Mod A

TCO (Terminal Course Objectives) (A). Understand the Basics of Composite Materials Technology. Introduction. In any subject it is easier to obey the rules if you have a basic understanding of the underlying science and technology. As an example, if you have ever conducted lap shear type adhesively bonded joint strength tests, using very poor surface preparations and very good ones, you will have found that the poor ones give almost no strength at all. The good ones fail at high loads although they all use the same adhesive. If you have never done such tests, or seen them done, you may find it much harder to believe that surface preparation is important. The background knowledge given in these pre-requisite modules will help you to understand the rules, quoted in data sheets and by instruction on the practical course, so that you will know why they are necessary and must be observed. They will try to give you an introduction to, and some understanding of, the critical points that need to be remembered when performing maintenance actions on composite parts. Always try to remember that the rules are not there to make life difficult, even if they are hard to obey at times. They are there to make sure that the job is well done and that failures do not occur as the result of either lack of knowledge or of the discipline required to apply sound knowledge to the job. These lessons have been learned, the hard way, by others and you don't want to repeat their mistakes yourself. Remember a good old saying, "Learn from the mistakes of others, you will never live long enough to make them all yourself". Foil of differences between composites and metals. KBA supply. New words One of the first things to be appreciated is the need to learn the meaning of new words. Every activity, sport or hobby has its own language that may mean very little to those not involved. In most languages the same word can have several different meanings depending on the subject under discussion. Most subjects will also have their own new words. To understand a new occupation we must learn new words and new meanings for old words. This takes some time and may cause confusion for the first few weeks. There is no way around this except time, patience and some effort. SAE AIR 4844 Composite and Metal Bonding Glossary will help considerably. New materials There is an immediate need to appreciate a significant range of new materials, Most people are familiar with wood, metals and several types of plastic. Entering the world of composites means learning about carbon, glass and aramid fibers, resins and adhesives, bagging films, release films, potting compounds, honeycomb core and foams of various types. It also means learning how vacuum systems work to apply pressure, how thermocouples measure temperature and how hot-bonders are used to control temperature, vacuum pressure and curing times. We will begin with resins and

adhesives and then move on to the three main fibers carbon, glass and aramid and also core materials of various types. For more information see Reference 1. A1 Distinguish among resin, fiber and core applications and uses.

Resins A resin is often called a matrix when used in conjunction with reinforcing fibers, i.e. a composite consisting of the fibers and the resin. In modern composite materials applications there are many resins available and numerous products within each type. Epoxy resins are the ones most commonly used in aircraft structural components, but Phenolic resins are used for internal composite parts such as floor panels, galleys and overhead storage bins because phenolic resins are less toxic in the event of fire. Polyimide and Bis-maleimide resins are used where higher service temperatures are needed, Polyester, Vinyl Ester and Epoxy resins are used extensively in boat building and for many other uses. First it should be noted that the term "resins" is normally used to describe relatively low viscosity liquid materials that form the matrix of a composite when they are cured. The word viscosity is used to describe how "runny" the materials are, for example, water is a "thin" (low viscosity ) fluid that flows easily, treacle is a much thicker (high viscosity) one. Wet-layup resins: These resins are typically two-part systems, where a curing agent is mixed with a base resin and the mixture brushed onto layers of Fibreglass, Carbon fiber, Aramid fiber or occasionally other less commonly used fibers. These liquid resins may then cure to a solid product at room temperature or they may need heat to be applied to achieve the chemical reaction required to make them cure to a solid. It is essential that the base resin and curing agent are weighed out in the correct proportions, to the nearest tenth of a gram using electronic scales and then mixed thoroughly for at least three minutes. This is important in order to achieve the required strength and temperature resistance of the composite part. . When the required amounts of base resin and curing agent have been taken from their cans, or other containers, always remember to replace the lids immediately. This is important because moisture absorption, especially into the curing agent degrades the material and reduces the strength of the final product. These two-part systems must be weighed accurately to ensure the correct mix ratio. Check the data sheet each time as different systems have significantly different ratios. They must then be thoroughly mixed for at least three minutes. Trials have shown that this is essential. Three minutes feels a very long time when mixing a resin, rather like waiting for a bus. Use your watch and check the time or you will be in danger of mixing for less than three minutes and then you will not achieve a good mix. Some two-part systems contain dyes to color the two parts, say red and white or blue and yellow. When a completely uniform color is achieved with no streaks the resin can be considered mixed but the three minute rule is worth using to be on the safe side. Remember that the resin matrix must be fully cured to give the final composite part the compression strength it needs.

