Composite Theory & Fibre Science PDF

Composite Theory Fibre Science The selective placement of fibres to give the greatest amount of strength in various applications is known as fibre science. The strength and stiffness of a composite build-up depends on the orientation of the plies to the load direction. A sheet metal c...

Composite Theory Fibre Science The selective placement of fibres to give the greatest amount of strength in various applications is known as fibre science. The strength and stiffness of a composite build-up depends on the orientation of the plies to the load direction. A sheet metal component has the same strength no matter in which direction it is tested. For example, a helicopter rotor blade has high stress along its length because of the centripetal and centrifugal forces. If it is made of metal, the strength is the same in all directions, giving strength in directions that are not needed. If the blade is fabricated of composites, it may have the majority of fibres running its length to give more strength in the direction where the most stress is concentrated. These vectors of strength might be referred to as 0° plies to react to an axial load like that to which a rotor blade is subjected, 45° plies to react to shear vectors or 90° plies to react to side loads. For example, suppose a wing in flight bends upwards as well as twists. The part can be manufactured so that the fibres run the length of the wing and reduce the bending tendency, and then have a layer with the fibres running at 45° and at 90° to limit the twist. Each layer may have the major fibres running in a different direction. The strength of the fibres is parallel to the direction the threads run. This is how designers can customise the fibre direction for the type of stress the part might encounter. Composite Layup Composites are engineered materials composed of a matrix material (e.g. polyester or epoxy resins) and a reinforcing material (e.g. glass mat or woven fabric). The process of making a composite is termed Composite Layup, which is derived from the original method of making these materials. The following video is supplementary, however covers some terminology surrounding composite materials and the layup process. Relevant Youtube link: Prepreg Layup and Vacuum Bagging 2023-11-16 B1-07d Maintenance Practices Page 134 of 335 CASA Part Part 66 - Training Materials Only  Solid Laminate Panels Solid laminate panels are typically built using either woven cloth or unidirectional tape. Unidirectional tape yields the highest strength-to-weight ratios, but requires more labour to build. These types of laminates are best used where tensile and/or compressive strength needs are great. They are often seen in highly loaded structures such as F-18 wing skins, B-2 wing skins, etc. They tend to be more durable and damage tolerant than sandwich panels, which are discussed below. The loads these structures can take depend heavily on details of the ply orientations, especially in unidirectional tape. Woven cloth ply orientation is also important (but perhaps less so than in tape) as the majority of fibres run in the warp direction. Heavily loaded, solid laminate structures are often reinforced with stiffeners. The most common types are blade and hat-section stiffeners, named for their shapes. These may be created as an integral part of the structure or secondarily bonded in place after the solid laminate skin is built. 2023-11-16 B1-07d Maintenance Practices Page 135 of 335 CASA Part Part 66 - Training Materials Only  Composite Lay-Up Terms and Conditions Warp Clock Warp clock The warp clock can be used to reference the orientation of the warp fibres. The warp fibres are used as a reference in the process of properly aligning the fabric of a patch with the original part. For most composite repairs, the surface layer of the original part is considered the reference or 0° warp angle, which corresponds to the warp of the fabric. The structural repair manual supplies information about the ply direction of each layer of the part. Anti-Clockwise Warp Clock Anti-clockwise warp clock The anti-clockwise warp clock is drawn from the manufacturing standpoint where the plies are viewed from the inside looking towards the tool surface. Note that the 0° axis is parallel to the long axis of the rectangular panel and the +45° is located by moving anti-clockwise from 0° to 90°. 2023-11-16 B1-07d Maintenance Practices Page 136 of 335 CASA Part Part 66 - Training Materials Only  Clockwise Warp Clock Clockwise warp clock The clockwise warp clock is drawn from the repair standpoint where the plies are viewed from the outside of the structure, or the tool surface, looking in. Notice that the 0° axis is still parallel to the long axis of the rectangular panel and the +45° is located by moving clockwise from 0° to 90°. The symbol is a mirror image of the anti-clockwise warp clock. Symmetry A symmetrical laminate is one in which all of the ply orientations are symmetrical about the mid- plane of the laminate. Another way of looking at this is to describe the ply orientations of the laminate as a mirror image from the centreline of the lay-up. Symmetrical Lay-ups Symmetry helps to avoid thermal twisting during cool-down after the cure cycle. Symmetric and un-symmetric laminate 2023-11-16 B1-07d Maintenance Practices Page 137 of 335 CASA Part Part 66 - Training Materials Only  Symmetrical laminate A symmetrical laminate is one with equal numbers of + and – angled plies. This arrangement helps to avoid twisting under an applied load. Weaves Symmetrical Weaves If one has a symmetrical plain weave cloth, then the two faces are identical to each other. Therefore 0° or 90° plies are the same thing, and are called ‘zero nineties’ (0/90s). Similarly, +45° and -45° plies are the same thing and are simply called ‘forty fives’ (45s). Balance no longer applies since every 45° ply is automatically both + and -45°, and so is inherently balanced. Symmetry in terms of ply orientation sequence from the mid-plane still applies. A quasi-isotropic lay-up should have equal numbers of 0°/90° and 45° woven fabric plies, Non-symmetrical Weaves Balance and symmetry are of utmost importance to produce a laminate that will not warp after a cure cycle or twist during loading. 2023-11-16 B1-07d Maintenance Practices Page 138 of 335 CASA Part Part 66 - Training Materials Only  Unidirectional Tapes Unidirectional tapes are fibres impregnated with a resin matrix; all fibres run in the same direction. They can be made in a variety of weights and thicknesses to meet customer requirements. Composite Consumables Release Films Release films come in two forms. Perforated release film is a plastic film which has been perforated. The holes in the film allow excess resin to flow through and into a bleeder. If a non-perforated film is used, the excess resin will not flow out and will create a brittle, heavy repair or part because of the excess resin. Non-perforated release films are used when a barrier is needed between other parts of the vacuum bagging process. These films are often used under a heat blanket, but over the bleeder material. This prevents the bleeder material and resins from coming into contact with the heat blanket. Before painting a surface that was cured with a release film, the glaze should be removed by hand sanding. Peel Plies A nylon or polyester release fabric may be used next to the wet resin during the curing operation. It is used to transfer excess resin to the bleeder material without sticking to the part. After curing, the peel ply is peeled off the part and causes a slightly rough surface. This is important if the part is to be repainted. Peel plies are extremely helpful over seams or where layers of fabric overlap. They ‘feather in’ the layers and eliminate the need for sanding. Peel plies come in different finishes; some are very smooth, while others are coarser. Some may be treated with mould release, corona-treated or Teflon-coated to release better. Breather Cloth Breather cloth should not be confused with bleeder cloth. They are two different products with different uses. Breather cloth is used for allowing the vacuum source to extract as much air as possible from the repair area inside the vacuum bag. It thus allows the full atmospheric pressure to be applied to the repair. Various grades of breather cloth are available which are designed for particular applications. Some are for tight corners or small radii and are thin, while others are suitable for less stringent applications and are thicker. 