An Introduction to Advanced Composite Structures Repair

An Introduction to Advanced Composite Structures Repair

Technicians must have a broad knowledge of materials and the knowledge of how these materials can and cannot be used to achieve high quality repairs By David L. Brewer

By David L. Brewer

October 2000

Over the past quarter century, the use of Advanced Composite Material (ACM) has become almost commonplace in the aerospace industry - first proven through military applications, then increasingly in commercial and general aviation. As with many high tech systems, they were implemented aggressively into military aircraft production 10 to 15 years prior to widespread use in commercial aircraft. Years of utilizing composite technology naturally precipitated a need to develop repair procedures.
ACM repair requires more than just the ability to read and follow the often vague steps in a technical manual (chiefly the Structural Repair Manual (SRM). Performing quality repairs demands more than mechanical skills - it requires the skill of a dedicated artisan. The technician must have a broad knowledge of materials and the knowledge of how these materials can and cannot be used to achieve that high quality repair - one that returns the part as close as possible to the original strength and stiffness without adding extra weight or protrusions that could affect airflow and/or the balance of critical flight controls.

Pamper your composite structures
Unlike metallic materials, ACM requires a certain amount of "pampering." They require acute cleanliness and precise control over critical shelf life and processing. They require dedicated and meticulous record keeping. Most materials repair require storage at low temperature refrigeration (freezing) to retard the curing of prepregs (tape or fabric pre-impregnated with resin and partially cured to a B-stage condition), and specially- formulated adhesives such as films and foaming adhesives. These materials are only allowed to remain out of the freezer environment for a short period of time before they have advanced through chemical reaction that would disallow these materials to be incorporated into a repair. Therefore, the technician must document the time a material is removed from the freezer and when it is returned to the freezer after use. This time is referred to as out-time and is typically limited to 7 to 21 days.
There are many ancillary materials that are used in the repair process as well. Ancillary materials are those that are used to aid and assist the curing process to tailor the cure to achieve the required fiber-resin ratio. They also provide the ability to produce a patch that bonds to the part and not to other materials. The most common ancillary materials include:
• Release films/fabrics
• Bleeder and breather cloth
• Vacuum bagging films
• Vacuum bag sealant (tacky) tape
Each of these materials is available in various composition, thickness, and temperature ranges. A release film/fabric is used in contact with the resin/adhesive or where control of resin flow is required. Non-porous release film/fabric is often referred to as a separator, in that if resin/adhesive contact is made on one side, it does not pass through to the other. Porous or perforated release films/fabric allows resin and air to pass through, yet it is easily removed from the part after the cure.
Bleeders and breathers may be the same material, but used in different applications. They are absorbent, highly porous, and are typically made of polyester random mat type material (often referred to as baby blanket). Bleeders are used to extract excess resins from the part and to provide a path for solvent and chemical volatiles, as well as for air to be withdrawn from the layup during the cure. Breathers are used where contact with the resin is not likely - usually between the vacuum bag film and the rest of the bagging materials to provide a chamber for air to be withdrawn from the vacuum bag. Therefore, the breather and bleeder must always be in contact to create this path.
Graphic

figure 1

Sealant tape (tacky tape) is used to provide an air-tight seal between the part and the vacuum bagging film, thus creating a vacuum chamber that allows atmospheric pressure (14.7 psi at sea level) to apply compacting pressure to the repair patch. (See Figure 1 above).

The repair process
The actual repair process involves many different steps. Any given repair can be very different from another depending on the type of structure, type of damage, and the materials used for construction. Since most aircraft structures are "sandwich" type construction, this is what we will discuss here.
When an ACM part is suspected of having damage, the first step would be to inspect the part to determine the type, size, and location of the damage. Small damage to the surface may be allowed to continue in-service with no repair or only minor repair action until a convenient time in which the repair can be accomplished. The more severe the damage, the more likely the part will require some lengthy repair prior to being flown again. Sometimes, the repair can be accomplished directly on the aircraft, or the part may have to be removed and repaired in the shop. The most common types of damage include:
Surface Damage: Cuts, gouges, scrapes, scratches, pits, etc. on the top ply of material or resin coating. Usually does not penetrate the top ply.
Delamination: Separation of two or more plies of material. Typically the result of impact or an inclusion during fabrication.
Disbond: Letting go at a bond line. Usually at the bond between the core and skin or between the skin and substructure.
Core Crush: Damage to the core usually caused by impact on the skin that depresses the skin into the core and causes it to be crushed. Skin may or may not have visible damage.
Puncture/Penetration: Damage through one or more plies to any depth or through both sides of the part.