Pre-preg resins: Alternatively the resin can be supplied already applied to the fabric in a form called Pre-preg, which means pre-impregnated fibers. It is then supplied as a roll between two plastic release sheets, which must be removed before application or the layers of pre-preg will not stick to each other during the curing process. The resin is then at what is known as the "B"stage and must be kept in a freezer at -18oC(0oF) until it is required for use. In this condition it has a shelf life of six to twelve months depending on the type. Pre-preg also has an open time limit at room temperature of about one month. After this period the material must be tested for acceptance before use or be scrapped. Records must be kept of shelf life and open time for each roll. When required for use a roll must be allowed to warm to room temperature and must not be opened until all moisture, due to condensation from the shop atmosphere onto the cold roll, has been completely evaporated. The total time out of the freezer must be recorded on each occasion plus 15 minutes and when the open time limit has been reached the material must be tested before further use. Pre-preg materials used in the fabrication of aircraft structural components normally come in two types, those that cure at 120oC (250oF) and those that cure at 180oC (350oF). Some types can be cured at 150oC (300oF). Matrix resins come in a wide range of chemical formulations and some can work at much higher temperatures than others. The maximum temperature at which a composite part can be used depends on the choice of resin and fiber. Carbon and glass fibers can be used at quite high temperatures but aramid and polythene fibers can be limited in maximum temperature by the fibers rather than the resins. In repair work one major requirement is that only the OEM (Original Equipment Manufacturer), SRM (Structural Repair Manual) specified resins and fibers or OEM or DER (Designated Engineering Representative) agreed alternatives may be used. This is necessary to maintain the original design strength of the part. The first problem in repair, once a decision has been made that the part will be repaired in preference to being replaced, is to check the SRM or part drawing and find out the materials the part is made from and hence the materials that must be used for repair. Adhesives Adhesives are used to bond composite parts together and are often used for bonding metal parts, usually parts made from aluminum alloys but also titanium alloys and occasionally stainless steel. Metal reinforcements are sometimes used in composite parts so there is a need to understand the adhesive bonding of metal parts as well as composites. The surface preparation of metal parts to be bonded is even more critical than the surface preparation of composite parts therefore the technician must be very careful to ensure that the surfaces to be bonded are properly prepared. The surface treatment of metal parts will be covered more fully later. Adhesives are chemically similar to composite matrix resins but are of higher viscosity to avoid the adhesive flowing out of joints and leaving them "resin starved". Two types of adhesives are typically used in bonding composites, paste adhesives and film adhesives. Paste Adhesives: Two-part room temperature curing adhesive systems are usually of about "toothpaste" viscosity, when completely mixed. They also need to be weighed out

accurately and the two parts mixed thoroughly for at least three minutes. Epoxy adhesives are the most common type used for bonding aircraft structural composite parts. The maximum service temperature of paste adhesives varies considerably with their chemical composition so it is essential to ensure that only the OEM approved type or approved equivalent is used for each specific purpose. One part paste adhesives are also available and these need to be heat-cured in a similar way to film adhesives Curing of paste adhesives takes place above a minimum temperature that depends on the chemical formulation. See the data sheets for curing times, pressures and temperatures. They also require low temperature storage. Lids must also be immediately replaced on containers, after removing the required amount of adhesive, to avoid moisture absorption. One part resins must be returned to the freezer as soon as possible after the required amount has been taken Film Adhesives: Adhesives can also be supplied in the "B" stage, or partly cured condition, when they are known as film adhesives because they are a thin film of adhesive between two plastic release sheets. They come in various weights per square foot or square meter and can also be cured at 120oC (250oF), 150oC (300oF)or 180oC (350oF) depending on the type. These cure temperatures must be accurately maintained over the entire bondline as too low a temperature means an incomplete cure and too high a temperature may make the bondline more brittle. Each adhesive data sheet will provide the temperature limits for that particular product. Some have a wider band of acceptable cure temperature than others. In some cases, but not all, a lower temperature may be used for a longer time but all film adhesives have a lower temperature limit below which a full cure will not take place. Check the data sheet for the material you are using. When bonding aluminum alloy skins to aluminum alloy or aramid honeycomb it is essential to use a fairly heavy weight of film adhesive in order to produce a good size of fillet where the honeycomb joins the skin. When bonding composite pre-preg to honeycomb it is required to add a thin layer of film adhesive and not rely completely on surplus resin from the pre-preg being adequate to ensure good fillets to the honeycomb core. Fibers used in composite structures There are three fiber types that are most commonly used in composite aircraft structures. These are carbon, glass and aramid. They are easily distinguished from one another because carbon is black, glass is water clear and aramid is yellow in color. Quartz fiber is sometimes used for radomes and Boron fiber patches are often used to repair metal parts that exhibit fatigue cracks. Carbon fibers have the best all-round properties when all factors are considered and so are the most commonly employed fiber for critical structural aircraft components. They are used for wing, fuselage, stabilisers (horizontal and vertical), elevators, ailerons, main wing flaps and rudder structures. Carbon fiber is also used in undercarriage doors, engine cowlings, helicopter rotor blades and can even be found in undercarriage components for some helicopters. Carbon fiber can be supplied in several grades of strength and modulus and forms. Carbon fibers are often woven into fabric (or cloth) forms for original part and repair pre-pregs or dry fiber mats for wet layup repairs.