2023-11-16 B1-07d Maintenance Practices Page 139 of 335 CASA Part Part 66 - Training Materials Only  Bleeder Cloth Bleeder cloth is used solely for absorbing excess resin from the repair. For repairs that use film adhesive, there is usually no requirement to absorb adhesive. When pre-impregnated (pre-preg) materials or wet lay-up repairs are specified, excess resin has to be removed to provide the correct fibre-to-resin ratio. The amount of resin that is absorbed is regulated by the number of thicknesses, or plies, of the bleeder cloth that are used. When the adhesive gets hot and begins to flow, it is easily soaked up by the bleeder cloth, so the more there is in contact with the repair, the more the adhesive will be absorbed. If there is too much absorption, the repair will be resin dry, and if not enough, the repair will be resin rich. In either case, the repair will be weaker than required, so it is important that the correct amount of bleeder cloth is used. This will be detailed in the repair procedure described in the repair authority being used. To prevent the bleeder cloth from becoming an integral part of the repair, a release film must be placed over the repair area before the bleeder cloth. Sealant Tapes Sealant tapes maintain a positive seal between the surface of the original part and the bagging films. This seal must be leak-proof to ensure maximum atmospheric pressure is held on the part. Most sealant tapes have a limited shelf life, so storage and labelling are required. If the shelf life is exceeded, the seal will not be as good and the sealant tape may not clean off of the surface easily. The tape should hold tight even if the bagging film shrinks during the curing process. The sealant tape should be able to withstand the temperatures of the cure. Pleats, or ears, are made with sealant tape to provide extra bagging film over the part if it has contours. To make the pleats, a 3- to 4-in. piece of sealant tape is cut. The middle of the sealant tape is pinched together, and the ends are attached to the sealant tape which has been placed around the part. Pleats allow for extra room in the bagging film to conform to the shape of the part and achieve a good seal. If extra vacuum bagging material is not available in some places, a bridging effect may take place. Bridging is when extra bagging film does not conform to the shape of the part, and the excess resin flows into these areas during the curing process. If enough pleats are added around the vacuum bagging area, the excess material should easily conform to the shape of the part when the vacuum is applied. 2023-11-16 B1-07d Maintenance Practices Page 140 of 335 CASA Part Part 66 - Training Materials Only  Vacuum Bagging Films Vacuum bagging films are used to cover the component and seal out any air. They must be made with absolutely no voids or pinholes of any size. If there are small holes in the film, air will leak through and less pressure will be applied to the part while curing. Bagging films come in a variety of temperature ranges. It is important to use the correct temperature rating for the required cure temperature. The vacuum bagging film should remain flexible at high-temperature cures, especially around highly contoured shapes. If the bagging film becomes brittle, it may develop air leaks which decrease the amount of atmospheric pressure to the part. Composite Solvents Only approved solvents may be used for bonded and composite repairs. The current approved solvents for bonded and composite repairs are methyl ethyl ketone (also known as butanone or ethyl methyl ketone) and Acetone. Methyl ethyl ketone (MEK) – used for removing dust, grease and mould release agents from composite components Acetone – used for general clean-up of tools and equipment and to clean composite parts after sanding as a pre-bond prep; recommended for solvent cleaning of composite surfaces as it less aggressive than MEK. Solvent Properties Solvents used for repair surface preparation are to meet the following engineering requirements. The material must: Readily dissolve surface contaminants; Have a rapid evaporation rate to prevent re-contamination of large areas; Produce no residue on the surface; Have no deleterious effect on the material being cleaned; and Be of a guaranteed high grade of purity. Chlorinated solvents must not be used on titanium alloys because stress corrosion cracking may result. 2023-11-16 B1-07d Maintenance Practices Page 141 of 335 CASA Part Part 66 - Training Materials Only  Solvent Wipe Cloths Only clean, lint-free white facial tissues or approved aircraft wipes should be used for wiping the surface with solvents. Wipe cloths must be changed frequently during solvent degreasing. Heavily soiled cloths will transfer contamination over the surface. The frequency of changing cloths must increase as the degreasing process progresses. Solvent wiping is to continue until no trace of contaminant remains on the wipe cloths. The final solvent wipes are to be performed using a fresh wipe cloth for each pass over the surface, and each wipe cloth is to be inspected for the presence of any contamination before being discarded. Any contamination detected will require repeated solvent degreasing. Precautions When Using Water Because the surface preparation process is susceptible to contamination, significant variation of water quality between repair venues dictates that water quality is to be controlled. Distilled, de- ionised water is required. In cases when it is not available, distilled water may be substituted after verification that the contaminants do not exceed the above limits. 2023-11-16 B1-07d Maintenance Practices Page 142 of 335 CASA Part Part 66 - Training Materials Only  Composite Maintenance Procedures Surface Preparation Repair Lay-Up Application Procedures Almost all adhesive bond failure in service may be attributed to incorrect application procedures. Application processes must therefore be undertaken with the utmost attention to detail. Variation from specified procedures or materials must not occur without specific approval of the Engineering Authority for the aircraft type. All application procedures are to be performed in a manner that excludes contamination from the bonding surface and bonding materials. Materials and equipment used for adhesive bonding should be used for that purpose only. Where equipment is used for other purposes, it must be thoroughly cleaned before use on bonding tasks. 2023-11-16 B1-07d Maintenance Practices Page 143 of 335 CASA Part Part 66 - Training Materials Only  Surface Exposure Times Surface exposure time is to be controlled during adhesive bonding processes. Because the surface preparation process for repair consists of several steps, there will be occasions when partially or fully prepared surfaces must be stored while other processed are undertaken. Care is required to ensure that process steps are not compromised by excessive surface exposure time, which may lead to contamination or formation of loose oxide films. To maximise surface preparation quality, exposure times between process steps are to be kept to a minimum. The maximum time allowed to elapse between steps is given in the structural repair manual (SRM). This is the maximum exposure time before the surface preparation must be repeated from the start of the first process step (i.e. solvent degreasing). The times stated are not to imply approval for repeated exposure of surfaces for those periods. All bonding processes must be performed as rapidly as possible, with breaks between steps kept to the absolute minimum. Maximum Surface Exposure Times for Work Environments Example Maximum Exposure Exposure Condition Time (Check SRM) Controlled area with air conditioned, filtered air and humidity 1 hour. control Controlled area, air conditioned with no humidity control 30 minutes Hangar floor inside a temporary controlled area with air 30 minutes conditioning and humidity control Hangar floor inside a temporary controlled area with air 15 minutes conditioning and no humidity control Open air 5 minutes All procedures for surface preparation must involve the following steps: Thorough solvent degreasing until no trace of contamination is detected on a clean wipe cloth A water break test to verify surface cleanliness Generation of a fresh chemically active surface. Metal surfaces require an extra step in the surface preparation process. They are required to be chemically modified to enhance bond durability. 2023-11-16 B1-07d Maintenance Practices Page 144 of 335 CASA Part Part 66 - Training Materials Only  Solvent Degreasing The primary purpose of solvent degreasing is to remove contamination from the surface prior to the generation of a chemically active surface. If contaminants are not removed, bond durability will be compromised. Heavily contaminated areas require a solvent scrub with Scotch-Brite™ pads prior to wiping. Wipe cloths used during the solvent degreasing process must be changed frequently during solvent degreasing. Heavily soiled cloths will transfer contamination over the surface. Solvent wiping must continue until no trace of contaminant remains on the wipe cloths. The final solvent wipes are to be performed using a fresh wipe cloth for each pass. Any contamination detected on the final wipes requires repeated solvent degreasing. Water Break Test The approved method of evaluating the surface condition of prepared metals and composites is the water break test, which assesses the surface tension of water placed on the surface. Water break testing is to be performed after solvent degreasing and prior to generation of a fresh chemically active surface, to ensure the surface is properly cleaned by solvent action. If the surface is properly clean and passes the water break test after solvent degreasing, the remaining steps only improve the wetting properties of the surface. Water break testing is to be performed by spraying, pouring or squirting distilled water onto the surface such that a complete thin film of water covers the surface. The film of water must remain intact for 30 s without any evidence of surface tension breaking the film. The surface is to be observed for further water break during drying because excessive thickness of water film may mask the presence of defective regions. Failure of this test due to water breaks is cause for rejection of the process. Solvent degreasing must be repeated before continuing with the preparation process. 