Computational Mechanics
and
Virtual Engineering
COMEC 2007
11 – 23 OCTOBER 2007, Brasov, Romania
BONDED JOINTS IN WOODEN ULTRA LIGHT AIRCRAFT STRUCTURES
Eng. Botond Varga-Orban1
1Transilvania University of Braşov, Romania,
1. ADHESIVE REQUIREMENTS
Aircraft wood structural joints are not the complex mechanically interlocking joints used in joinery and cabinet making. They are very simple in construction relying only on the cohesive strength of the bond between the wood and the adhesive to transmit flight loads between structural members. In forming joints the adhesive does not bind one wood surface to the other, rather each surface is individually bound to the glue. In wooden structures the strength of the glueline must be greater than that of the wood whereas in adhesive bonded metal structures the glueline will most likely be weaker than the metal.
Adhesive bonding operations are much simpler than incorporating metal fasteners and provide a lighter structure and a more uniform stress distribution.
Airframes require proven adhesives that provide waterproof gluelines without any degradation in extreme environments. The gluelines must also be highly resistant to cracking with age, attack from fungi, other micro-organisms, fuel, oil and other chemicals. The adhesives must be tough enough to resist vibratory and cyclical stressing in an exposed environment over perhaps 40+ years.
The adhesive must also have these workshop capabilities:
•Be easy to work with; i.e. readily applied to properly prepared surfaces and allowing ample time [15-25 minutes] for the joint to remain 'open' for final positioning of the members.
•Bonds and cures in normal home workshop environments: temperatures 15° C – 35° C with relative humidity up to 70%.
•Optimal results achieved when moisture content of the wood is in the 6% to 18% range.
•Fills gaps up to 1.5 mm to cater for less accurate joint preparation.
In addition two part glues should be easily measured out (for example a 1:1 by volume mix ratio rather than 2:1 by weight), mix readily, have a reasonable pot life (30-60 minutes), a reasonable curing time, not be a risk to health – where the user takes reasonable precautions in a ventilated workshop – and be easy to clean up.
1.1. Suitable adhesives
Two part epoxy resin adhesives. The first choice for aircraft structural bonds: Epoxy resin adhesives require the precise and thorough mixing of a resin and a hardening agent immediately before use. There are quite a number of different resin and hardener combinations available offering ample choice for a particular job and/or a particular environment. Some epoxy systems allow a selected adhesive filler to be added to the resin/hardener mix to achieve a particular viscosity or gap filling requirement.
Epoxy adhesives generally require only sufficient clamping pressure to remove voids, squeeze out excess glue and to hold the pieces immobile during curing at normal room temperatures.
Reaction commences once the two parts have been mixed together and the reaction rate roughly doubles/halves for every 10° C change in temperature from 20° C. If initial cure time is 60 minutes at 20° C then it will approximate 30 minutes at 30° C or 120 minutes at 10° C. The cure times at room temperature to achieve maximum cohesion might require several days. Shrinkage during cure is slight (probably under 1%) so that machined accuracy in mating joint surfaces is not necessary but still desirable – to produce a maximum quality bond.
Epoxy adhesives will bond very well when the MC of the wood ranges from perhaps 6% to 20% and will exceed the shear strength of all woods so that, in a properly prepared joint, the wood will fail before the glueline. Of course it is absolutely necessary to follow the manufacturer's instructions regarding application and usage.
Resorcinol-formaldehyde resin [RF]. Requires MC in the 12% - 15% range and higher clamping pressures for good bonding. Temperatures during cure should be above 22° C. Glue shrinks while curing so gap filling capabilities are not so good thus accuracy of fit is vital for a high strength joint, however RF was the adhesive of choice for several decades. Mix is by weight perhaps one part of the hardening powder to four parts of the resin liquid.
Phenol-formaldehyde [PF]. Requires MC in the 8% - 12% range and controlled heat and pressure to set permanently and thus not appropriate to the home workshop. Gap filling capabilities are poor and there may be age hardening/cracking problems if not used under controlled conditions. The Type A marine plywood bond is produced from a phenol-formaldehyde resin.
1.2. Unsuitable adhesives
Urea-formaldehyde resin [UF] glues were extensively used in aircraft but it is now considered that they may deteriorate badly over time in hot, humid climates such as found in northern Australia. Other glues that must not be used in aircraft structural joints are melamine fortified urea-formaldehyde resin [MF], acid catalyzed phenolic [ACP], polyvinyl acetate [PVA], casein and animal glues.
2. WOOD BONDING JOINTS
Adhesive joints are designed to provide a continuous bond over as much surface as feasible whereas bolted or screwed joints apply pressure over a smaller area – and the associated holes tend to weaken the structure. Modern adhesives work very well in tension, compression and shear so airframe joints should be designed to take advantage of that.
2.1. Flat grain joints: in such jointing the grain of all members should be close to parallel and the strength of the joint is then dependent on the shear strength of the timber species.