Typical repair procedure
Graphic

figure 2

The following is a description of a "typical" repair procedure for one side skin penetration damage with core crush and disbond. Not all repairs will follow these exact steps. Always refer to the applicable SRM.
Where one side skin penetration damage with core crush and disbond is incurred, the first step is to perform an inspection to map the damaged area. This may be accomplished by an acoustic "tap test" or by ultrasonic A-scan pulse-echo Non-Destructive Inspection (NDI). The map is drawn to outline the damage. (See Figure 2).
Once all damage is mapped, the repair layout is drawn to enclose all of the damage in a geometrical pattern; either circle, rectangular, or oval, etc. The pattern should enclose all the damage, but take in as little of the undamaged structure as possible. Square repairs are discouraged due to high stresses at right angles. All corners should have a radius with no right angles.

Removing the damaged skin
Once the pattern has been laid out, a controlled-depth pneumatic router with router bit (material specific) can be used to remove the damaged skin. The preferred method is to use a router and router guide to steady the tool and prevent kickback that may cause more damage. The depth of the cut should be regulated to cut through the skin but no deeper into the core than is necessary. Typically, the router is set to approximately 1/8-inch below the skin's lower surface. After cutting around the layout pattern, the skin can be peeled off using a pair of duckbill pliers or pried off with a flat-tip screwdriver on a block of wood or other rigid material to act as a fulcrum. Depending on the extent of damage and the thickness of the core, a full core plug can be removed or a partial core plug can be routed out to a depth to just remove the damage. Full core plug is more common on thinner structures (up to 2-inches ±).
Before removing the damaged core, the honeycomb ribbon direction should be marked to ensure the replacement core is installed in the same direction for strength and stiffness. If full core plug is used, a core cutter or X-Acto™ knife can be used to cut around the outer edge of the core to be removed. Cut to the surface of the lower skin, using care to not damage the lower skin. Once the core has been cut all around, use a needle-nose or duckbill pliers to grasp the core cells and twist to fracture the cell node bond to the lower skin. A twisting motion is recommended to prevent delamination of the lower skin. Remove all old core. Use a sanding disc and arbor to lightly sand the core node adhesive bond on the lower skin surface. Take care to not sand into the fabric of the skin ply. It is not necessary to remove all adhesive as long as the surface does not have any high spots that would prevent a good, level seat for the new core plug.

figure 3

Obtain the specified core material for the repair and cut a replacement plug to fit the hole. The replacement core should be slightly thicker than the original core to allow approximately 3/8-inch above the top skin. Allow approximately 1/8-inch gap between original core and core plug. Align the core ribbon directions when sizing the core plug fit the hole. (See Figure 3).

Clean the repair area with an approved solvent (e.g. MEK, acetone, or other approved solvent). If possible, dry the repair area in an oven or use a heat blanket at 180 ±10 degrees F for 2 to 24 hours until all moisture is evaporated. This is difficult to determine in most cases, so, as a precaution, two hours is the recommended minimum dry time.
Moisture is a major concern in sandwich structures since the presence of water vapors during a cure can cause porosity and voids in the adhesive matrix.

Performing the cure
Wrap the core plug or cavity wall with foaming core splice adhesive and insert the plug into the hole. Cure the foaming adhesive per the manufacturer's recommendations. Curing involves heating the foaming core splice adhesive to the cure temperature that activates the foaming agent in the adhesive. Foaming action expands the adhesive from one to five times its original thickness to fill the open cells of both original core and core plug, which bonds the plug in place. After the core is bonded in place, it is machined to trim the excess core height to near flush with the surface by using a core cutter router bit and router holder. The router holder will stabilize the tool during cutting operations. Allow a slight oversize so as not to cut into the top ply of material.
Finish sand by hand with 220-grit or finer abrasive paper on a flat wooden block. Vacuum the repair area to remove any sanding debris. Clean with clean, lint-free cloth and approved solvent. Allow to air dry.