Glass fiber is used for radomes because it has good transmissivity and is not electrically conductive. Aramid fibers are used for some radomes and Quartz fiber for some others. These materials can also be used for galleys, floor panels and overhead stowage bins. Carbon is sometimes used for floor panel skins. Glass fibers are available in two basic types, "E" glass and "S" glass. "E" glass fiber is cheaper and by far the most commonly used but "S" glass fiber, although more expensive, has higher mechanical properties and is used where the additional cost can be justified. Aramid can be supplied in several types. "Kevlar" 49 is the most common for aircraft use and "Kevlar" 149 can be supplied if required. “Kevlar” 149 has a much lower water uptake than “Kevlar” 49. All these fibers, except Boron, which is too thick to be woven, can be made into fabrics having many different weave styles or made into unidirectional tapes each having a wide range of areal weights. To assist in good adhesion to matrices Carbon fiber is treated with an oxidation process and then an epoxy sizing/finish is applied. Glass fiber has the sizing applied after the fibers have been drawn from the bush and this helps to prevent damage during weaving. After weaving this size is burned off by heating to 600oF (315oC) in an oven and a finish is applied to improve the resin to glass bond strength and also to increase bond durability. It is important to note that the choice of finish used on the glass depends on the resin to be used to make the composite. Glass and aramid fabric manufacturers supply documentation that give tables showing the finish required for each resin type. Aramid fabrics and tapes may or may not have a finish depending on the resin to be used. It can be seen that it is essential to select the correct fiber type and weight of fabric or tape with the right surface finish in order to make a strong, durable repair. It is very easy to use a fabric of the wrong weight and great care must be taken to ensure that this does not happen. When fabric is cut from a roll a label with full identification details must be attached or included in the plastic bag. This information must give the surface finish details, as once removed from the roll there is no simple means of identifying the finish used on a fabric. A further point that should be noted is that some composite lay-ups may use fabrics and tapes of different weights at certain points in the lay-up and sometimes layers of aramid or glass are added at special positions. This means that all the plies in a lay-up may not be the same materials or the same weight so careful identification and the correct location and orientation of each ply is a serious matter. In should be noted that a fiber reinforced composite is only strong in the direction of the fiber. The angle of each ply is important to strength and stiffness and each layer or ply must be laid up in the direction given on the drawing or SRM page. Drawings and SRM's give ply tables showing the material type and lay-up direction of each ply in a part. An orientation clock (sometimes called a warp clock) is also shown on each drawing sheet to give the lay-up direction of each ply relative to a key direction on the part. It

should also be borne in mind that although composites are very strong in the fiber direction their transverse strength may be only about one thirtieth of their longitudinal strength. This is a similar ratio to wood, which has cellulose fibers in a matrix called lignin. The low value of transverse strength needs to be considered in design. This is why clean, dry composite surfaces, which have been carefully abraded with aluminum oxide or silicon carbide abrasive paper need to be used. Only the surface resin layer should be abraded without damaging the first layer of fiber. Use the grit size recommended in the SRM. Note that for repair work the first repair ply should be oriented in the same direction as the surface ply to which it is to be bonded. The transverse strength of a composite is limited by the lower of two properties. The first is the resin strength itself and the second, which may govern the end result, is the resin to fiber bond strength. If the resin comes unstuck from the fiber then the transverse strength will be only that of the bond and not the resin. Ideally the strength of the bond to the fiber needs to be greater than the strength of the resin itself. This requires the use of the correct finish on the fibers and dry fiber surfaces. Core materials Aircraft currently employ many parts such as flight control surfaces, undercarriage doors, engine cowlings, many fairings and wing trailing edge panels, which utilise sandwich composite construction. Sandwich parts consist of two thin face sheets, with a core material between them, to provide stiffness and strength at low weight. These thin-faced structures are easily damaged but are, fortunately, relatively easy to repair. Anyone involved in composite repairs is likely to deal with many of these components.

In a similar way to an "I" section steel beam used in buildings, the skins take the tension and compression loads that are taken by the flanges in an "I" beam and the core takes the shear loads like the web does in an "I" beam. Figure 2.26 from Ref: 1 or Sandwich construction diagram (use SRM diagram of a sandwich cross-section). Sandwich panels are near to optimum design when the weight of the two skins is equal to the weight of the core material. However, in applications such as passenger, and especially cargo floor panels in aircraft, the indentation or damage resistance may be very important to the life of the part in service. It has been found that the damage resistance can be improved at the lowest weight increase by using a heavier, and hence stronger, core material rather than by adding to the weight of the top skin. The size of the indenting object is also important. Small indentors tend to cause compression failure of the core and larger ones cause shear failure. Therefore, core materials for these types of applications need both good compression and shear strengths. These factors need to be taken into account when designing wheels for food carts. The largest practicable “footprint” on the panel is desirable. It is also helpful for the food cart wheels to have

“tyres” of the lowest possible coefficient of friction to allow the wheels to swivel easily on the carpet in the cabin. A considerable range of core materials exists, each serving its own specific purposes, although many are used for a range of components. The factors that determine choice are strength, upper temperature limits, moisture absorption and cost. There is no such thing as a bad natural material, although a bad material can be produced when its formulation is not correct or a process is not carried out correctly or it has degraded in some way before use. Materials are what they are, when made or selected properly, and we have to use the right ones in the right places. As an example, balsa wood can be a good core material but only if water is prevented from ingressing into it. When foam cores are used in boat hull construction they must be of the "closed cell" type to avoid water absorption if the outer skin is damaged. Typical core materials used in composite aircraft structural sandwich parts are: Aramid honeycomb Glass cloth honeycomb Aramid cloth honeycomb Aluminum alloy honeycomb Other core materials currently available are, Balsa wood Polystyrene foam (Styrofoam) Polyvinyl chloride foam (cross-linked and uncross-linked versions). Polyurethane foam Polyimide foam Polymethacrylimide foam (Rohacell). Carbon fiber cloth honeycombs All honeycombs can be supplied in a range of cell sizes, cell shapes and core densities. The normal hexagonal cell does not bend easily except when aramid is heat formed. Aramid honeycomb with hexagonal core is often heat-formed by an OEM as this can be cheaper than buying over-expanded or flex core. Over-expanded honeycomb is made to bend one way and to wrap around leading edges or to make tubes. Flex core, or double flex core, cell shapes are used for the nose of radomes and other double curvature requirements. Only electrically non-conductive core materials may be used to make or repair radomes. One problem that can occur with radomes, and other parts in service, is that hexagonal honeycomb, that is supplied flat, will not always take up the shape required in a radome if it cannot be heat-formed. It may be necessary to request OEM approval to use over-expanded or flex core of equivalent strength and stiffness in these positions for repairs. Aluminum alloy honeycomb must be anodized to ensure a good bond to the adhesive that joins it to the skin. The bondline area, end-on to the cells, is so small that good fillets of adhesive are essential to a good bond (0.5 mm or .020 inch minimum fillet size) is recommended. For this reason only the correct type of film adhesive may be used to

