Increase the Quality and Durability of Your Next Composite Repair * * This reprint is based on an article by Michael J. Hoke, President, Abaris Training, Reno, NV and Griffin, GA, that appeared in the Huntsman Advanced Materials, FLIGHTLINE Newsletter, Volume 6, Number 1. Wipe abraded surfaces with a clean, lint-free cloth and reagent-grade solvent. With the growing use of composite components on a broad range of aircraft today, maintenance technicians must be thoroughly skilled in composite repair procedures. Complicating this requirement is the rapid proliferation of advanced materials used in manufacturing and repairing aircraft. Among the key parameters that affect the quality and durability of a composite repair are resin selection, damage removal and surface preparation, and fabricating methods and materials. The following guidelines have been developed to assist technicians in wet lay-up repair of laminated aircraft parts. Resin Selection In preparation for repairing a composite component, select an appropriate resin system from the list of approved products in the structural repair manual (SRM). In evaluating specific resins, technicians should review the physical properties of the potential systems along with the handling and performance requirements of the project. Primary factors in material selection include: • Size of the repair • Work life of the resin system • Flow of the resin system Damage Removal and Surface Preparation When delamination is located on a part, carefully rout out and/or sand the area until the damage is completely removed. Then clean surfaces thoroughly. During these processes, remember the following: The freshness of the abraded and cleaned surface directly affects the durability of the repair. If damage is removed and the part is covered for repair the next day, the patch will not be as strong as a patch installed immediately after the part was abraded and cleaned. For thorough cleaning of abraded areas, wipe surfaces using a clean, lint-free cloth and a reagent-grade solvent. Conventional solvents, even when dispensed from a new can, can contain traces of oil that will contaminate rather than clean the area. Laminate Patch Lay-up To prevent stretching and fraying of fabric and facilitate application of successive plies on the repair area, a preferred method for patch preparation is pre- impregnating cloth with mixed laminating resin as follows: • Cut templates to size for each layer of the repair. • Place a section of cloth large enough for the full repair on top of a piece of clean plastic. Wet dry fabric with mixed resin. After putting down a layer of clean plastic, spread resin uniformly across the cloth surface using a squeegee.
Transcript
Increase the Quality and Durability of Your Next Composite Repair*
* This reprint is based on an article by Michael J. Hoke, President, Abaris Training, Reno, NV and Griffin, GA, that appeared in the Huntsman Advanced Materials, FLIGHTLINE Newsletter, Volume 6, Number 1.
Wipe abraded surfaces with a clean, lint-free cloth and reagent-grade solvent.
With the growing use of composite components on a broad range of aircraft today, maintenance technicians must be thoroughly skilled in composite repair procedures. Complicating this requirement is the rapid proliferation of advanced materials used in manufacturing and repairing aircraft.
Among the key parameters that affect the quality and durability of a composite repair are resin selection, damage removal and surface preparation, and fabricating methods and materials. The following guidelines have been developed to assist technicians in wet lay-up repair of laminated aircraft parts.
Resin SelectionIn preparation for repairing a composite component, select an appropriate resin system from the list of approved products in the structural repair manual (SRM). In evaluating specific resins, technicians should review the physical properties of the potential systems along with the handling and performance requirements of the project. Primary factors in material selection include: • Size of the repair • Work life of the resin system • Flow of the resin system
Damage Removal and Surface Preparation When delamination is located on a part, carefully rout out and/or sand the area until the damage is completely removed. Then clean surfaces thoroughly. During these processes, remember the following:
The freshness of the abraded and cleaned surface directly affects the durability of the repair. If damage is removed and the part is covered for repair the next day, the patch will not be as strong as a patch installed immediately after the part was abraded and cleaned.
For thorough cleaning of abraded areas, wipe surfaces using a clean, lint-free cloth and a reagent-grade solvent. Conventional solvents, even when dispensed from a new can, can contain traces of oil that will contaminate rather than clean the area.
Laminate Patch Lay-upTo prevent stretching and fraying of fabric and facilitate application of successive plies on the repair area, a preferred method for patch preparation is pre-impregnating cloth with mixed laminating resin as follows: • Cut templates to size for each layer of the repair. • Place a section of cloth large enough for the full repair on top of a piece of clean plastic.Wet dry fabric with mixed resin.
After putting down a layer of clean plastic, spread resin uniformly across the cloth surface using a squeegee.
• Pour mixed resin onto the cloth and cover with a second piece of plastic. • Spread resin uniformly over the fabric with a squeegee. • Using the templates, cut plies required for the repair. • Put down each fabric layer following the orientation schedule called for in the SRM until the damaged area is filled.
