Composite Repair Durability

How long will composite repairs last? If properly protected, they can last decades. There are fiberglass composite structures that have lasted 50 years without failure, but these have also been sufficiently protected from environmental degradation, including:

• Heat (infrared radiation) and ultraviolet (UV) radiation, which can damage resins and fibers
• Moisture ingress, which can occur if fibers are not adequately surrounded and impregnated with a matrix
• Chemical attack from fuels and oils, hydraulic fluids, deicing fluids, acids, paint strippers, and other solvents

The best defense against heat and UV degradation is to properly paint repaired surfaces with coatings that contain UV-inhibitors, which also have a sufficiently light color to reflect infrared radiation. Studies done in the 1970s showed composites painted black could achieve surface temperatures 115°F to 120°F (29°C to 35°C) above ambient air temperature. Repairs in areas near exhaust or other high temperature regions must also use resins and adhesives that can provide the required properties at higher service temperatures. Galvanic corrosion, such as the swift degradation observed when carbon fiber is in direct contact with aluminum, is also an issue that requires consideration, especially with bolted repairs where the proper selection of fasteners is critical. The corrosion from this direct contact between two materials at opposite ends of the galvanic scale occurs only in the metal, not in the carbon fiber, with moisture exacerbating it. Another way to prevent galvanic corrosion is to isolate the carbon fiber from the aluminum with an interface ply of fiberglass, film adhesive, or other non-reactive material.
 

Surface Preparation
For adhesively bonded repairs, the most important key to durability is proper surface preparation as well as application and curing of the adhesive. Abaris Training provides hands-on instruction in all of these skills for the repair of both composite and metal aircraft structures, including a course specifically on adhesive bonding.Photo Courtesy of Abaris TrainingThough bolted repairs are sometimes used on composite structures, the most common permanent solution is a flush scarfed bonded repair which restores strength and stiffness while maintaining aerodynamic smoothness and, if needed, radar transparency. The durability of bonded repairs relies on the quality of the adhesive bond between the repair patch and the surface of the structure repaired. The major variables affecting this bond include suitable selection and curing of the adhesive, uniform bond line thickness at the proper dimension (not too thin, not too thick), and preparation of the surface to which the repair will bond.

In the 1950s, the Fokker F27 twin turboprop airliner used aluminum skins bonded to aluminum structures with phenolic adhesives. Longtime bonding expert, R.J. Schliekelmann, Fokker, Schiphol, The Netherlands, described this in his presentation at the 15th Annual Conference, International Federation of Airworthiness, November 4 – 6, 1985, Amsterdam, The Netherlands. Use of an autoclave was to cure the adhesive bonds under elevated temperature and pressure. This construction performed well and the adhesive bonds did not degrade with heat and moisture, many lasting for several decades. The aerospace industry developed epoxy adhesives in the 1960s expecting similar performance, but was surprised when degradation of adhesively bonded joints due to hot/wet conditions was first seen in the 1970s. A tremendous amount of research over the next 20 years resulted in what we know today: surface preparation is the difference between adhesive bonds that last days and those that last decades. The goal is to have such a strong adhesive bond that failure occurs within the adhesive itself, known as cohesive failure, versus peeling of the adhesive from either the repair patch or the repaired surface, e.g. adhesive failure.

It is important to point out that though a cohesive failure is the goal in bonded joints, it is possible to have a cohesive failure and not have a good bond. One example would be if the adhesive never fully cured because the proper curing temperature was never reached. The resulting joint would fail in cohesion, as the uncured adhesive would separate and fail within itself. Thus, the goal is to have a cohesive failure where the adhesive is fully cured and exhibiting optimum properties, while an adhesive failure is always the result of insufficient bonding at the joined surfaces, indicating issues with surface preparation (surface prep).

The steps involved to achieve good surface prep with composites are fewer and less onerous than with aluminum; however, the small details become much more critical. The surface must be very clean, meaning no contamination from bond-degrading dirt, dust, grease, or oil, including oil from fingerprints. Therefore, the use of gloves is required, but it is important that they be powder-free, as the powder will contaminate the bond line. In addition, gloves often become dirty with substances in the shop without the technician noticing because his/her sense of touch is less through the glove. Therefore, technicians must be aware of this and in practice, change gloves much more often than they typically do.