2023-11-16 B1-07d Maintenance Practices Page 145 of 335 CASA Part Part 66 - Training Materials Only  Drying Procedures CAUTION: Inadequate drying prior to application of adhesives can lead to serious degradation of adhesive strength due to micro-void formation in the adhesive. Durability may also be impaired. Following water break testing and before application of adhesives, removal of surface moisture is to be performed. (Drying procedures for surface preparation are not to be confused with drying procedures for moisture removal from laminates and sandwich panels.) Inadequate drying of the surface before bonding is a major contributing factor to micro-voiding in adhesive joints. Drying is usually performed by use of hot air from either a handheld hot air gun or an external hot air blower fitted with air filters, or by placing the item in a recirculating air oven. Compressed air must not be used, as even minute amounts of oil can significantly degrade the repair’s durability. Care is required to avoid overheating the surface and reduce subsequent excessive oxide growth. After water break testing, the surface is to be heated to a temperature with reference to the repair procedure or SRM. Exposing Fresh Material Following solvent degreasing, water break testing and drying, surfaces being prepared for adhesive bonding are to be abraded by hand abrasion or grit blasting to generate a fresh, chemically active surface. Subsequent treatment must be applied as soon as possible to prevent unwanted contamination or oxide formation. The exposure time during this step is critical to the bonding process. The purpose of this phase of surface preparation is often misunderstood. Hand abrasion or grit blasting is not just a process whereby the surface is roughened to allow the adhesive to ‘key’ into it. The main purpose for abrasion is to generate a fresh, chemically active surface. The procedure must therefore be thorough and the following procedures must commence as soon as practical. 2023-11-16 B1-07d Maintenance Practices Page 146 of 335 CASA Part Part 66 - Training Materials Only  Abrasion Processes Hand Abrasion Hand abrasion is the most common form of surface generation specified in repair manuals and is also the less effective of the two methods (Hand and Grit blasting). Because abrasion involves repeated contact over the surface, contaminants are widely distributed by the process. The degree of contamination of abrasion equipment also directly influences surface quality. Because the degree of abrasion required is difficult to specify, the standard of surface preparation produced is highly dependent on the training of the operator. Removal of sanding debris from the surface after hand abrasion is to be performed by dry wiping with a clean wipe cloth. This removes some debris, but a significant amount of contaminant remains embedded in grooves and scratches produced by abrasion. Solvent degreasing to remove sanding debris after abrasion is not approved because it spreads any residual contaminants, which inhibit later surface treatments. Grit Blasting Grit blasting is the preferred procedure to generate a fresh chemically active surface. Provided the grit and propellant gas are clean, the process does not cross-contaminate the surface. Grit blasting aims to remove only sufficient material to generate a fresh surface. It provides clear visible control of the process due to the change in reflectivity of the exposed surface. Scrupulous containment and clean-up of grit material must be undertaken to prevent contamination of aircraft systems/components. The approved method of extraction of grit during grit blasting or during clean-up is by use of vacuum systems in which exhaust air is ejected away from the aircraft. Use of calico bag ‘FOD guns’, or vacuum systems which vent into the aircraft, are prohibited because fine dust escapes through the bag. Precautions are also required to prevent abrasion damage and contamination of bearings, seals and transparencies. 2023-11-16 B1-07d Maintenance Practices Page 147 of 335 CASA Part Part 66 - Training Materials Only  Surface Treatment of Metals Following solvent degreasing and generation of a fresh chemically active surface, metals require chemical modification to promote adhesion. Chemical modification procedures for adhesive bonding to metallic surfaces usually fall into two categories: Coupling agents, which attach to the surface and bond to the adhesive, effectively coupling the adhesive to the metallic surface. The preferred chemical surface treatment for most common metallic materials is the use of silane-based organo-functional coupling agents. Acidic processes, which control the thicknes of the oxide layer to produce a thin well-bonded film onto which the adhesive bonds. Primers Corrosion-inhibiting primers should be used on metallic adhered surfaces only where facilities are available for control of thickness, containment of over-spray and disposal of toxic waste products. In field repair conditions, use of primers is difficult. Surface Treatment of Composites Chemical modification is not required for composite surfaces. Irrespective of later surface preparation tasks, composite materials are to be given a thorough solvent degreasing as the first step in surface preparation. For painted composites, initial solvent degreasing should be performed before the paint is removed. This solvent degreasing must not be considered part of the surface preparation. Final thorough solvent degreasing must be performed immediately before abrasion as part of the surface preparation process for bonding. A water break test must be performed immediately after solvent degreasing. Following successful water break testing, measures are required to generate a fresh chemically active surface. Grit blasting is the preferred method for abrasion of composite surfaces. Excessive or repeated abrasion which exposes bare fibres is to be avoided. Exposure of bare fibre material removes the surface coating applied during fibre manufacture. As a result, an inferior bond to the fibre will be achieved. Surface Treatment of Honeycomb Core Surface preparation by solvent degreasing is to be performed on honeycomb core material by immersion, flushing or vapour degreasing. Core must be dried by heat lamps or recirculating air ovens (connected to an approved fume extraction system) for at least 10 min prior to bonding (Ref SRM). 2023-11-16 B1-07d Maintenance Practices Page 148 of 335 CASA Part Part 66 - Training Materials Only  Composite Lay-Up and Curing Impregnating and Laying Up Raw Fabric Prior to impregnating fabric with resin, fabricate ply templates to use in cutting the plies to shape. Then pour the liquid resin onto the fabric and work it in using a squeegee. The resin must fully permeate the fabric. Be careful not to damage the fibre orientation or fray the edges of the patch with the squeegee. Using the prepared templates, cut out the plies, observing correct fibre orientation. Once the plies are cut, lay them up in the correct sequence. As plies are laid up, use a resin roller or squeegee to remove trapped air, which will cause voiding. Apply flash breaker tape or a release film around the repair site to facilitate repair clean-up. Place a peel ply or perforated release film over the final ply. CAUTION: If the plies are being laid over honeycomb core, do not squeegee or roll between plies as the resin will be forced out of the fabric and into the core. Resin starvation will result. 2023-11-16 B1-07d Maintenance Practices Page 149 of 335 CASA Part Part 66 - Training Materials Only  Laying Up of Pre-Impregnated Fabric (Pre-preg) Ensure that the pre-preg has thawed to room temperature before opening the bag. Using pre- fabricated ply templates, cut out the plies, observing the correct fibre orientation. If required, cut a layer of film adhesive to the patch dimensions. Place the pre-cut film adhesive over the repair site. Lay up the plies in the correct sequence, ensuring that the release ply is removed as each ply is laid up. Using a heat gun, gently tack each ply in position, observing the correct ply orientation. Apply flash breaker tape or a release film around the repair site to facilitate repair clean-up. Place a peel ply or perforated release film over the final ply. Patch Lay-Up Using Film Adhesive Place pre-cut film adhesive onto a pre-cured patch. Place the patch in the correct position, observing orientation requirements. Ensure that the release ply is removed prior to patch installation. Using a heat gun, gently tack the patch in position. Apply flash breaker tape or a release film around the repair site to facilitate repair clean-up. Place a peel ply or perforated release film over the final ply. Patch Lay-Up Using Paste Adhesive Brush paste adhesive onto a pre-cured patch and onto the repair surface to ensure adequate adhesive wetting of both surfaces. Place the patch in the correct position, observing orientation requirements. Gently shimmy the patch to expel entrapped air. Secure the patch in position with flash breaker tape. Apply flash breaker tape or a release film around the repair site to facilitate repair clean-up. Place a peel ply or perforated release film over the final ply. 2023-11-16 B1-07d Maintenance Practices Page 150 of 335 CASA Part Part 66 - Training Materials Only  Curing Pressure Applications Vacuum Bagging Vacuum bagging is arguably the most popular and cost-effective method of applying pressure to a repair site on composite materials. Vacuum pressure works by using atmospheric pressure to apply even pressure over the surface of the repair. Vacuum bagging is effective with the following curing methods: Heater blankets Room-temperature curing adhesives Oven curing Autoclave curing. Vacuum bagging Shot Bags The use of shot bags or sandbags should be limited to field applications on room-temperature curing adhesives when no other means of pressure application are available. Correct and even pressure distribution cannot be ensured, and this method of pressure application should not be used when heat curing composites. 