **** ****** ******
Willow ** * *** *** * *
Poplar *** * **** **** *** *


Links Providing information on Wood
  1. Design-Bites... .... Clear, informative, concise downloadable guides on wood (for a reasonable fee)
  2. Woodzone..Informative site providing general information
  3. Woodweb..Woodworking Industry Information
  4. Wood Handbook -- Wood as an Engineering Material..Downloads ..Comprehensive Document (American ) Excellent
  5. Wood Guide.. Friends of the Earth Guide - Including very useful information
  6. Canadian Wood Council ..Excellent site on Wood Engineering - my words
  7. Timber Trade Federation ..The Timber Trade Federation is the official voice of the UK timber trade.

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Last Updated 17/02/2010