bond honeycomb. If in doubt always use a slightly heavier than specified film adhesive when bonding to honeycomb to ensure a good fillet. A2 Describe various composite processing parameters. Composite components are normally made by the OEM, or an approved subcontractor, to very high standards. 1. Production quality tooling is used as a significant number of parts have to be made and good tooling is essential. 2. All film adhesives and pre-pregs are stored to their manufacturer’s recommendations and records are kept to ensure that materials are within their shelf life and in good condition at the time of use. 3. The staff are trained and make many parts and therefore become familiar with the needs of the job they are doing. 4. The workshop temperature is maintained within specified limits and so is the relative humidity of the atmosphere. 5. The part is laid up in a clean room with a positive pressure where temperature and humidity are also controlled. No cutting or sanding is permitted in a clean room and staff must wear clean overalls. No smoking, drinking or eating is permitted in the clean room. 6. The work is planned to run smoothly so there are no long delays between layup of the part and the cure cycle. 7. Normally an autoclave is used to ensure the required pressure and temperature and a computer controlled program will control the heat –up rate, the cure cycle, with any steps in temperature if required, and the cool down rate. 8. The pressure will also be monitored throughout the cure. Repair processing See section B1. It may be a surprise to know that good repair is actually a more difficult process.

Correct processing of composites and adhesives is absolutely vital. OEM's have as one of their greatest concerns the quality of repairs made at outstations around the world, where they have no control over the quality of the work. This is the basic reason for this training course. On the positive side, the better we do the work, the larger the repairs we are likely to be allowed to make. When metal sheets or large billets are purchased from approved and long-established companies with the full quality control test data supplied, OEM's and airlines can have a high level of confidence that the material is of the right quality. When composite repairs are performed by other than the OEM’s it is essential that the repair materials, repair processes and the repair personnel are fully approved and qualified. The processing parameters that need to be controlled are many.

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The workshop must be clean, dry and at the right temperature and humidity. The recommended conditions are a clean room with a positive air pressure to minimise the entry of dust and dirt, room temperature 18oC (64oF) to 30oC (86oF)and 60% maximum relative humidity. A figure of 35% relative humidity is preferred. Tests at QinetiQ, Farnborough, UK have shown that relative humidity below 35% does not lead to any significant improvement in results and is uncomfortable as a working environment. Good lighting should be provided. Any tooling required must be available, clean and in good condition. The component must be clean and dry and the surface must be correctly prepared for bonding the repair patch. Check with a moisture meter if components are fiberglass or aramid. Dry until readings in the green band are obtained. Dry the surface to SRM requirements as a minimum if the surface is carbon fiber. No moisture meter for carbon fiber is available at a reasonable cost for general use. The required repair materials must be available and in the correct condition (i.e. dry and warmed to room temperature) and within their specified shelf lives. Each ply of composite must be of the correct material type, weave and weight and it must be laid in the right place and in the right direction. If using pre-preg material, both release films must be removed. Place all release films in one pile and check that they have all been removed before proceeding with the bagging process. Remove one release film, lay the ply and then roll the ply down before removing the top release film for each layer. If new honeycomb is used in a repair it must be of the right type, weight and cell size with the specified finish and must be undamaged. It must also be oriented to drawing and in line with the core being repaired. It must be bonded to the skin with the specified adhesive and bonded to the existing core with potting compound using the specified materials. If other core materials e.g foams are used they must be of the right type and weight and they must be joined to the existing core with the adhesive specified in the SRM. If vacuum pressure is to be used the bagging film and sealant and the breather cloth lay up, together with all release films, perforated and non-perforated must be correct and in the required positions and all the materials must be clean and dry and in good condition. Thermocouples must be correctly located. Remember that thermocouples are often accurate to about plus or minus five degrees. To ensure a good cure add five degrees to the cure temperature. See Ref:1, Chapter 10, for more details of thermocouples. Use the correct type as several types are available and their calibration requirements are very different. Your hot-bonder will only work with the type for which it was designed. If you are not familiar with thermocouples a simple description may help. AIR 4844 defines them as,”A device which uses a circuit of two wires of dissimilar metals or alloys, the two junctions of which are at different temperatures. In special laboratory cases, where great accuracy was required, it was common practice many years ago to place one junction, the cold junction, in melting ice and the other at the point of temperature measurement. In portable equipment, such as hot-bonders, a cold junction compensation circuit is used, and only the hot junction employed for temperature measurement is visible