2023-11-16 B1-07d Maintenance Practices Page 151 of 335 CASA Part Part 66 - Training Materials Only  Mechanical Clamping As with shot bags, mechanical clamping should be restricted to applying pressure to room- temperature curing adhesives only. Mechanical clamping devices 2023-11-16 B1-07d Maintenance Practices Page 152 of 335 CASA Part Part 66 - Training Materials Only  Vacuum Ports The vacuum ports or, as they are also called, sniffer ports are a very useful and important piece of equipment used in vacuum bag type repairs. They are a two-part valve system that consists of a base plate, usually made from aluminium or heat-resistant plastic, and a valve section which screws into it and can be attached to a vacuum gauge or the vacuum source. The base is placed over the breather cloth but under the vacuum bag, and a suitably sized hole is made in the bag. The valve part of the sniffer port is then inserted through the hole and firmly tightened to the base. There is also a rubber seal on the base that makes an airtight seal on the vacuum bag. The valve allows the vacuum supply to be disconnected from the lay-up without the vacuum being lost from the bag. This is extremely useful for checking the bag for leaks before the cure cycle is started. When the vacuum source is removed, the vacuum gauge can be monitored to make sure the leak-down rate is within the limits specified by the repair authority. Vacuum port Vacuum Hoses Depending on whether you are curing in an oven; in an autoclave; or with heat blankets, heat guns or lamps, there are different types of hoses to be used. If the hose is enclosed in the heating environment, such as with ovens or autoclaves, the hose must be able to withstand the high heat. If your application of heat is by another method, plastic hoses can be used as long as they are equipped with an internal wire to keep them from collapsing. 2023-11-16 B1-07d Maintenance Practices Page 153 of 335 CASA Part Part 66 - Training Materials Only  Curing of the Composite Material Composite matrix systems cure by chemical reaction. There are systems which will cure at room temperature, but can be accelerated by applying external heat. Two-part epoxy resin systems, are typical of room-temperature curing systems. Some matrix systems require heat to cure the composite and achieve the maximum strength. Failure to follow the proper curing requirements, or improper usage of curing equipment, can cause defects leading to rejection of the repair. Improper curing or handling during the cure has a direct effect on the strength of the repair. During the curing process, humidity may cause a problem unless the repair is vacuum bagged. Room-Temperature Cure Some repairs may be cured at room temperature, depending on the type of resin system used. The curing process can be accelerated by applying low heat to some room-temperature resin systems. Check the applicable cure time for the specific material used. Full cure strength is usually not achieved until after 5 to 7 days. If the repair calls for a resin system that can be cured at room temperature, it is for parts used in areas where there is no exposure to high operating temperatures (usually above 71 °C). Such room-temperature cures are usually suited for composite parts used on lightly loaded or non- structural parts. Humidity may cause problems with repairs if they are not vacuum bagged. 2023-11-16 B1-07d Maintenance Practices Page 154 of 335 CASA Part Part 66 - Training Materials Only  Heat Curing The most widely accepted method of curing structural composites employs resins which cure at higher temperatures. These adhesives and resins require elevated temperatures during their cure to develop full strength and reduce the brittleness of the cured resin. Heat also reduces the curing time. Heat curing can be achieved with a temperature-controlling system in conjunction with any of the following: Autoclave Recirculating air oven Heater blankets Heat lamps. When a part is manufactured at a high temperature, the repair patches used on it may have to be cured at the manufacturing temperature to restore its original strength. Although the fibres will withstand higher temperatures than the matrix, the recommended curing temperature should not be exceeded to avoid material disintegration or further delamination of the existing structure around the repair. When a part is to be cured with heat, it is not enough to simply apply heat at the final cure temperature. The resins must be allowed enough time to flow properly before they go through their curing process. Otherwise, a resin-rich area may result. In conjunction with heat curing, there must also be some method of applying pressure to the composite. Applying pressure consolidates the resin, assists in the wetting action of the resin, reduces void content and porosity in the bond line, and helps remove volatiles during cure. This applies to room-temperature curing adhesives as well. Controlled curing of resin systems is essential to realising the full strength and durability of the resin system. It is also important to allow a repair to cool at the proper rate. Composites gain much of their cure strength in the cooling down process. A slow rate of temperature rise and a gradual cooling process are desirable, but not usually possible, unless a monitor or controller is available. This device regulates the temperature in a specific way. The ‘step cure’ and ‘ramp-and-soak’ are probably the most commonly used with composite repair. They ensure a slow rate of temperature rise and decline. 2023-11-16 B1-07d Maintenance Practices Page 155 of 335 CASA Part Part 66 - Training Materials Only  Step Curing Step curing is used when a manually operated controller is used. It requires the technician to make the adjustments manually at specific time intervals. Step curing is the process of elevating the temperature slowly by raising it at intervals, holding it for a time at each interval, until the cure temperature is reached. This allows the slow heating process which is critical in curing the composite. After the cure time has elapsed, the temperature can be stepped down by lowering the temperature the same way until room temperature is reached. This slow cool-down gives a stronger final cure to the component. Ramp-and-Soak Curing More sophisticated and accurate curing may be done with a programmable controller. A controller may be programmed in ramp-and-soak mode, which is used to heat or cool a repair at a specific rate. 2023-11-16 B1-07d Maintenance Practices Page 156 of 335 CASA Part Part 66 - Training Materials Only  Vacuum Bag Lay-Up Sequence With the repair in position, the vacuum bagging process can commence. Place the bleeder cloth over the repair site to facilitate excess resin soak-up during cure. A release ply should be placed over the bleeder to prevent the breather cloth from soaking up resin. Heater blankets can be installed during this process if this mode of cure is to be utilised. Place a layer of breather over the release film. The breather ensures that vacuum draw over the repair site will be even. Surround the repair with vacuum bag sealant tape and install the thermocouples and base plates for vacuum gauge and suction where required. Remove the white release paper from the vacuum bag sealant tape and place the vacuum bag film over the repair. With the vacuum bagging film in place, the vacuum suction and gauge ports can be installed. Connect the vacuum pump or transducer, then connect the gauge and check the bag for leaks. Ensure that the vacuum being drawn is at least 25 inHg as indicated by the gauge. NOTE: A vacuum leak check is done to make sure that the bag has sealed. Typical vacuum bagging assembly 2023-11-16 B1-07d Maintenance Practices Page 157 of 335 CASA Part Part 66 - Training Materials Only  Composite Heat Curing Equipment Heat Lamps The use of heat lamps to cure composite parts is not recommended. The temperature cannot be accurately controlled, and the heat may localise in one spot. Scorching or blistering of the part may occur if the heat lamp is too close or is left on too long. Heat lamps generate high surface temperatures, which tend to cure a repair too rapidly. Heat lamps 2023-11-16 B1-07d Maintenance Practices Page 158 of 335 CASA Part Part 66 - Training Materials Only  Heat Guns When a heat gun is used to cure a composite part, it must be controlled with a monitor. A typical heat gun can generate very high temperatures when it is left on constantly. If the cure temperature is 180 °C and a heat gun is used to cure the component, the heat gun should be monitored with a controller to maintain a constant temperature. Heat guns 2023-11-16 B1-07d