An understanding of proper inspection together with repair of airframe components and structures is crucial for a mechanical aircraft technician, especially one dealing with maintenance, manufacture or even overhaul of aircraft (CNMACA et al, 1996). The composite scarf repair procedure outlined in this paper shall follow structural repair manual and adhere to ATA l00 system specification.
A composite material refers to a combination of two or more varied materials which do not entirely merge or dissolve into one another, yet work as one to provide superior properties. A composite repair needs to be fully identified prior to performing a repair. There must be clear understanding of such significant details as material specifications, core materials, ribbon direction, ply count, ply orientations, ply buildings, drop-offs, etc (Kedward & Kim, 2004).
Damage of a composite laminate often affects a single region of the laminated, which necessitates removal of some of the material at the damaged area and applying a repair patch. Using the scarf technique, the damage is solved by inserting repair material into the laminate in the area of material extracted. A scarf repair is necessary for laminated composite because undertaking a complete replacement of damaged areas with new parts is highly expensive.
Before making selection of repair materials for a damaged composite laminates, it is essential to determine the materials originally used in the manufacturing process, as well as how they combined to produce the overall composite. The aim of the repair is to return the structure to its original strength, shape, stiffness and surface finish. Such determination can be conducted through careful taper sanding via a small sample of the particular damaged part, followed by careful reading of the information provided on the composite itself. The following materials are used in the composite scarf repair:
- The resin system (thermal & structural properties)
- Cure cycle requirements & available equipment
- Liquid/prepreg resin formulas (Kedward & Kim, 2004)
- Fiber orientation
- Fiber reinforcement – carbon or fiberglass
- Fiber-to-resin ratio
- Fiber reinforcement from woven cloth or unidirectional tape
- Core orientation
- Core type e.g. balsa, aluminum honeycomb, foam, etc.
- Potting or core adhesive compound
- Lighting strike materials
- Coatings (ceramic coatings, nickel coatings, conductive paints)
- Fiber and mesh materials e.g. copper mesh, conductive fibers, aluminum mesh, etc.
- Grounding strips (often bonded to the outside surfaces on aircraft)
- Paints & Sealants (for post-repair)
- Polyurethane (hybridized coatings that are less brittle and more damage tolerant & newer paint systems)
- Epoxy-based paints and primers (weather & UV protection)
- Rubberized coatings ( products preventing damage from erosion)
- Polysulfide (often used as fuel tank sealant)
Composite Repair Process:
- Inspecting to assess the extent and degree of the damage. This is conducted by tapping on the laminate and listening to any resonant changes in sound. Evaluating the damage is important so as to undertake the appropriate repair to give reliability improvement of the particular airframe structure (Mallick, 2006).
- Marking out the damaged part by drawing a circle around it. A second circle is marked beyond the first circle to identify the area that will help support the patch. A third circle outside the second circle is drawn to help define the total area that needs to be cleaned and prepared for patching.
- Removing the damaged material. Damage removal can be done by grinding or cutting (routing). During cutting, a wheel or circular saw is used so as to avoid cutters with reciprocating blades. The appropriate cutting method is “high-speed, low feed,” where a high blade speed and gentle pressure are moved along the cut a rather low feed rate. The damage is then removed in a circular or oval fashion in order to limit stress concentrations at corners (Mallick, 2006).
- Drying or removing contaminated materials.
- Preparing the repair area
- Performing composite repair itself i.e. carefully tapering (scarfing) sand away from the edge to the damaged area. The ideal repair requires trying to match (not exceed) the strength, stiffness and weight of the original structure.
- Inspecting repair for quality assurance such as delimitations, inclusions and proper cure. It must ensure that the scarfing has been done slowly and carefully.
- After completion of scarfing, the surface is covered with a smooth, taped-down sheet of clear plastic film or Kraft paper to shield it from dirt, grubby fingers, moisture, etc.
- Tracing out on extra pieces of clear film is the outline of every repair ply, so that they neatly fit into the contour map formed on the surface of the scarfing.
- Marking the ply orientation of every ply on the templates while tracing its outline.
- Cutting out replacement plies from the same material similar to the original structure or very close substitutes (Mallick, 2006).
- Applying a layer of film adhesive onto the whole clean scarfed area, when using a prepreg.
- Laying down the innermost, smaller ply, then bond on, via oval hole, a larger oval to serve as back side reinforcement for the filler ply.
- Continuing to lay up one ply after the other, with each ply measuring about ½ inch larger than the original top ply. The orientation of the plies must compliment the original outer ply.
- Vacuum bagging the repair using a depressurized plastic sack so as to ensure good contact during the curing process (Mathews & Rawlings, 1999).
- Curing the repair, at appropriate temperature, by carefully using the heat lamps.
- Debagging the repair after completion of the cure, and inspecting for any delaminations and disbands.
- Restoring surface finish, prime and paint.
Vacuum bagging is a simple, inexpensive and most popular method for applying the pressure required to compact and bond the composite repairs. A vacuum bag is used to apply vacuum over larger areas (Duong & Wang, 2007). Vacuum bagging is relatively simpler than using clamps and weights for positive pressure. In addition, it results in very uniformly distributed pressure in the vacuum bag area.
Vacuum bagging is attained by covering the repair area with as sealed Nylon bag-film, followed by extraction of air from the bag by use of vacuum pump. As such, the atmospheric pressure is used to push together the composite repair/laminate (Mathews & Rawlings, 1999).
In order to conduct a desirable vacuum bagging, the following requirements are needed to be fulfilled:
- The vacuum bag, seal and tool should not be porous (i.e. hold a vacuum or be airtight)
- Leaks through the area of repair must be as minimal as possible
- The vacuum pump capacity should be sufficient enough to accommodate the entire volume of the bagged part.
- The vacuum hose used to draw air from the vacuum bag should be big enough to accommodate the necessary air flow requirements, and its length to the pump needs to be minimized (Duong & Wang, 2007).
- A vacuum gauge must be used to ensure that the desired vacuum is attained.
Problems and Solutions in Composite Scarf Repair
There are a number of issues encountered during the composite repair process. First, there is possibility of some damages going unnoticed. These include hidden manufacturing defects such as poor adhesion that could result in the sandwiched structure to disband between the core and the skin, in case of a low velocity impact. When delaminations and cracks occur, they may spread in a cone-shaped from the impact point and spread to backside of the laminate (Duong & Wang, 2007). Secondly, there could be unprecedented damage sources, such as composites designed to resist other impacts, but not removal for inspection.
Thirdly, composites are typically designed to meet particular needs which translate that there is a risk of not getting universal materials and techniques to undertake the scarf repair. A remedy to this is to conduct investigation before a composite repair, so as to establish the composite repair specifics.
Finally, there is a risk of increasing damage to the composite part in the course of the scarf repair. Therefore, it is necessary to take caution so as to remove and replace only the damaged materials in the structure (CNMACA et al, 1996). Successful composite repairs call for proficiency in such skills as non-destructive inspection, taper sanding, ply orientation /replacement, vacuum bagging, as well as processing.