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to the user. A net electromotive force (emf) or current, occurs as a result of this temperature difference between the two junctions. The minute electromotive force , or current, is sufficient to drive a galvanometer or potentiometer”. These can be calibrated in terms of temperature. The most common thermocouple types are Iron/Constantan, Chromel/Alumel and Platinum/Platinum Rhodium alloy. They are listed in order of cost. Vacuum pressure must be correct and the leakage rate check requirement must be met. Bonding should commence as soon as possible after the materials have been laid up, to avoid degradation of film adhesives and pre-pregs at room temperature. Some large jobs may take three days to lay up and this is a significant amount of the permitted open time if the material is anywhere near the end if its shelf life. For such work the remaining open time for the material should be checked before lay-up starts. The specific pre-preg material working life is provided in the supplier data. The temperature readings from all the thermocouples must be within the required limits and maintained correctly for the specified length of time to achieve full cure. Local heating at any cool spots may be needed to do this. Vacuum pressure must be maintained for the whole cure time and until the component temperature has fallen to 50oC or less. After curing, the release films and peel plies must be removed with care to avoid damage after so much good work has been done.

Some very practical lessons learned the hard way. 1. Some pre-pregs need a perforated release sheet to allow volatiles to escape in addition to its use for bleeding off excess resin. One sample did not cure with non-perforated release sheet on both sides but it did cure when the nonperforated release sheet on one side was removed and replaced with a perforated sheet. In this case the volatiles seemed to be inhibiting the cure. 2. Shell Epikote 828 seems to get more viscous with time in storage. Any crystallisation can be removed by heating the base resin alone to 60oC maximum for one hour, see the data sheet. The same applies to Shell Epikote 815, which is 828 with a diluent. 3. Breather cloth MUST be placed under the vacuum extract fitting. This is to ensure a good airflow and to prevent resin going up the vacuum line. If resin gets into the extract fitting it may also block it and solid resin is difficult to remove. 4. Do not locate the vacuum extract fitting too near the part or it may draw resin into itself. 5. Do not place the vacuum extract fitting on the part or it will leave its mark, permanently.

6. Tools (molds) must be absolutely clean and smooth. All defects will be repeated on the part. 7. Release sheet should extend well beyond the end of the part to avoid resin sticking to the mold face. 8. Composite laminates will not bond together properly at zero pressure. Just rolling them together, even with a good roller pressure, will not be sufficient. This is especially true when time-expired material is being used for training purposes. A good vacuum is essential for a good bond and to extract volatiles. Some of the finer points can only be learned from experience and may not be applicable to all materials. It is worth making your own check list and recording experiences to save going through the whole process again another day. A3 Describe composite design parameters and effects of processing

Design parameters include strength, stiffness, impact resistance, fatigue resistance, creep resistance, temperature limitations and relative thermal expansion coefficients between different layers of fabric or tape and between the component and the tooling used. 1. 2. 3. 4. 5. 6. Strength in the fiber direction depends almost entirely on the fibers. Stiffness in the fiber direction depends almost entirely on the fibers. Impact resistance depends on the toughness of the fibers and the strain to failure of the resin matrix. Also on the strength of the matrix to fiber bond. Composite compression strength depends on the matrix resin modulus. Interlaminar shear strength (ILSS), or short beam shear strength, depends on resin matrix properties. See ASTM-D-2344. Fatigue resistance depends on the relative fatigue strain to failure of the fiber and resin. If the fiber has a fatigue strain to failure higher than the resin then the resin will fail first and laminate failure will be progressive. If the resin fatigue strain to failure is higher than that of the fiber then the fatigue properties become fiber dependent. The correct finish on all types of fiber is important to give a good resin to fiber bond. The maximum operating temperature of a composite is defined by the maximum operating temperature of the resin matrix. This is related to the glass transition temperature, which often depends on the cure temperature. Hence the correct resin must be used and it must be cured at the correct temperature for the required time. Creep resistance depends on the creep characteristics of the fiber. Temperature cycling casn have a significant effect on a composite part. If, for example, aramid and carbon fibers are mixed in a laminate then if the component

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suffers a wide range of temperature cycling e.g. in an engine cowling, then disbonding may occur due to the stresses imposed by the differential thermal expansion and contraction because these characteristics are different for the two fibers. Delamination has been found to occur over a period of time in engine cowlings of this design. Cracking due to linear expansion differences has been reported and must be considered in design. The resin must be fully cured. If the resin matrix is not fully cured then the performance of the composite may be seriously affected. The resin matrix may have a lower modulus than required and the compression strength could be reduced. The moisture absorption into the matrix could be increased . The glass transition temperature could be lower than the specification requires. The plies must be de-bulked prior to cure. If the plies are not de-bulked sufficiently to remove air before heat is applied, and the vacuum is not sufficient to remove any solvent vapors that outgas from the resin during cure, then voids may be present in the cured product. These can result in reduced compression strength. The correct de-bulking procedure must be followed and the required vacuum or autoclave pressure MUST be maintained for the full cure time. The correct perforated release films and breather cloths must also be used.