Maintenance Practices Page 159 of 335 CASA Part Part 66 - Training Materials Only  Heat Blankets Heat blankets are probably the most widely accepted method of applying heat to a composite component for repair work. They uniformly heat the repair area without heating a larger area than necessary. They are used with a controller or hot patch bonding machine, which means the accuracy of the cure is higher. They can be used with vacuum bagging to hold the heat directly onto the surface. Heat blankets are made of a flexible silicon and come in a variety of forms and sizes. Heating coils within the blanket are powered by a controller regulating unit. A thermocouple is used with the blanket to monitor the heat and to control the temperature. The ramp-and-soak method of heating is easily accomplished with the heat blanket method and results in a stronger cure. The heat blanket must cover the repair completely and usually is an inch or two larger than the largest size patch. However, if the heat blanket is too large, the heat may damage surrounding areas of the part. Heat blankets 2023-11-16 B1-07d Maintenance Practices Page 160 of 335 CASA Part Part 66 - Training Materials Only  Oven Curing Ovens offer controlled, uniform temperature over all surfaces. Some ovens have vacuum ports installed to provide vacuum pressure while curing. Oven curing is frequently used by manufacturers. When using an oven for repair work, the part must be removed from the aircraft and must be small enough to fit into the oven. Ovens may also present a problem by heating up the whole part, not just the repair area. The areas which are not being repaired are subjected to very high temperatures and may deteriorate the existing bond. Ovens which are used to cure composites must be certified for that purpose. Curing oven 2023-11-16 B1-07d Maintenance Practices Page 161 of 335 CASA Part Part 66 - Training Materials Only  Autoclaves Autoclaves are used in the manufacture and repair of composite structures and components. In this case, the part is vacuum bagged and heated to the curing temperature at a controlled rate while additional pressure is applied within the autoclave. If the damage is large and extensive enough, the part may be sent to a remanufacturing facility. Large manufacturing facilities have the moulds and capabilities to repair large damaged surfaces. If an extensively damaged component is not cured with the moulds and high heat and pressure, the part may not regain its original strength. Autoclave 2023-11-16 B1-07d Maintenance Practices Page 162 of 335 CASA Part Part 66 - Training Materials Only  Hot Patch Bonding Equipment Hot patch bonding equipment Simply stated, a hot patch bonding machine performs two functions: It applies atmospheric pressure by means of a vacuum pump. It applies heat, usually in the form of a heat blanket. Hot patch bonding uses heat blankets which have electrical coils bonded into a rubber pad or blanket. The heat blankets can heat up quickly unless they have a monitoring unit to control the rate of temperature rise and to set the temperature. When working with composites, the temperature must be controlled at a constant (Set Point) and specific rate of change (Ramp up and ramp down). It is critical to perform these functions with a minimum of effort and maximum of efficiency in order to achieve effective results. The controller allows the temperature to slowly rise at a specific rate, holds the temperature constant, and then allows a slow decline of temperature at a specific rate. Recording the temperatures of the curing process can be accomplished using hot bonding units equipped with a temperature recording unit. Such controllers may have an included recorder or use a separate recording unit. All aircraft repairs require that permanent records of the cure cycle be included in the aircraft log book. To use a controller, a thermocouple is placed beside the repaired area under a heat blanket and under the bagging film to sense what temperature is being delivered to the part. The thermocouple sends the temperature information to the controller. The controller adds heat or stops heating depending on how it is set. 2023-11-16 B1-07d Maintenance Practices Page 163 of 335 CASA Part Part 66 - Training Materials Only  Initially applying heat at the final cure temperature will not allow the resins enough time to flow properly before they go through their curing process. The resin and catalyst mixtures need time to slowly start their chemical reaction before the final cure temperature is reached. NOTE: It is also important not to turn off the heat and allow the part to cool too quickly. Composites gain much of their strength during the cool-down process, which also prevents the part from becoming brittle. A slow rate of temperature rise and decline is desirable, but can usually be achieved only if a monitor or controller is available. Thermocouples Thermocouples Correct placement of thermocouples is one of the most important factors to be considered when beginning the bagging-up sequence. During the cure cycle, it is essential for the operator to have accurate temperature monitoring. This means we need to know exactly what temperature the repair is at, any time throughout the cure. This enables us to adjust the heat course and keep our temperature within the specified limits. To do this effectively, we need to have more than one thermocouple available. The more we have on the repair area, the more accurately we can monitor the temperature. Before using any thermocouple wires, always ensure they are in good condition. The success of the repair depends on them working correctly. The actual placement of the thermocouples of course depends on the physical configuration of the panel being repaired. Often large repairs have heat-sink areas, such as attachment points, nearby. This must be considered when placing thermocouples. We need to know the exact temperature at any part of the repair during the entire cure cycle, and this can only be done with reliable equipment, which relies on the thermocouples giving accurate temperature readings. 2023-11-16 B1-07d Maintenance Practices Page 164 of 335 CASA Part Part 66 - Training Materials Only  Composite Repair Environment Handling and Storage of Materials The environment in which composite and adhesive materials are exposed in an uncured state is to be controlled within the limits detailed below. Adhesives and composite materials absorb moisture from the atmosphere. This moisture may result in voids in bonds and laminates, and lower material properties. Composites and adhesives are also susceptible to contamination by dust and airborne particles, which may lead to voids or delamination. Composites and adhesives must always be handled while wearing clean disposable gloves to minimise contamination by oils from the skin. Gloves also prevent personal exposure to resins and adhesives, which may cause skin irritation. All materials associated with adhesive bonding and all composite materials are to be stored in such a manner as to prevent contact with contamination sources in particular oils, greases and waxes. Storage methods must also prevent cross-contamination of materials stored in the same location. Quality Measures During Repair Quality assurance requires compliance with the following basic requirements: The bond integrity achieved during application must be controlled by certification of compliance with approved procedures. The assurance of adhesive cure is provided by reference to hard-copy records of temperature and cure times. The performance of repairs is limited to appropriately qualified personnel. Facilities The construction, layout and management of a repair facility have a direct and significant influence on the integrity of composite and adhesive bonds. These factors must minimise contamination and satisfy the requirements of occupational health and safety. 2023-11-16 B1-07d Maintenance Practices Page 165 of 335 CASA Part Part 66 - Training Materials Only  Repair Environment Most adhesives and resins, absorb atmospheric moisture, which may cause void formation during elevated temperature cure. Moisture also degrades elevated temperature properties of adhesives and resin systems. Wherever possible, repairs should be performed in a controlled area to reduce contamination and prevent degradation of adhesives, resins and prepared surfaces. The atmosphere in the controlled area must have regulated temperature and humidity within the limits prescribed above. Measurements to verify compliance are to be made at the location of the repair and/or where adhesives are exposed. Measurements of humidity and temperature are to be taken before commencement of work and monitored during repair application. Temperature Control For humidity and temperature control limits for working with adhesives and uncured composite materials, refer to the graph above. Any controlled area where adhesives or prepared surfaces are exposed should be kept at a pressure above all surrounding areas by use of a continuously operating air handling system. Repair activities should be restricted to times when atmospheric conditions fall within prescribed limits. Application of repairs or exposure of adhesives or uncured composites under conditions outside those prescribed will compromise the integrity and durability of the repair. 