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.

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

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 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".

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 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.

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

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

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.

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.

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 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