It can be seen from this list of things that can go wrong, unless the above factors are understood and dealt with, that incorrect processing will affect the strength and maximum service temperature of the final product. Hence the need for this training course or an equivalent. Many of the words in the above section may be new to you. Please see section A6 Composite and Metal Bonding Glossary SAE AIR 4844 latest issue for definitions of these terms. Time spent reading the glossary is an important educational process in itself. Always keep a copy handy, it can be very helpful. A4 Describe various composite machining, assembly and finishing processes. Composite machining. This is well described in a very informal video supplied by Du Pont. Ref: 2. SRM’s from Airbus and Boeing cover the design of drills and other cutting tools very well. As with any material the cutting tools must be sharp and use the angles, found by much experiment, to be the best for each process. Cutting speeds and feed rates must also be correct. Always consult source documentation, e.g the SRM for the correct cutting instructions. See Ref: 1, Chapter 13. Simple factors that apply equally well to cutting wood, plywood or composites are the need to use a backing plate of wood, plywood or plastic to avoid "break out" on the rear face. If you doubt this just put a piece of wood or plywood in a vise and push an ordinary twist drill through it at a high feed rate. The back face will have large splinters broken from it. Likewise if you need to chamfer a piece of wood you must cut it with a plane or chisel so that the wood fibers are in tension. If you don't you will split the wood across the grain. Composites behave in a similar way but are much more expensive if you damage them. Always cut or sand a composite so that the fibers are cut in tension.

This is especially important when cutting aramids. Another important point with aramids is that they are softer and need a sharper cutting tool. Any tools used to cut aramids must be kept for aramids only. If tools have been used to cut glass or carbon fibers they will not be sharp enough to cut aramids afterwards. Carbon fiber, although not itself hard, wears steel tools quite rapidly. Frequent sharpening of all cutting tools is necessary. Routers Routing is a very coarse cutting system and only sharp tools of the right design can be used with the support of the right templates. Special band saws High speed diamond grit-coated band saws are good for cutting composites but backing support must be provided. Dust extraction must be provided on all power tools and all dust produced by hand tools should be removed with a vacuum cleaner as soon as practical. DO NOT, under any circumstances use a pressurised air line to blow dust away as this only adds to the particle concentration that you are breathing and is an unacceptable health hazard. Cutting honeycomb cores Aluminum alloy, aramid and other honeycomb materials can be supplied in sheets cut to the thickness required. They can be cut down the cells with a sharp knife, e.g. Stanley Knife or a razor blade, but if they need to be cut to thickness from a block then a very high speed band saw is needed. A suitable band saw needs to run at a blade speed of 5,000 metres (16,000 ft) per minute. The reason for this high blade speed is that the only resistance to the cutting force of the blade teeth is the inertia of a very lightweight material. Consequently only a very high speed can help to produce a clean cut before the material is bent out of shape. A special blade is also needed with only a 0.05 mm (0.002 inch) offset on the teeth to avoid tearing the honeycomb. This blade MUST NOT be used to cut any other materials. For sandwich panel repairs it is common practice to cut a piece of honeycomb, slightly in excess of the final thickness, to the required shape to fit a hole using a sharp knife. You can then apply a suitable adhesive to the bottom skin, a generous amount is recommended to ensure good filleting, and then to "pot" the honeycomb in place around the edges to join it to the existing honeycomb with a suitable potting compound. Once the adhesives have cured, the skin should be protected, and then the honeycomb core that projects above the skin can be carefully sanded flush with the skin in preparation for the application of the skin repair patch. Cutting foam cores

Some can use hot wire cutters but others e.g. polyurethane foams, give off dangerous fumes and the use of hot-wire cutters is not permitted. Check with the material data sheet and SRM to ensure that you use the correct, and safe, cutting method. Water jet cutting This uses very high pressure water containing an abrasive grit and is a very good production line system when large numbers of parts need to be cut. It can also be used for single cuts. The machinery is very expensive and needs good safety systems as a water jet can remove a finger or an arm very rapidly. It has the advantages that accurate profiles can be cut quickly and the temperature is low so that no damage is done to the matrix resin during the cutting process. It also washes away the machining dust. Other cutting and sawing processes generate considerable frictional heat. This causes unwanted fumes to be given off during cutting. Extraction systems should be used to remove these vapors and the dust. Oscillating saws (Cast cutters) This method uses a circular saw blade that oscillates through a very small angle. If the blade touches the skin, the flexibility of the skin is sufficient to allow it to move by the same amount as the blade or more so that no cutting action takes place. When the same saw is used on a rigid composite it will cut the material because it is rigid compared to skin and flesh. This technique is often used to cut out damaged sections of skin from a sandwich panel with honeycomb or other core prior to the removal of damaged core. The following recommendations are made to ensure safety and quality. 1. Use the correct blades . 2. Never use a dull (worn) blade, it may cause injury. 3. Tighten the blade retaining nut before each use. 4. Clamp the composite to minimise vibration and to improve the quality of cut. 5. Too much pressure may cause the blade to break. 6. Clean the blade before each cutting operation. 7. Do not use near solvents or other flammable fluids as these cutters are electrically powered. Tank cutters These are like circular hacksaw blades and are used by plumbers to cut holes in water tanks. They are very useful for cutting large radii at the corners of holes cut in damaged sandwich panel skins. It should be noted that when cutting out sections of skin, to remove impact or disbonding damage, square corners should not be used as they can induce fatigue cracks at a later stage . Generous corner radii should always be used for skin repairs. i.e 25mm (one inch) or larger. Use suitable tank cutters Grinding burrs