2023-11-16 B1-07d Maintenance Practices Page 166 of 335 CASA Part Part 66 - Training Materials Only  Controlled Areas The functions of adhesive bonding and composite fabrication must be performed in a controlled area consisting of a Preparation Room and a Bonding Room. Access should be restricted to essential personnel only. The preparation area is not to be used as a thoroughfare to non-bonding activities. Preparation Room A totally enclosed controlled environment, separated from the open workspace, preferably by an airlock, must be provided for surface preparation. This area must be separate from the Bonding Room. Given the sensitivity of these processes to contamination, the Preparation Room must be reserved exclusively for adhesive bonding purposes. No other functions are permitted in the room, including composite fabrication and adhesive mixing. Bonding Room A totally enclosed controlled environment, separated from the preparation area, preferably by an airlock, must be provided for composite fabrication and adhesive application. Storage Areas Provision should be made for storage of composite and adhesive bonding materials and components awaiting repair. Store the following in flame-proof approved cabinets when not in use: resins hardeners catalysts accelerators solvents. Store resins, solvents, catalysts and accelerators in original containers or in clearly labelled containers so they will not be mistaken for something else. Resin systems give off harmful fumes and are hygroscopic; ensure lids are securely fitted when not in use. Resin systems and curing agents must be stored in cool, dry conditions. Do not inter-mix resin and hardener batch/lot numbers, and do not use out-of-life resin systems. Let resin systems come to room temperature before opening containers. Adhesives and pre-preg composite materials must be stored in cold storage. This may require refrigeration to -18 °C or lower temperatures. Frost-free freezers are required. 2023-11-16 B1-07d Maintenance Practices Page 167 of 335 CASA Part Part 66 - Training Materials Only  Because most composite materials are subject to storage/shelf-life limits, monitoring is essential to guarantee that materials used in composite repairs perform to specifications. Recording criteria should include Batch Identification, Vendor/Supplier Information, Storage Life, Time In and Out of Storage, and Recertification Information. In some instances, storage life of products may vary markedly from that recommended by the manufacturer. Rolls are to be stored on racks supported by the central tube through the product. Storage on-end or lying flat should not be permitted. Suitable precautions are required to prevent loose fibres and off- cuts from contaminating other products stored in the region. Fibre material storage racks If the storage area is not humidity controlled, fabrics must be oven dried at a specified temperature before use. Storage and handling of bulk honeycomb materials require care, and are to be stored in sealed plastic bagging material, within a clean environment. Storage is to be such that crushing of the cells or edges of the core block does not occur. Non-metallic core materials, such as Nomex, paper or fibreglass cores, must be stored in a region with humidity control to prevent moisture absorption and subsequent dimensional changes. 2023-11-16 B1-07d Maintenance Practices Page 168 of 335 CASA Part Part 66 - Training Materials Only  Moisture in Sandwich Cores Moisture that has entered the cells of honeycomb or foam cores in damaged sandwich composites is quite a problem. Expansion and contraction of the moisture during freeze/thaw cycles in flight can break down the cells and also dis-bond a facing from its core. The presence of moisture can prevent adhesion of the core to a repaired or new facing. Moisture present during a hot-cure repair will turn to steam, and if it is trapped, the resulting pressure may prevent bonding of the repair. It may also dis-bond facings that were previously soundly bonded. 2023-11-16 B1-07d Maintenance Practices Page 169 of 335 CASA Part Part 66 - Training Materials Only  Machining Composites Machining Definition Machining is the drilling, cutting, sanding or grinding of material. Composite materials act differently than traditional aluminium when machined. Each different type of fabric machines differently than other types of fabrics. Cutting Uncured Fabrics Before a fibreglass or carbon/graphite fabric is combined with a matrix and cured, it can be cut with conventional fabric scissors. Aramid fabric in its raw state is more difficult to cut. Special steel-blade scissors with serrated edges are used to cut through aramid. Ceramic-blade scissors with serrated edges will also cut through aramid with ease and last many times longer. These types of scissors are used because the serrated edges hold the fabric while cutting without fraying the fabric edges. NOTE: A conventional pair of scissors will separate the weave, but will not cut the fabric unless they are very sharp. The scissors’ edges quickly become dull and need to be sharpened frequently. Different fabrics tend to dull the cutting surface in different ways, so sometimes engineers have specific scissors for specific materials. Pre-impregnated materials can be cut with a razor blade in a utility knife following a template or straight edge. The resin tends to hold the pre-impregnated fibres in place while the razor edge cuts through the fibre. Very sharp, defect-free cutting edges are necessary to work with composite fabrics. Reserve your scissors and tools for specific materials and they will last longer. Machining Cured Composites Because of the high strength of cured composites, different machining tools and techniques are used than for metal structures. Air tools used for cutting, trimming, drilling and finishing composites should be rear-exhausting to prevent contamination of surfaces. Many fibres absorb cutting fluids if they are used. If the wrong type of cutting fluid is used, the area may not bond properly. Water is typically the only accepted cutting fluid. Machining characteristics of fibre-reinforced plastics or composites vary with the type of reinforcement fibre being used. Again, if a cutting tool is used with aramid, it should be used only on aramid. If it is used on a carbon/graphite or fibreglass structure, it dulls the cutting surface differently than the aramid does, and will never again cut the aramid in the manner it should. 2023-11-16 B1-07d Maintenance Practices Page 170 of 335 CASA Part Part 66 - Training Materials Only  Drilling and Countersinking Countersink drill Drill stop fixture The production holes in composite materials present different problems from those encountered in drilling metal. Composites are more susceptible than metal to material failures when machined, making hole quality important. Proper selection and application of cutting tools can produce structurally sound holes. Delamination, fracture, breakout and separation are types of failures that may occur while drilling composites. Delamination most often occurs as the peeling away of the bottom layer when the force of the drill pushes the layers apart rather than cutting through the last piece. A fracture occurs when a crack forms along one of the layers due to the force of the drill. Breakout occurs when the bottom layer splinters as the drill completes the hole. Separation occurs when a gap opens between layers as the drill passes through the successive layers. 2023-11-16 B1-07d Maintenance Practices Page 171 of 335 CASA Part Part 66 - Training Materials Only  Whenever possible, the material being drilled should be backed with wood to prevent these problems. Wherever possible, use the correct style of drill for the composite being drilled to prevent the initiation of defects. High-speed drills can be used; however, shortened drill life can lead to rejectable conditions such as delaminations and fibre pull-out. Carbide drill bits work on all types of composites and have a longer life than a standard steel drill. Diamond dust-charged cutters perform well on fibreglass and carbon, but they produce excessive fuzzing around the cut if used on an aramid component and for this reason should be avoided. Drilling Aramid Aramid (Brad) drill Machining and drilling materials reinforced with aramid or Kevlar™ fibres requires different tools from those made of fibreglass or carbon/graphite fibres. The physical properties of aramid fibres are unique. When a conventional sheet metal drill bit is used on aramid, the fabric tends to fuzz around the drilled holes. Because of the flexibility of the aramid fibre, the drill will pull a fibre to the point of breaking instead of cutting it. As each fibre is pulled before it is cut, a fuzzing appearance is produced around the edge of the drilled hole. A brad point bit is available which is designed specifically for aramid fabric. This is used to produce a clean, accurate, fuzz free hole. 2023-11-16 B1-07d Maintenance Practices Page 172 of 335 CASA Part Part 66 - Training Materials Only  Drilling Fibreglass or Carbon Drilling fibreglass or carbon can be accomplished with most conventional tools; however, the abrasiveness of these composite materials reduces the quality of the cutting edge and shortens the life of the drill drastically. Carbide, diamond-charged or carbide-coated tools are used to obtain better results and longer tool life. Diamond-charged tools are usually steel drills which have a coat of diamond dust to cut through the material. This