Such joints are the strongest and generally associated with lap joints as in the layout image; or adding strengthening/stiffening blocks either side of a structural member where a particular load will be applied; or in fabricating lengthy components by lamination.
2.2. End grain to end grain joints: in basic end grain jointing the squared ends of two members are butted together, end grain against end grain, to produce a longer length from two shorter pieces.
The surface area bonded is minimal and the joint is very weak so the normal practice is to bond a piece of the same dimension material centered over the butt joint or plywood strengthening plates over opposing faces: much the same as the lap joint above.
2.3. End grain to surface grain joints: the square/angle cut end grain of one piece is butted against the surface grain of the edge/face of another to create a T-joint or an angled joint – a node connection.
End grain joints are weak so strength is usually provided by bonding rectangular plywood gussets on one side (or both sides) of the node and/or adding solid wood corner blocks within the interior angle/s. It is important that all corners of the plywood gusset are supported on solid wood and the surface grain is appropriately oriented. The same grain orientation requirement also applies to corner blocks.
Simple scarf joints: to produce [splice] a longer length from two shorter pieces or to insert a repair. The matching ends are cut at an angle, providing a much greater bonding area where the grain is as close to purely surface grain as possible. When splicing solid wood the length of the scarf should be 12 to 15 times the thickness of the piece, a 1 in 12 slope will provide a joint strength around 90% of the natural wood and a 1 in 15 slope is the minimum in spars. However the slope of the scarf should match the grain slope and as wood in aircraft structures should have a grain slope better than 1:15 a 1 in 12 scarf slope should have no application. Smooth, accurately mated, grain slope matched surfaces and good adhesive penetration are necessary, see below. The bottom image shows the layout for a squinted scarf joint which avoids the feathered ends of a through slope; the squint is cut at an angle so that the butted areas are not solely end grain, however a squinted joint is unlikely to be applicable in an airframe primary structure.
Plywood may also be edge joined using a scarf technique [see 'joints in plywood' below]; in this case the width of the scarf should not less than 12 times the plywood thickness, i.e. 3 mm plywood, scarf width = 36 mm. If you recall the 1-in-60 rule a 1 in 12 slope equals a 5° angle while 1 in 15 is a 4° angle.
The term scarf is derived from an Old Swedish word meaning 'to join together' and there are many glueless scarf joint designs, particularly as splices in beams of old buildings or replications.
Extract from ANC–18
The following few paragraphs and images are an extract from chapter 4 of 'ANC–18 Design of wood aircraft structures' second edition issued June 1951 by the subcommittee on Air Force-Navy-Civil Aircraft Design Criteria of the United States Munitions Board Aircraft Committee.
4.60 GENERAL. Glue joints should be used for all attachments of wood to wood unless concentrated loads, cleavage loads, or other considerations necessitate the use of mechanical connections.
4.61 ECCENTRICITIES. Eccentricities and tension components should be avoided in glue joints by means of careful design. Figure 4-33 illustrates an example of an eccentricity and a method of avoiding it.
4.62 AVOIDANCE OF END GRAIN JOINTS. End grain glue joints will carry no appreciable load. Strength is given to such a joint by using corner blocks or gussets as shown in figure 4-34. These sketches are typical of joints encountered in joining rib members, in attaching ribs to beams or intercostals [stiffeners terminating at a rib or frame] to frames, or any other similar application.
[Extract from ANC–18 ends.]
3. FACTORS AFFECTING JOINT STRENGTH
The shearing strength of epoxy adhesives is greater than that of the strongest woods; if accurately mixed, correctly applied to prepared surfaces and allowed to cure in appropriate environmental conditions. Other factors that ensure maximum strength of a glued joint are:
•Minimum glue line – the joint surfaces of the structural members should make close contact throughout the joint when dry mated although gaps up to one millimeter or more may be OK, depending on the viscosity of the epoxy mix.
•Continuous and even glue distribution on all surfaces throughout the joint with no trapped air. (Careless mixing of the resin and hardener will introduce air bubbles to the adhesive.)
•Maximum contact surface particularly when end grain is included in the joint. Some schools of thought say planed surfaces should be slightly roughened by sandpapering 30° to the grain direction, particularly if the surface has been burnished by dulled cutting tools, while others say the surfaces should be kept smooth; which is probably correct if the surface has been freshly cut with a very sharp plane blade. (Dulled cutting edges on any woodworking tool are incompatible with quality work.) One problem with sandpapering is that imprecise action results in rounding/beveling of edges which, in effect, reduce the contact area.
•Joint cleanliness – no dust or other foreign material included.
•Evenly distributed application of the necessary clamping pressure over the appropriate time relative to the type of wood and the type of adhesive.