These come in a wide range of shapes and sizes for various tasks and a convenient size can usually be found to smooth the edges of cut-outs in skins and to blend the sides with the corners produced by tank cutters. See Chapter 13 in Ref:1, Hand Tools. Hand power drills The first point to be made about these is that they should be air-powered and not electrically powered. The reason for this is that solvents, such as acetone, MIBK and IPA are used for cleaning off surplus resin from parts and for cleaning resin or adhesives from tools and molds and these solvents are flammable. They are always present in composite and metal bonding workshops and fire is a serious risk to be carefully avoided. These drills are used to carry drill bits of various types, tank cutters and grinding burrs. If working directly on aircraft, fuel or fuel vapor may also be a problem. Pneumatic tools with rear exhausts are recommended and are beneficial for two reasons, firstly air is directed away from the work and dust is not blown around the shop and secondly venturitype vacuum attachments to the tools can be used to provide dust extraction. This applies to high speed grinders below and all other compressed air powered tools. See Ref:1. Machining a hole in carbon fiber or glass fiber is particularly gruelling on bits. Carbide or cobalt-tipped spade bits are good, but diamond polycrystalline coated tools are better. For carbon fiber, Boeing recommends drilling dry if possible. If necessary use filtered air, CO2, non-oil-containing Freon or Boelube as lubricants. It is important not to exceed the glass transition temperature of the resin during drilling. Always check the SRM for the correct drills and tools. In general a high cutting speed and a low feed rate is recommended. The temperature of the component must not exceed 60oC during machining of any kind. High Speed Grinders These machines are used for light sanding, feather edging and cutting. They must be small and light enough to be handled easily. A high skill level is required to achieve good results. Use the correct disc with the right grit type and size. Generally 120 grit should be coarse enough to remove paint and 240-320 grit is suitable for preparing surfaces for adhesive bonding. A delicate touch is needed because the objective is to lightly abrade the resin at the surface without cutting into any of the surface fibers. This really does need care and practice. Orbital Sanders These sanders are normally compressed air powered and should be used carefully and the correct type of grit and grit size must be used. Also new abrasive paper should be used when the first piece has become worn out or clogged. While it is important not to remove too much material too quickly, by using a grit size that is too coarse, it should be noted that the grit on a worn paper slowly breaks up into a smaller grit size and so becomes a finer grade with time. This will generate a lot of frictional surface heat on the part and the cutting rate will fall to near zero.

Abrasive papers and grits When using abrasive papers and grits it is important to use the grit sizes recommended in the SRM. Too fine a grit size will cause frictional heat and a slow rate of material removal. However, too large a grit size will cause deep scratches and may remove more material than intended and hence lead to a larger repair. Choosing the optimum grit size is essential and also the correct grit type. Aluminum oxide and silicon carbide are the most commonly used grits. Suitable grades of 3M Scotchbrite abrasive pads may also be recommended in the SRM. Reamers When mechanical fasteners are used that require either a close tolerance hole, or a small degree of interference fit, a very accurate hole needs to be made and a jig may be needed to ensure accurate hole alignment in addition to the precise hole size required. One of the problems of serious structural repairs with mechanical fasteners is going to be tooling to ensure accurate alignment of holes and close tolerance holes. Lubrication and cooling Another factor to be considered when machining composites with any of the tooling mentioned above, but especially when drilling holes, is the need to avoid heating the composite to the point where the glass transition temperature of the resin is exceeded. Drilling above the resin Tg may cause clogging and prevent material removal. The composite should not be heated to above 60oC (140oF) to minimise this risk. See Ref:1. Composite parts are not good at conducting heat away from drilling, thus keeping the drill bit cool is a major concern. Air cooling, using clean, filtered air may be used, or CO2 gas, or non-oil-containing freon. Approved lubricants are alcohol lubricants from the BOELUBE family of cutting agents. These will not ingress the fibers or resins, will not cause outgassing in honeycomb structures, will not contaminate the adhesive or bonding agents, and may be removed easily by alcohol solvents or a mild detergent rinse. The Boelube range are based on cetyl alcohol (C16H34O) also known as Hexadecanol or nHexadecyl alcohol. This a waxy solid of melting point 49oC (120oF), which is a good indicator of the temperature around the drill and helps to avoid exceeding 60oC (140oF). See Ref:1. Speed and feed rates The wrong cutting speed or feed rate can cause heat or mechanical damage and /or delamination of composite parts. Check the SRM for the correct values. As a general rule, a high-speed, slow-feed technique is preferred. See Ref: 1 for more detail. Diamond Wheel trimmers

These are used mainly for cutting panel edges to shape and use diamond-tipped cutting wheels. They are designed to cut quickly and to give a good edge finish. Assembly processes. Composite components can either be; a) Bonded together using suitable film or paste adhesives b) or they can be mechanically fastened using a range of special fasteners for this process. Quite often a combination of these is used and assembly may be by bonding in some areas and fastening in others or joints can be both bolted and bonded to provide extra security and fatigue resistance. Complex assembly jigs may be required to ensure the precise location of critical parts such as hinges in undercarriage doors or attachment lugs for empennage components. Parts that may need frequent removal for inspection or maintenance may need very accurate jigging so that any replacement parts will be certain to fit. Finishing processes The quality of finish required is likely to depend on whether or not the part can be seen by passengers. It may also be a wing or empennage leading edge and a smooth finish may be needed to ensure smooth airflow. Special finishing films of adhesive or resin are sometimes applied when a particularly good finish is required. The quality of surface finish on the mold used to make the part is also critical as the slightest mark on the mold will be repeated in the surface resin of the cured part. Fillers can be used and the surface rubbed down by hand in some cases. Primers and paints are the first thought when finishes are mentioned. They are needed to protect composite parts from environmental effects such as moisture and ultra-violet rays. However, many other coatings are used for a range of purposes 1. Conductive coatings are added to give protection from lightning strikes and take several forms. Metal foils, flame-sprayed aluminum, and metal-coated fabrics or expanded foil mesh may be used. It is important, when repairing composites, to remember that any such coatings must be replaced and their electrical conductivity checked as part of the repair process. Erosion-resistant coatings are used on radomes, wing leading edges, tailplane and fin leading edges and the leading edges of helicopter blades. They may be neoprene or polyurethane rubber boots for radomes or erosion resistant paints of various types or they may be titanium or stainless steel sheets formed to the shape required. Some interior panels may be coated with Tedlar, Polyvinyl fluoride(PVF) to keep out moisture and provide an easily cleaned surface. Speedtape or a thin coat of sealant may provide temporary protection for a small amount of allowable damage that is awaiting repair.