type of drill works well on carbon and fibreglass components. For fibreglass or carbon drilling, a dagger or spade bit can be used to reduce the tendency of the fibres to break rather than be cut. This type of bit has a single cutting edge. The best results for drilling and countersinking carbon materials are obtained when using a carbide dagger bit. Uni-drills can be used to drill and ream carbon and fibreglass, but they do not produce an acceptable hole in aramid materials. Uni-drills fuzz the aramid fibres excessively. Dagger or spade drill bits Countersinking Composites Countersink for composites 2023-11-16 B1-07d Maintenance Practices Page 173 of 335 CASA Part Part 66 - Training Materials Only  Countersinking fastener holes is as important in composites as it is in metal parts. A countersunk hole should be produced to the proper fastener angle, proper depth and proper finish. Always use a pilot with cutters and ensure that the pilot has been relieved. This ensures a firm seat in the cutter. Countersink cutter geometry 2023-11-16 B1-07d Maintenance Practices Page 174 of 335 CASA Part Part 66 - Training Materials Only  Composite Maintenance Practices and Equipment Fasteners A common type of fastener used in composite structures is the removable fastener, such as those used around door edges. Probably the most common damage to these areas is from wear around the edges of the hole, causing the part to improperly transfer the structural load. Fasteners may pull out of the edges of the composite if placed too close to the edge. Therefore, it is important to locate fasteners well enough inside an edge to prevent excessive wear or pull-out. The direction of the fabric weave should also be considered to prevent the fastener from pulling out. Many times, the material around a drilled hole is sealed to prevent the fabric from wicking moisture. One way to seal a hole is to use an insert that is coated with resin. The resin, in combination with the insert, permanently seals the hole against moisture. Special composite fasteners can be used which are very similar to a Hi-Lock. These fasteners are used where a blind fastener is needed. Some of these fasteners have a very small bearing surface with the composite part. This may allow the fastener to puncture through a thin face sheet if too much pressure is imposed on it. If possible, it is best to use a composite fastener that has a large bearing area. These fasteners must be ordered in the proper diameter and length if they are to work properly. Sanding Sanding is used widely during repair operations to remove single layers of fabric at a time. Do not use aluminium oxide for sanding carbon fibres. Small particles of aluminium from the sandpaper may become lodged in the fibres. Silicon carbide or carbide should be used to prevent deterioration due to electrolytic action. Hand sanding is done when only one layer, or a very thin coat of paint, is to be removed. Because the fabric layers of composites are very thin, they may sand off very quickly. Be careful not to sand through too many layers of fabric. If sanding a layer of paint off, do not sand into the top layer of fabric, or an additional repair may have to be performed. Wet sanding is preferred, using a fine-grit sandpaper of about 240 grit. 2023-11-16 B1-07d Maintenance Practices Page 175 of 335 CASA Part Part 66 - Training Materials Only  Mechanical Sanding A right-angle sander is used for scarfing and step-cutting the repair. Some sanders blow exhaust air out the front, while others vent the exhaust out the back. Rear-exhausting tools are preferable due to a lower risk of contaminating surfaces with moisture and oil. If sanding carbon, the dust should not be allowed to blow away as it may lodge on an aluminium surface, causing an electrolytic action. If a dust collector or downdraft table is available, it should be used. The dust should be vacuumed up and disposed of properly. If you are repairing composites frequently, a right-angle sander will help you do a professional job. Standard composite safety procedures call for a dust respirator to be worn when sanding. Each material sands differently, and various techniques should be used as appropriate. When sanding aramid, expect the material to fuzz. When the sanding is almost through the layer, a lighter colour of fuzz will be visible and spots of gloss area may appear. During the sanding process, it is important to look carefully for a gloss area. When an area begins to gloss, one layer of laminate has been removed and the sander is just above the following layer. Carbon material produces a very fine powder when it is sanded. It is usually easier to see the layers of carbon than those of aramid. Another way to tell if sanding through one layer has been completed is to look at the weave. Since most composites are made with each layer’s weave in different directions, a change in weave direction may indicate a new layer. Trimming Cured Laminates Standard machining equipment can be used to trim composites; however, some modifications to the tooling may be necessary. All cutting surfaces should be carbide-coated whenever possible. Diamond- edged blades work well on carbon and fibreglass. 2023-11-16 B1-07d Maintenance Practices Page 176 of 335 CASA Part Part 66 - Training Materials Only  Routers Routers They are used to trim composite laminates and to rout out damaged core material. For routing honeycomb, carbon or fibreglass laminates, a carbide blade ‘diamond-cut’ router bit works best. In order to rout out damaged core material, a circular or oval area of the top laminate skin over the damaged core must first be routed using a pointed router bit. If the damage penetrates one skin and the core, care should be taken not to rout into the opposite laminate. Some repairs require only a portion of the honeycomb to be removed in the repair area. In this case, adjust the depth of the bit accordingly. If the damage penetrates both skins as well as the core, the router depth can be set to completely remove the damaged area. Hole Saws Hole saw 2023-11-16 B1-07d Maintenance Practices Page 177 of 335 CASA Part Part 66 - Training Materials Only  Holes may also be cut by the use of a hole saw, but the saw will tend to tear out the honeycomb core. The teeth of a hole saw usually do not cut through the fibres of an aramid composite, but fray the edges. Because of this, hole saws are not recommended on aramid laminates. For carbon/graphite cutting, the saw may be fitted with a blade that has diamond dust on the cutting edges, which tends to produce a cleaner cut. Roto–bores effectively cut composite materials because they use peripheral cutting. Only the outside edge of the hole is cut, greatly reducing friction and controlling diameter. Rotor-bore Bandsaws Bandsaw blade A bandsaw may be used if the blade has a fine tooth, 12 to 14 teeth per inch. Blades made for composite sawing are available in carbide or with diamond dust on the cutting edges for use when cutting carbon material. Bandsaw cutting produces some fuzzing on aramid, but this can be cleaned up by hand-sanding the edges. Blades with an alternating tooth pattern and wave set should be used when cutting aramid. 2023-11-16 B1-07d Maintenance Practices Page 178 of 335 CASA Part Part 66 - Training Materials Only  Counterbores When a larger hole is needed, counterbores can be used with all types of composite materials except aramid, as this may create excessive fuzzing. Safety Equipment Dust masks or respirators with fine filters are required when sanding, drilling or trimming some composite structures. A dust collector or downdraft table is desirable to use while sanding to pull the fine particles out of the air. These alone should not be relied on; use them in conjunction with dust masks or respirators. Depending on the types of resin systems you use, some may require respirators when working with them. A mixing booth may be desirable but not practical in the field as many repair parts will not fit under them to accomplish the work. Eye protection is necessary when working with chemicals or power tools. 2023-11-16 B1-07d Maintenance Practices Page 179 of 335 CASA Part Part 66 - Training Materials Only  Quality Assurance Methods Available Repair Quality Assurance Can the quality of the repair be inspected? How do we know, for example, that a bonded repair will not fall off tomorrow, next week or next year? This is a difficult problem. There are currently no methods of establishing the strength and durability of a given composite repair without destroying the repair. There are, however, methods of establishing that the cured resin matrix should perform as designed. These are: Lap shear testing – establishes whether a given resin system has fully cured Barcol hardness testing – establishes whether a resin matrix has achieved full strength. Following repair, the adhesive layer is to be inspected for voids, dis-bonding, cracking in the adhesive, poor adhesive flow and porosity. A thorough visual inspection will detect many defects before resorting to more sophisticated methods. Voiding and Dis-Bonding Large voids and dis-bonds may be detected by tap hammer inspection. If these flaws are suspected but cannot be detected by tap hammer inspection, then ultrasonic inspection (Pulse-Echo) should be used. Voiding and dis-bonding are not cause for immediate repair rejection. If the voiding or dis-bonding is within pre-established limits, the repair is considered acceptable. Cracking Cracking in the adhesive bond is usually caused by thermal shock due to excessive heat or rapid cooling. Cracking is cause for immediate repair rejection. Adhesive Flow Inspection of the adhesive flash produced during the cure is to be undertaken to detect the absence of adhesive flow. Poor flow can be attributed to inadequate pressurisation, too slow a heat-up rate or an adhesive which is out of useable life. This condition causes poor bond durability and the repair must be rejected. 