Properties of plywood (xhtml w3c 01/10)

Builders guide to safe aircraft materials

Properties of plywoods

Rev. 5 — page content was last changed 6 January 2010
consequent to editing by RA-Aus member Dave Gardiner www.redlettuce.com.au

Content

4.1 Plywood design
4.2 Australian plywood standards
4.3 Veneer quality
4.4 Gluebonds
4.5 Plywoods available in Australia

4.1 Plywood design

Plywood is manufactured by slicing or rotary peeling thin (<2.5 mm) sheets of material (veneer) from a flitch or log, and then laminating three or more veneers into a rectangular sheet — perhaps 1200 mm × 1200 mm for very thin sheets otherwise 1200 mm × 2400 mm. Laminating an odd number of plies (3, 5, 7 ... ) reduces warping. Increasing the number of plies increases the resistance to shearing forces.

The plies are generally laid so that the grain directions alternate between lengthwise and crosswise, with the grain direction of the two surface veneers parallel to the longer dimension of rectangular sheets. Three-ply is stronger along the surface grain axis, But as the number of plies increases, the lengthwise/crosswise strengths and stiffness of a plywood sheet will become more equal. Three-ply bends easier along the grain direction of the surface plies because only the core ply will have cross-wise grain. To ensure the strength and stiffness characteristics of three-ply in the face grain direction, it is required that the thickness of each surface veneer is between 25% and 33% of the total sheet thickness; i.e. the two surface sheets comprise 50% to 66% of the total sheet thickness.

Three-ply manufactured from very thin veneers is predominantly utilised as aircraft fuselage and wing panels (stressed skins), for example. Finnish birch plywood sheet thicknesses start as low as 0.4 mm (1/64"). Such panels are supported/restrained on all sides. Thicker three-ply has many uses — spar webs for example, where just two opposite edges are supported or restrained. Five or more plies are usually available in sheet thicknesses of 5 mm or greater and such plywood designs should be balanced; that is, the total thickness of the odd-numbered plies (number 1 being the face ply) should be about the same total thickness as the even-numbered plies. The centre ply is known as the 'core', the outer plies as the 'faces' (or the 'face' and 'back') and the intermediate plies are the 'crossbands'. The density of plywood is usually much the same as the parent wood, slightly higher if it's softwood.

(When softwood plywood has to be bent to form a curve perpendicular to the surface grain, some checking is likely to occur. To protect the surface it may be necessary to apply a light fabric coating to the skin.)

The stiffness of plywood can be increased without an increase in weight by laying the two outer plies so that their grain direction is at 45° to the long axis of the sheet. Forty-five degree plywood is slightly stronger than the normal 90° sheets but it is much more expensive and difficult to acquire. Humidity effects may be less apparent in the 45° plywood.

4.2 Australian plywood standards

Wing constuction: the leading edge D box is being formed using webbing clamps to hold the plywood in position

In Australia there is no national standard for 'aircraft grade' plywood. An old document ANO 108.23 'Specification: marine plywood for aircraft use', refers to the AS/NZS 2272 – 1979 'Plywood – Marine', which has been superseded.

Australian-produced plywood used for aircraft construction should be chosen from that manufactured to the standard AS/NZS 2272 – 1996 Plywood – Marine — and no other, even if stated to be an equivalent standard. Marine plywood manufactured to the British standard BS 1088 should not be substituted for AS/NZS 2272 marine plywood, nor should exterior plywood manufactured to AS/NZS 2271.

AS/NZS 2272 – 1996 provides a purpose-designed structural plywood intended for use in marine applications and manufactured from timber species selected for density, bending strength, impact resistance and surface finishing characteristics. The standard also specifies veneer qualities, bond quality, joints, veneer construction, moisture content, dimensions and tolerances, finishing, and branding. A series of performance requirements relating to species, and a procedure for evaluating and admitting new species are included. The density of marine plywood should be between 450 and 720 kg/m³, and MR should not be less than 75 MPa.

In the USA plywood sold as 'aircraft grade' should meet the specifications in the MIL-P-6070 standard and could be African mahogany or American birch hardwood veneers that are hot press laminated to a hardwood core with waterproof glue. 'Marine grade' plywood, if softwood, should meet products standard PS 1-74, which allows face and back veneers A-A, A-B or B-B while 'marine grade' for tropical hardwoods must meet British Standard specification BS 1088.