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Whatever the coating if it has to be removed, in part or in whole, to repair a component then it must be replaced before the repair is signed off as completed. A5 Describe stiffened laminate and sandwich applications and structural properties. Stiffened laminate applications Stiffened laminate applications are usually wing and empennage main torque box skins and spars, and fuselage skins, stringers and frames. These components employ skins that are themselves of thickness greater than needed for the skins used for sandwich panels. Fuselage skin laminates will be fairly thick in some places where bending or torsion loads are high. The latest aircraft are using very thick composite laminates for wing spars in addition to the heavy skin panels used in wing torque box structures previously made from aluminum alloy. The wing skins themselves are stiffened with composite stringers which may be bonded, bolted or attached by both methods. Wing skin profile is maintained and air loads transferred to the spars by ribs, which are attached to the skins and to the spars at each end. The wing skins use a ply lay-up designed to give good torsional stiffness to the wings as well as good bending strength. The lower wing skin takes tension loads in flight and some compression load on the ground whereas the upper skin is in compression during flight but carries some tension loading on the ground and more in the case of a heavy landing. Twisting (torsion) loads are applied to the wing by engine thrust and reverse thrust and by the use of ailerons and spoilers. Similarly, the use of rudder, especially in the event of engine failure on one side, will apply severe torsion loads to the fuselage. Likewise the horizontal stabiliser will apply bending loads to the fuselage when the elevators are used as will a heavy landing. Sandwich structure applications These are much lighter structures often using skins of only two or three layers of a fairly thin carbon, glass or aramid fabric. They may use aramid honeycomb, aluminum alloy honeycomb, various types of foam or balsa wood as core materials. Usually, but not always, aluminum honeycomb is used in panels with skins of aluminum alloy and composite-skinned panels usually use aramid or glass honeycomb or foam cores. There are many applications of sandwich structures. Some of these are ailerons, elevators, rudders, floor panels, trailing edge falsework panels, wing to body fairings, flap screw jack fairings, engine cowlings, undercarriage doors, radomes, galleys, overhead stowage bins etc. These are the items, which until recently, have been the concern of those repairing composites. They are used because sandwich panels are light, stiff structures, which can be obtained at a low weight. Often no great strength is required in some fairing panels but stiffness is needed to maintain their shape. Most of these items are also readily removable so they can be replaced with a spare part while the repair is done and no delays are incurred. Because of their light construction, it is likely that these parts will continue to be those most often in need of repair and the techniques for these have been available for a long time. Thoughts for the future

Any major structural parts that do get damaged will take a lot of time and money to repair and new skills will have to be developed as experience dictates. Repairs will have to be made because these parts are too large and costly in time and money to replace. It is certain that for major repairs using mechanical fasteners it will be the drilling of accurately aligned holes to the tolerances required that will give some problems. No hand held drills for this job! Portable jigs that can hold the drills steady will be needed and the drills themselves will need to be of the controlled feed and speed type. Reamers will almost certainly be required to achieve the tolerances needed for holes for mild interference type fasteners. They too will need locating jigs. Ready made stringer sections will be needed or techniques for making these on site. A6 Glossary of terms This document was compiled some time ago and has been updated several times. This should be available on line and you can make reference to it any time you need a definition of some word that is new to you. Inevitably you will need to learn many new words. This is always the case when learning any new activity so keep that glossary handy. Useful acronyms AC: Airworthiness Circular AD: Airworthiness Directive ADL: Allowable Damage Limit BMS: Boeing Material Specification BVID: Barely Visible Impact Damage DER: Designated Engineering Representative (of the FAA) FAA: Federal Aviation Administration FAR: Federal Aviation Regulations M&P: Materials and Process (as in M&P Engineer) MPD: Maintenance Planning Document NDI: Non-Destructive Inspection OEM: Original Equipment Manufacturer P/E: Pulse Echo ultrasonic inspection equipment PSE: Principal Structural Element RDL: Repair Damage Size Limit SRM: Structural Repair Manual

References 1. "Care and Repair of Advanced Composites", Second edition, Keith.B. Armstrong, L.Graham Bevan and William F. Cole II, Published by SAE International, 400, Commonwealth Drive, Warrendale, PA 15096-0001, USA. 2005. ISBN 0-7680-1062-4.

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Cutting, Machining and Repairing Composites of Kevlar, Du Pont External Affairs, 1111, Tatnall Street, Wilmington, DE 19898, USA.