2023-11-16 B1-07d Maintenance Practices Page 180 of 335 CASA Part Part 66 - Training Materials Only  Porosity Porosity is difficult to detect using tap tests and ultrasonics. The presence of a large number of small bubbles in the adhesive fillet line indicates a porous bond. Porosity is caused by too rapid a heat-up rate, incorrect pressurisation, or moisture contamination of the adhesive or surfaces. Bond durability is adversely affected and any repair with this condition should be rejected. Manufacturers’ Repair Instructions In military or type-certificated civilian aircraft, repairs must be performed in accordance with approved repair instructions issued by the aircraft manufacturer or military organisation responsible. In the airline world, these instructions are located in SRMs. Classification Criteria Typically, these repair instructions specify limits on the size and location of ‘repairable’ damage. The limits vary considerably from part to part, depending on such things as location of the damage, size and severity of the damage, adjacent damage, part construction details, criticality of the structure, ease of subsequent inspection, temperature requirements, etc. Engineered Repairs/Instructions If the damage falls outside the allowable repair criteria, then either the part must be replaced or a specific repair must be designed for that particular component and damage level by an engineer specifically trained and qualified to design composite repairs. This type of engineered repair design is very often done with the close cooperation of the aircraft manufacturer, or may be done by engineers from the manufacturer. Composite Process Defects Defects in composite materials generally manifest themselves as resin defects, finishing process damage and lay-up defects. Their causes can be generally attributed to the following: Inadequate or poor surface preparation Non-observance of procedures Use of time-expired materials Inadequate storage and incorrect handling of composite materials Lack of, or inadequate, training Contamination of materials Lack of control over the process Inadequate quality control over written procedures. 2023-11-16 B1-07d Maintenance Practices Page 181 of 335 CASA Part Part 66 - Training Materials Only  Resin Defects The following are examples of resin defects: Bond line too thick Bond line too thin Resin is rubbery Porosity Voiding Thermal stress Cracking Crazing. Resin defects 2023-11-16 B1-07d Maintenance Practices Page 182 of 335 CASA Part Part 66 - Training Materials Only  Finishing Process Damage The following are examples of finishing process damage: Incorrect sanding materials Incorrect use of sanding materials Failure to remove lay-up materials Inadequate or poor surface preparation Surface contamination Incorrect handling of composite materials. Lay-Up Defects The following are examples of lay-up defects: Resin starvation Resin richness Warped patch Crushed core. 2023-11-16 B1-07d Maintenance Practices Page 183 of 335 CASA Part Part 66 - Training Materials Only  Composite Painting Painting the Composite Part After manufacture, the part is painted to seal the surface from moisture and for cosmetic purposes. For most aircraft, the same type of paint that is used for the metal portions of the aircraft is suitable for use on the composites. Some companies, such as Boeing, use a layer of Tedlar on the composite before painting. Tedlar is a plastic coating which serves as an additional moisture barrier. Paints The new-generation paints are used on composites just as on aluminium parts. The flexibility and wear resistance of the paint will not deteriorate like some gel coats. The component is primed and painted in the same manner as aluminium. 2023-11-16 B1-07d Maintenance Practices Page 184 of 335 CASA Part Part 66 - Training Materials Only  Protective Treatments Protective treatments offer protection from: Ultraviolet (UV) rays Heat soakage Erosion Lightning. UV Protection UV radiation degrades resins. This can be greatly reduced by special blocking additives in the liquid resin, or more usually by a special UV-blocking dark paint applied after cure. Reducing Heat Soakage White or light-coloured paint finishes on fibreglass-reinforced plastics lessen temperature rise through heat soakage from the sun or high ambient temperatures. The top surfaces of aircraft, in particular, should be white. It also helps if the aircraft is kept out of the sun while it is on the ground. Combating Erosion A well applied and maintained polyurethane paint provides good primary protection from erosion. It should be white or light-coloured to reduce temperature rise as discussed in the last section. Polyurethane paint is not waterproof, so it must have the right waterproofing undercoat. Ultraviolet Ultraviolet (UV) light, primarily from sunlight, slowly attacks cured epoxy and causes the resin matrix to become more brittle and prone to cracking over time. In addition, aramid fibres are subject to weakening with prolonged UV exposure. One can easily protect composites from UV by using an opaque paint with a UV barrier. Protection from ultraviolet light 2023-11-16 B1-07d Maintenance Practices Page 185 of 335 CASA Part Part 66 - Training Materials Only  Infrared Infrared light is heat radiation and is a particular problem with the bottoms of wings and tail surfaces while the aircraft are parked on a black asphalt ramp on a hot summer day. These surfaces receive direct infrared radiation from the ramp. One can protect components from this condition by painting them white, which will reflect away most of the infrared radiation. Moisture Moisture and aircraft fluids are the enemy of composite materials. Protection must be maintained throughout the service life of composite materials. Protection can be provided by: Applying bondable Tedlar to the internal faces of panels during manufacture to create a moisture barrier Maintaining the integrity of sealing media on composites, i.e. faying surface sealants, edge member closures, etc. Maintaining the integrity of surface finishes. Polyurethane paints provide superior erosion protection compared to other surface finishes. However, polyurethane is not waterproof, so a suitable waterproof undercoat must be applied. 2023-11-16 B1-07d Maintenance Practices Page 186 of 335 CASA Part Part 66 - Training Materials Only  Composite Protection Lightning Protection Aircraft require electrical contact between all metallic and composite parts to prevent arcing or fibre damage. Aluminium provides a conductive path for dissipation of electrical energy. The aluminium may be provided in a number of ways, depending on the manufacture of the aircraft. Whether an aircraft is aluminium or composite, when lightning hits, it needs a path for the electricity to flow through. On an aluminium skin, the electricity will flow through the skin and discharge out the static wicks. Since composites do not conduct electricity, lightning protection has to be built into the component. If there is no lightning protection in the composite and the lightning exits through the composite component, the resins in the composite will evaporate, leaving bare cloth. Carbon composite was at first believed to conduct enough electricity to dissipate the electrical charge, but this was later found false. Aluminium lightning protection may be found in carbon parts. A barrier, such as a layer of fibreglass, should be used to prevent a galvanic potential between the carbon and aluminium. 2023-11-16 B1-07d Maintenance Practices Page 187 of 335 CASA Part Part 66 - Training Materials Only  Electrical Bonding Different manufacturers use different methods to dissipate the electrical charge on composite structures. These are a few different methods: Aluminium wires may be woven into the top layer of composite fabric. This is usually done with fibreglass or Kevlar™ and not with carbon/graphite. A fine aluminium screen may be laminated under the top layer of fabric. If this method is used on a carbon/graphite component, it is usually sandwiched between two layers of fibreglass to prevent a galvanic potential. A thin aluminium foil sheet may be bonded to the outer layer of composite during the manufacturing process. Aluminium may be flame-sprayed onto the component. This is molten aluminium that is sprayed on like paint. Some companies simply paint the component with an aluminised paint. In some structures, a piece of metal is bonded to the composite to allow dissipation of the electrical charge out to another metal component or static wick. Using conductive adhesives when bonding is required from composites to bonding leads. After repairing any of the composite lightning protection methods, engineers must carry out electrical continuity checks to verify conductivity between the repair and the surrounding mesh/wires, etc. 2023-11-16 B1-07d Maintenance Practices Page 188 of 335 CASA Part Part 66 - Training Materials Only