4.3 Veneer quality

The following information is from the Engineered Wood Products Association of Australasia.

There are five veneer qualities specified for plywood in the Australian and New Zealand grading system — A, S, B, C and D. Only A-grade surface veneers are suitable for aircraft construction

'A' — a high-quality appearance-grade veneer suitable for clear finishing. Marine plywood to AS/NZS 2272 has two A-grade faces and an A or B-grade core.
'S' — an appearance-grade veneer that permits natural characteristics, such as knots, as a decorative feature subject to agreement.
'B' — an appearance-grade suitable for high-quality paint finishing.
'C' — a non-appearance grade with a solid surface. All open defects such as knot holes or splits, are filled.
'D' — a non-appearance grade with permitted open imperfections. Limited numbers of knots and knot holes up to 75 mm wide are permitted.

The plywood veneer quality is specified as the front face veneer quality first, followed by the back face veneer quality. Typical examples are: AA for marine plywood, AD for a wall cladding where only the front face of the plywood is visible, CD for structural plywood flooring and DD for structural bracing.

4.4 Glue bonds

Four types of glue bonds are defined — A, B, C and D — in decreasing order of durability under conditions of full weather exposure. For more information on glues see suitable adhesives in the 'Wood joints and adhesives' module.

Type A bond is produced from a phenol-formaldehyde resin [PF], which sets permanently under controlled heat and pressure. It forms a permanent bond that will not deteriorate under wet conditions, heat or cold. It is readily recognisable by its black colour. Type A bond is specified in AS/NZS 2272 for marine plywood.

So in the Australian/New Zealand Standards system for veneers and bonds, marine plywood has a grading of AA–A bond and should have the 'PAA Tested marine' stamp applied to the sheet. If it is not stamped then it is likely to be material that has not passed final inspection.
Type B bond is produced from melamine fortified urea-formaldehyde resin [MF], which sets under controlled heat and pressure.
Type C and Type D bonds are interior use bonds produced from urea-formaldehyde resin [UF] and should not be used in aircraft because UF could deteriorate in hot, humid climates.

Home-built laminae: sliced and rotary peeled veneers are available for building your own laminates, and are particularly useful when it is necessary to build up high strength/weight ratio formed shapes such as fairings. When very thin veneers are used, epoxy adhesives will make a very strong mechanical structure through the wood fibres, the same effect as a fibre reinforced polymer [FRP] composite.

4.5 Plywoods available in Australia

The only commercially available Australian timbers suitable for producing aircraft-quality plywood are coachwood and hoop pine. The latter is plantation-grown and produced as marine plywood to AS/NZS 2272 in sheet thicknesses of 1.5, 2, 3, 4, 6, 9, 12 mm and above. Coachwood is a rainforest timber and supplies are probably very limited.

Other plywoods available from merchants are:

Finnish birch — aircraft plywood manufactured from silver birch (Betula pendula) hardwood and complying with the requirements of Germanischer Lloyd GL1. It is available in three-ply thicknesses as thin as 0.4 mm, with a density around 575/650 kg/m³. A slightly lower-quality Finnish birch, complying with the requirements of Germanischer Lloyd GL2, is also available. For more information regarding stock thicknesses and sheet sizes contact the Australian distributor DMK Forest Products.
US-manufactured African mahogany or American birch hardwood veneers with hardwood cores of poplar or bass wood to US specification MIL-P-6070; thicknesses commence at 1/16" (1.6 mm). Supplies may have to be obtained directly from the USA.
Okoume (gaboon) is a hardwood plywood but may only be available in limited thicknesses. Density is around 400/450 kg/m³.

The next module in this group is 'Wood joints and adhesives'

Builders guide to safe aircraft materials – wood, plywood and adhesives modules

| Guide contents | Properties of wood | [Properties of plywoods] |

| Wood joints and adhesives | Wood beams in aircraft | Selecting aircraft timber |

| Basic strength and elastic properties of wood |

Other information about plywood

ANO 108.23 Specification: marine plywood for aircraft use. This is an out-of-date document that is still of interest (pdf document 17 kb).