•Where end grain is involved additional glue [and time] must be allowed for capillary action (assisted by 'working in') to fill the cut cells to avoid a glue-starved joint. This also applies to scarf and edge joints in plywood where there are alternate layers of face and end grain in the joint. With some epoxy systems the manufacturer may recommend that when end grain is involved the surfaces should be pre-coated with the resin/hardener mix without fillers.
Pressure is applied using various types of clamps but staples or nails may be used to hold three-ply gussets, and then removed after curing – particularly if likely to be in contact with fabric. It is important that excessive pressure is avoided; it may lead to joint starvation and/or crushing of wood fibers.
A 'break piece' should be made with each pot batch from scrap pieces of the timber and, when fully cured, tested to destruction to ascertain that the wood fails not the glueline. Obviously it is necessary to take the same care with the test piece joint as with a structural joint.
Further information on wood bonding is contained in the FAA advisory circular chapter 1-1 AC 43.13-1B sections 1.4 to 1.11 below.
4. JOINTS IN PLYWOOD
The following section is an extract as is from chapter 4 of 'ANC–18 Design of wood aircraft structures' second edition issued June 1951 by the subcommittee on Air Force-Navy-Civil Aircraft Design Criteria of the United States Munitions Board Aircraft Committee.
4.10 GENERAL. Nearly all wood aircraft structures are covered with stressed plywood skin. The notable exceptions are control surfaces and the rear portion of lightly loaded wings. Shear stresses are almost always resisted by plywood skin, and in many cases, a portion of the bending and normal loads is also resisted by the plywood.
4.11. JOINTS IN THE COVERING. Lap, butt, and scarf joints are used for plywood skin.
When plywood joints are made over relatively large wood members, such as beam flanges, it is desirable to use splice plates, often called aprons or apron strips, regardless of the type of joint. It is desirable to extend the splice plates beyond the edges of the flange so that the stress in the skin will be lowered gradually, thus reducing the effect of the stress concentration at this point.
Splice plates (fig. 4-1 above) can be made to do double duty if they are scalloped corresponding to rib locations so that they may act as gussets for the attachment of the ribs.
Scarf joints are the most satisfactory type and should be used whenever possible. Scarf splices in plywood sheets should be made with a scarf slope not steeper than 1 in 12 (fig. 4-2 below). Some manufacturers prefer to make scarf joints in such a way that the external feather edge of the scarf faces aft in order to avoid any possibility of the
airflow opening the joint.
If butt joints (fig. 4-3 below) are made directly over solid or laminated wood members, as over a spar or spar flange, experience has indicated that there is a tendency to cause splitting of the spar or spar flange at the butt joint under relatively low stresses. A similar tendency toward cleavage exists where a plywood skin terminates over the middle
of a wood member instead of at its far edge.
Lap splices (fig. 4-4 below) are not recommended because of the eccentric load placed upon the glue line. If this type is used it should be made parallel to the direction of airflow, only, for obvious aerodynamic reasons.
4.12. TAPER IN THICKNESS OF THE COVERING. Loads in the plywood covering usually vary from section to section. When this is so, structural efficiency may be increased by tapering the plywood skin in thickness so that the strength varies with the load as closely as possible (fig. 4.5 below). To taper plies in thickness plies should be added as dictated by increasing loads. In doing so, the plywood should always remain symmetrical. For example, plywood constructed of an odd number of plies of equal thickness can be tapered, and at the same time maintain its symmetry by adding two plies at a time. This method is suitable for bag molding construction. Stress concentrations should be avoided by making the change in thickness gradual, either by feathering or scalloping. In bag molding construction, the additional plies are often added internally so that the face and back are continuous. (Bag molding refers to the
molding of shaped laminates using a vacuum or pressure bag to hold the material in a form whilst curing.)
When flat plywood is used, the usual method of tapering skin thickness is to splice two standard plywood sheets of different thicknesses at an appropriate rib station with a slope of scarf not steeper than 1 in 12 as shown in figure 4-6.
4.63 GLUING OF PLYWOOD OVER WOOD-PLYWOOD COMBINATIONS. Many secondary glue joints must be made between plywood covering and wood-plywood structural members having plywood edges appearing on the surface to be glued. Wood-plywood beams or wing ribs employing continuous gussets are examples of such members. The plywood edge has a tendency to project above the surface thereby preventing contact between the plywood covering and the wood portion of the plywood of the wood-plywood surface. This condition can be the result of differential shrinkage between the wood and plywood or may be caused by the surfacing machine having a different effect cutting across the grain of the plywood from cutting parallel to the grain of the wood. Figure 4.35 shows this condition and shows and shows how it can be eliminated by beveling the edges of the plywood.
ANC–18 extract ends.
5. GLUED TRUSS STRUCTURES
The structure of a simple wooden aircraft must cope with the basic stresses; compression, tension and shear. These are always present throughout the airframe in normal cruising flight (because of the forces generated by thrust, drag, lift and weight) but greatly increased when maneuvering or gust loads are applied.