NOTE: Preparing the resin system is a critical step in fabricating a durable laminate. Begin by verifying the precise ratio of resin to hardener called for on package labels. Then, use an accurate scale to weigh system components. Mix resin and hardener completely for about three minutes or until a uniform color has been attained. During the blending process, scrape container bottom and sidewalls frequently to incorporate any unmixed resin or hardener. After the resin system is fully mixed, apply it to the reinforcing fabric.
Vacuum Bagging ProcessWhen the lay-up is complete, vacuum compact the laminate to reduce air voids that can cause blistering and delamination. This procedure also helps control resin bleed within the laminate layers to maximize repair strength.
Many different bagging materials are used in vacuum compaction. Selection of specific product types and combinations must be made according to the requirements of each project. If the wrong sequence of materials is used, it can ruin the repair by, for example, bleeding out too much resin. Key factors affecting choice and application of bagging materials are the type of resin being used for the repair and the temperature and pressure under which the part will cure. Typical bagging materials selected for aircraft composite repair include:
• Peel ply facilitates separation of bagging materials from the laminate after compaction. This ply leaves a matte finish on the part that is easy to paint. NOTE: If the component will be bonded, the surface may require further preparation even if peel ply is used.
• Perforated release film controls resin bleed and air removal from the laminate. Holes are either pierced or perforated and a variety of center distance spacings is available. Some films bleed a percentage of the resin and allow air to escape while others retain nearly the full resin content.
For elevated temperature curing, add a heat blanket over the repair area.
Breather material is added over peel ply, release film and a bleeder to permit a vacuum to be pulled evenly over the laminate surface.
Cut repair plies from pre-impregnated fabric after using prepared templates to mark required sizes on the plastic.
Put pre-cut repair plies down on the prepared surface.
• Non-perforated release film retains all resin and air. The film is usually used to separate the bleeder and breather layers or to prevent resin from bonding to the caul plate or heat blanket that may be added after the bleeder and breather materials are installed.• Bleeder absorbs excess resin during compaction. A number of fabrics, including fiberglass, are used in this application.• Breather allows a vacuum to be drawn over the entire laminate. During installation, the breather must completely cover the area being vacuum bagged to ensure uniform pressure and to optimize air removal.• Caul plate can be placed over the breather blanket to produce a flat surface and promote uniform heat transfer from the heat blanket across the composite surface.• Heat blanket can be used to elevate the temperature of the laminate, thereby decreasing cure time. • Bagging film acts as a leak-proof membrane that is sealed over the part to vacuum compact the laminate. Use of a bagging film that is not coated with any oils is imperative.• Sealant tape holds the vacuum bag securely to the edges of the part and seals splices and folds in the bag.• Miscellaneous. Thermocouples monitor cure temperature and vacuum ports pull vacuum. In addition, a necessity for high-temperature curing of composites is a portable, computerized process controller. This equipment permits pre-programming of specific ramp and soak cycles and precise control of cure temperatures.
NOTE: The film and fabrics mentioned above are supplied in a number of grades and sizes with varied temperature and elongation characteristics to suit the requirements of individual applications.
Bagging TipsDuring the vacuum bagging process, as with the rest of the composite repair, attention to technique is crucial. Tips to remember:• Cover sharp areas on the caul plate to prevent puncture of the bag.
• Complete bagging and place the part under vacuum before the resin system begins to gel.• Install thermocouples so that they do not directly contact the metal caul plate to prevent calibration errors that can interfere with temperature readings.• Insert thermocouples adjacent to the repair area. If they are too far from the patch, readings will be inaccurate. If they are too close, they may print through the caul plate causing an indentation in the repair surface.
Vacuum CompactionStart with a low vacuum of approximately 10 inches of mercury to permit easy repositioning of the bag and fitting into corners as necessary. Then increase the vacuum to 22 to 26 inches of mercury until compaction is complete.
NOTE: Some resins may require use of less vacuum to prevent frothing and bubbling that can produce porous bondlines. Follow SRM instructions carefully.
CuringCure parts at the lowest temperature compatible with the resin system in use. If the SRM permits, it is preferable to cure composites at a temperature below 180ºF (83ºC). When high temperatures (350ºF/177ºC) are required, residual moisture in the composite part may form steam that can create enough pressure to disbond the laminate skin from the core.
ConclusionEffective repair of composite parts is not difficult. But, it requires a careful analysis of project requirements, a thorough understanding of materials being used, and close attention to techniques employed in each phase of the repair. The quality and reliability of repairs are directly related to strict adherence to proper fabricating methods.
During vacuum compaction and curing, excess resin is drawn up through the bleeder/breather materials.
Once cured, the repaired part is prepared for painting before being reinstalled on the aircraft.
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