The other critical step in surface prep is developing an electron-sharing covalent bond between the adhesive and substrate surface by raising the surface energy. This is critical to enabling adhesive wet-out of the surface and a good bond. The best way to do this is by lightly abrading the surface to be bonded. The goal is not to form mechanical roughness, but instead to shear the very top layer of resin matrix molecules to create broken atomic bonds without damaging or breaking the fibers underneath. While grit-blasting may reach down into the weave interstices of the surface reinforcing fabric, it almost always uses equipment powered by compressed air, which will contaminate the bond line with oil and water. Thus, abrading by hand using a ScotchBrite pad is the preferred method for achieving a sufficiently high surface energy with much less risk of contaminating the bond line. This high surface energy effect from light abrasion is very time-dependent. Application of adhesive must appliedwithin one to two hours, or else abrasion must be repeated before bonding. After abrading, remove dust from the bonding surface by gently wiping with a clean, unused cheesecloth. Do not use compressed air guns or solvent wipes because they may contaminate the bond line.

Another issue in adhesive bonding is foaming of the adhesive, which sometimes occurs with the high levels of vacuum used during the curing process. Before beginning the adhesive bonding process, it is important to read the adhesive manufacturer's technical data sheets to obtain the best curing temperatures, times, pressures, and vacuum levels. For example, phenolic adhesives require significantly higher pressures than epoxies to achieve a good bond.

Shop conditions are another factor that need to be addressed before bonding. Most adhesives absorb moisture from the air, which can then turn to steam during the elevated curing process and literally blow apart the repair bond line, or at the least, fill it with air bubbles, which will seriously degrade its strength and durability. Make sure that adhesive bonding shop work is completed in an area where the air is conditioned to be dry and maintained at a constant room temperature.
 

Measurement Versus Skill
The major issue with bonded composite repairs today remains that the strength of the bond cannot be measured in any non-destructive manner after full cure. Therefore, the true strength of the bond cannot be quantified. Thus, the only alternative is to develop the best materials and process possible and make sure that every step is followed with the utmost accuracy, attention to detail and skill.

It is possible to build test coupons, using the same mixed adhesive batch as in the repair and the same composite adherends (bonding surfaces) with the same surface prep. These test coupons can then be subjected to peel tests and wedge tests (originally developed for aluminum-aluminum bond evaluation, and provide good durability predictions for adhesives used in composite bonded joints) to quantify the bond strength and verify that the materials and process steps do produce a sufficient bond for long-term durability.

However, these are test coupons and not the bonded repair itself. As Roland Thevenin, senior composites expert at Airbus comments, "People doing this work [bonded composite repair] must be aware of the need to follow a rigid process. Moreover, as the fleets of more-composite aircraft grow, so will the number of people involved in their repairs – and only proper training and qualification will ensure composite repairs are conducted correctly."

Currently, only the organization contracted to complete the composite repair is certified, not the individual technicians actually performing the work. The Federal Aviation Administration (FAA) does certify that each maintenance and repair organization (MRO, e.g. airline, third party repair services provider) trains its technicians in how to properly perform repairs, but this currently relies on each FAA inspector's subjective review of the MRO's training documents against a recommended standard. That standard will soon be a final version of the draft advisory circular AC No. 65-CT Development of Training/Qualification Programs for Composite Maintenance Technicians, which was open for public comment early in 2011. Now that the public comment period is over, the FAA is reviewing received input against the draft and will issue a final document later this year. This AC will provide new guidelines for FAA inspectors to use in certifying that MROs do have training that teaches their technicians how to perform composite repairs. However, the individual technician will not be certified.

This is why Airbus has stressed the need for an approved individual qualification for composite repair technicians. This is also the goal being sought by the Commercial Aircraft Composite Repair Committee's (CACRC) Training Task Group. The envisioned certificate would be similar to an Aircraft & Powerplant (A & P) license, in that an individual would graduate from a certified composite repair training program and then pay a fee to be certified via a written and practical exam as a Level 3 or Level 4 composite repair technician, for example. This process is similar to current FAA certification for NDI technicians. The CACRC operates under the oversight of SAE International, Warrendale, PA, and SAE is possibly interested in managing this type of certification process. It is currently surveying the aerospace industry to determine needs and issues and will make a decision as to if, and how, it proceeds later this year.

Meanwhile, the best durability for bolted and bonded composite repairs, as agreed upon by both the FAA and European Aircraft Safety Administration (EASA), is achieved by following the appropriate aircraft manufacturer and aircraft industry composite repair guidelines and attaining the highest proficiency in the necessary skills through training. Both the CACRC and FAA have documents which outline the necessary components of such training, and hopefully the industry will soon have a certification to guarantee these are met by new composite repair technicians entering the workforce. Right now, the plan is that current technicians would be grandfathered in for such a certification.

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