Joining Structures

8/12/2019 Joining Structures

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7 - Joining Structures

ChapterJoining StructuresA dh esive Joints

The joining problem arises when we m ust connect large separate struc-tures like upper and lower fuselage halves or spar assemblies into awing, where there may be small gaps and misalignments.We use adhesive materials to join the large structural subassembliesthat are assembled to make a composite aircraft. One particular advan-tage of composite materials is that we can safely obtain really goodlong-term performance from bonded composite joints, using simpletechniques available to the home builder.

In contrast we see that the mainstream aircraft industry has suc-cessfully used bonded metal structures since WWII but has requiredcomplex chemical treatments and extraordinary cleanliness toachieve reliability. W ith aluminum this is because the chemically ac-tive surface reacts with air to form a soft oxide which is far weakerthan the bulk metal. Reliable bonding techniques require that thisoxide be removed and kept off during the bonding operation. It canbe done in a homebuilt environment but the work is hard to inspectand the consequences of spectacular failure are most unattractive.

Basic Requirements of an dhesiveRem embering the puzzle at the beginning of Chapter 2, consider whatis required to make an o verlapping lap joint between three smooth rec t-angular blocks. We ll use a matched pair of adhesive joints so that thesystem will be perfectly balanced - no question of anything other thanpure shear forces on the joints. See the sketch following.Now suppose that the blocks to be joined were - say - automobile tiretread rubber and that the adhesive is really strong stuff that has thephysical charac teristics of steel. Stress this specime n in a test mach ine.What happens'?

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Understanding ircraft Composite Construction

Blocks to Be Joined(The Adherends )

Adhesive Lines I> Adhesive Test

The joints break progressively along the adhesive lines at the rubberface. See that as the stress is applied, the rubber at the edge of the jointstretches (strains). The stiff steel glue does not pr oduce m uch strain,so the attached rubber (which must have the same strain) cannot de-velop much reaction force. The rubber at a distance from the joint in-deed stretches. The result is an extrem e stress concentration whichworks like a zipper to peel the rubber right out of the joint. This iscalled adhesive failure, because the glue and the adherend separated.Now reverse the prope rties of the two m aterials. Let the blocks be steeland let the adhesive have the properties of automobile tread. Apply astress. The steel strains (but not much ). At the glue joint, the rubbermust strain sufficiently to generate a shear stress which produces aforce equal to the total load. Given that the area of the glued j oint islarge relative to the cross-sectional area of the steel, quite a large totalshear force can be transmitted, perhaps more than the steel can carry.In fact, a rubber-like material might be quite a practical adhesive for thisapplication. Assum ing that the total strength of the adhesive is actuallyless than the strength of the steel, if the test force were increased to fail-ure we would see cohesive failure, or failure within the material of theglue joint. With sufficient bonded area, the steel part of the specimenwould fail first.

Joining WoodWe don 't see much wood used in highly stressed structures in compos-ite designs, but we do see it used as a sandwich core material and as asupport for molds. Some builders still prefer it to other materials forprimary structures in spite of the previously discussed disadvantages.

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7 - Joining StructuresWood has been used for aircraft since the Wright brothers and a greatamou nt of published standard practice has evolved. Wood w as used inWW II when aluminum was in very short supply and appropriate woodproduction facilities existed. (Remember the Mosquito bomber.) Mod-ern glues are available which are far mo re environmen tally stable thanthe wood s they are to join. (It will be assumed he re that a builder who in-tends to use wood as primary structure in her aircraft will go to pub-lished sources for wood adhesive techniques )The matrix resins used in composite layups can be good adhesives, butthe low viscosity that allows them to penetrate a layup also mak es themsqueeze out of a joint if much pressure is used to force alignmen t, as itoften is. A little bit of micro (glass bead s) can be ad ded as a filler to set aminimum thickness for the glue line. Also, the moisture inevitably pres-ent in wood m ay reduce the properties of the epoxy. A matrix resin maybe stiffer than ideal, as discussed above, but the com promise may be ac-ceptable. (See the section below on the Secondary Bond.)While it is convenient for the composite hom ebuilder who is using hismatrix ma terial for practically everything in the sh op to use it for woo dglue, too, if the application is critical it may b e better to go to formu la-tions intended for marine use. The West System materials are primarilysold as water-resistant marine adhe sives, but are offered by the aircraftsupply houses.