Patch must match
Graphic

figure 4

Fabricate a replacement patch from either prepreg or wet layup with liquid adhesive. Patch must match the original surface ply orientations. (Note that patch fabrication procedures would require another article on its own). As a minimum, the same number of plies as the parent structure must be used in the patch (See Figure 4 ).
The patch is usually cured in-place using a silicone rubber heat blanket and a Hot Bonder/Controller set to precisely apply power to the heat blanket at a rate that provides accurate cure temperature over the specified time. Vacuum is drawn over the repair through use of a vacuum bag made up of the materials discussed earlier. Maximum vacuum is drawn for maximum compaction pressure.
Precise control of the ramp-up, dwell and ramp-down temperature over a specific period of time is required to obtain maximum strength and stiffness of the patch. Hot bonders offer virtually worry-free control for most repairs, unless there are underlying heat sinks that can create cool spots. The technician must possess skills and knowledge of heat management techniques to be able to counteract these problems. This is another place where the "artisan" must be present as several methods are available to assist in heat sink problems if they are a factor. The repair area may be augmented by a second heat blanket, hot air or heat lamps, provided each is controlled properly and not allowed to overheat the structure. Some hot bonders also provide finite control of these accessories.

Special considerations for curing
The cure is extremely important to the success of the repair. If the resin is heated too quickly, air and volatiles may be trapped inside the resin, causing porosity and thus producing a weak repair. Another factor that may affect the repair is the lay-up sequence and construction of the ancillary materials in the vacuum bag. Bleeder material and release film porosity may affect the repair and should be given careful consideration. Always ensure the bag is built properly for efficient curing.
Once the patch has been cured, the repair is subjected to another NDI test to verify the repair is of acceptable quality. Then the area is prepped for finishing before the part may be returned to service.
Since the technician must have an acute understanding of all the factors involved in repairing ACM parts, it is highly recommended they attend one of the various schools that can provide the skills and knowledge needed to prepare the sheet metal mechanic to become the "composite artisan."
AMT

David L Brewer is the Technical Services Division Manager for Heatcon Composite Systems in Carrollton, TX.

Composite Listing
The following is a listing of companies that provide Composite Repairs, Composite Supplies, and Composite Training. Please contact them directly for information about products and services:


Abaris Training
5401 Longley Ln., Ste. 49
Reno, NV 89511
(775) 827-6568 or
800-638-8441
www.abaris.com

An industry leader in advanced composite training since 1983, Abaris Training offers 12 different courses in design, fabrication, repair and inspection of advanced composite structures. Classes held at Reno, NV and Griffin, GA facilities. Worldwide on-site training available at customer facilities - hands on training emphasis in all courses.

American Composite Education Inc.
4540 Sandpebble Trace #104
Stuart, FL 34996
(561) 225-6847

Develops materials and processes for masters, molds, tools and structures. On-site training using advanced composite mold making book. Consulting in all aspects of composites.

Conolite
325 DeSoto Ave.
Morristown, TN 32814
(423) 587-1842
www.conolite.com

Conolite is a division of Panolam Worldwide Industries and is a leading manufacturer of cargo bay laminate material. Conolite is manufactured in two types: Polyester and Phenolic. The company has on-site FAA DER.

Hartzell Propeller Inc.
One Propeller Place
Piqua, OH 45356
(937) 778-5713 (training)
(937) 778-4201 (supplies)
www.hartzellprop.com

Hartzell Propeller offers training in inspection, repair and overhaul of its composite propeller blades. Course descriptions can be found on their web site in the "Product Support" section. Also in this section is a listing of authorized distributors and service centers that provide supplies to repair and overhaul Hartzell composite blades.

Keystone Helicopter Corp.
1420 Phoenixville Pike
West Chester, PA 19380
(610) 644-4430
www.keystone-helicopter.com

Full-service maintenance and completion shops including composite molds, repair, and manufacturing. Specializing in aviation quality products and workmanship.

S.M.A.R.T. Inc.
509 Live Oak St.
Edgewater, FL 32132
(904) 426-5908 or
(800) 932-SMART
www.Smart-Inc.com

S.M.A.R.T. Inc. provides skilled quality composite technicians worldwide for short or long term contracts. We also provide all technical and engineering disciplines for the aerospace industry.

WichiTech Industries Inc.
8980-L Route 108
Columbia, MD 21045
(410) 715-1076 or (800) 776-4277
www.wichitech.com

Manufacturers and distributors of single and dual-zone portable hot bond composite repair systems used for repairing advanced composites.

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