Joining Formed Composite StructuresThe bu tt joint is the simplest way to join two co mp osite structures. Thetwo structures are broug ht together, held accurately in position, and es-sentially repaired to make a new continuous structure, as shown onthe next page.To ma ke a butt joint for a sandwich structure, we essentially replace themissing or dam aged core m aterial and add new skin material sufficientto carry the required stress across the joint. As shown, the added skinmaterial should be tapered to avoid stress concentration near the joint.The single-skin case is similar, except that there is no core to fill. (We as-sume that a single skin structure is not required to carry bend ing stress,but will only receive stress in the plane of the surface.)

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Understanding ircraft Composite ConstructionThree Layers of Skin Material

(Tapered Thickness to AvoidStress Concentration)

or riginal Skinilled Core at JointSimple Butt Joint - Core and Single Skin Ca ses

The simple butt joint has three important disadvantages: (1) The sketchexaggerates the thickness of the added skin, but this method usuallymakes a noticeable bulge in the finished surface, and (2) it requiresfull access to both sides of the joint (which might be difficult in a fuse-lage tail cone), and (3) requires that the parts be located very accurately.Where it is practical to cut away som e core m aterial before laying up theskin, the core material may be recessed near the joint and the joiningdoubler may be p laced in the recess, as shown below.

Tapered ore for Surface ReliefFacing Skinsoublers

CoreNo Taper Here) Filled Core t Joint rea

Simple Butt Joint With Relieved SurfaceThe disadvantage of requiring full access to both sides of the joint re-mains: e ssentially, the builder must do a complete layup on both sidesto make the joint, which is often impractical for a long structure.The tapered core lap joint shown o n the next page m ostly avoids thesedisadvantages and is typical of the method generally provided by com-posite kit manufacturers. 2



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7 Joining StructuresSkin

CoreLow Density Fill

Adhesive LineTapered Core Lap Joint

Low Density FillAdhesive Linene Doubler LayerSingle Layer Joggle Joint

The previous illustration shows the case where an additional doublerskin layer has been added to each of the joined sections to strengthenthe joint where the core is tapered. See that the technique provides avery large adhesive area to make a very secure joint. To assemble, thejoint may be clamped or held with a line of pop rivets until cured. (Af-ter cure, the pop rivets can be d rilled out an d the holes filled, with negli-gible loss of strength.) Alternatively, this joint can be mad e remo vableby using bolts to hold the structures together. (If bolts are used, caremust be taken that adequate bushings and washers are provided toavoid crushing the lap area at the bolt holes. See the section followingon bushings.)The pr ecision m olded sandw ich stru ctures offered by the kit manu-facturers always provide for bonding areas to join the par ts. Often,the edges of a structure are made with the core tapered to zero thick-ness and the inner and out er skins brought t ogether to form a singlesm ooth surface area for bond ing. If necessary, the designer may spec-ify additional reinforcing layers at the transition betwe en the sandw ichskin and the joint. The corresponding mating structure will then havethe joint joggle in the opposite direction so that the two parts fit to-gether to give a smooth outer surface. (Here, the similarity to the toyplastic snap-together models is quite apparent.) Wh ere it is not practicalto apply significant bonding pressure, adhesives such as Hysol 9330 .3or 3M Scotchweld 2216 A/B Gray are used, held in place by Clecos orby pop rivets. Again, the builder should mak e all joints using the meth-ods and matrials recomm ended by the original designer.

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Joining Structures

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