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3.9.4.1 Critical Bonded Repair Processing Parameters

3.9.4.2 CACRC Standards

The results also showed that when properly followed, the procedures following the CACRC standards yielded strong durable repairs. CACRC standards cannot be used as a sole document to repair a composite part. These standards represent best practices/techniques for repair and therefore a part specific document is required. The CACRC standards can however be used along with an SRM or other part specific repair document.

3.9.4.3 Bonded Repair Variability

Research work showed variability in the repair residual strength results between depots and mechanics and underscored that repair technician experience alone is not a predictor of repair performance and that some of the process deviations may have been avoided with more stringent quality control oversight.

3.9.4.4 Repair Process Development, Substantiation, and Knowledge Transfer (Records Keeping)

Results of the study also demonstrate the importance of repair process development, substantiation and proper execution. Process substantiation should include understanding of the critical process steps and parameters affecting the repair performance and the consequences of bad process implementation. Because of the chemical characteristics of the various systems used for bonding and repair, it is very important to understand the capabilities and limitations of the specific systems especially when they are close to the end of their storage and/or work lives. The use of adequate processes specific to the materials used is key to the structural integrity of the repaired part. Caution should be exercised when applying results from one material system to the next.

This should be used to demonstrate that the substrate will yield durable bonds in service, and must be conducted in the most aggressive environments the structure will be subjected to.

Knowledge transfer in the form of training, validated repair instructions and repair records and documentation is an integral part in ensuring repair process repeatability, stability and thus structural integrity of the repaired component. Process documentation is necessary to ensure strict adherence to the process. QA oversight is strongly advocated.

3.9.4.5 Repair Curing Process Simulation

The simulation of cure process for bonded repairs can be used for process development, to understand the evolution of cure as well as the development of residual stresses. For large repair areas on complex structures with multiple components, this can be used to identify the optimal layout for heat blankets and thermocouples.

3.9.4.6 Workforce Education and Training

The study demonstrates the importance of workforce education and training for the proper execution of bonded repairs to composite substrates. Part specific training of the composite repair workforce, taking into account the process learning curve, is strongly advocated. At this point, due to the potential for understrength bonded repairs, and limitations in inspection methods, repair sizes should be limited such that the repair failure will not cause the aircraft to lose its capability for continued safe flight and landing.

1.4.1 Bond Integrity and Standards

Adhesively bonded components are manufactured, and bonded repairs are conducted, without the benefit of a comprehensive set of effective nondestructive process control tests or techniques to fully assess the through-life integrity of the bonded product. Standard nondestructive inspection (NDI) techniques may be able to detect physical defects leading to voids or airgaps in bondlines, but they cannot detect weak bonds or bonds that may potentially weaken in service. Recently, however, there has been a proof test developed based on shock waves generated by a high-peak power, short-pulse laser, which provides some hope that a localised measurement technique with the ability to consistently verify bond strength will be available in the future [20,21]. In the meantime, the quality and integrity of the bonded component will rely on a fully qualified bonding procedure, together with the assurance that the process was carried out correctly. The Aloha Airlines Boeing 737 incident in April 1988, where the aircraft lost part of the cabin roof in an explosive decompression [22,23], illustrates the importance of bond durability and more importantly, the ease with which this issue can be overlooked.

In the repair environment, experience has shown that some bonded repair designs and application procedures have little chance of success and can, in some cases, decrease the service lives of components [24]. A survey of defect reports conducted at one Royal Australian Air Force (RAAF) Unit [24–26] indicated that 53% of defects outside structural repair manual limits were related to adhesive bond failure. In addressing the standards applied to adhesively bonded repairs, the RAAF [27] have established a substantial improvement in the credibility of bonded repair technology.

1.4.2 Adhesive Bonding Environments

The performance of an adhesive bond is sensitive to the adherend surface treatment and the environmental conditions under which the bond is prepared. Facilities located adjacent to operational airbases or in industrial environments need to have concern for the effect of hydrocarbon contamination. Facilities in tropical locations need special consideration for the effect of heat and high humidity.

Factory manufacture uses specialised facilities and staff. The facilities will include vapour degreasing or alkaline cleaning, etching tanks, anodising tanks, jigs, autoclaves and appropriate environmental controls. Adhesives will be stored in freezers, and monitoring procedures will be in place. There is a well-trained workforce with skills maintained through production volumes, and highly developed inspection procedures are available.

At the other extreme, field repairs are generally conducted with relatively unsophisticated facilities, minimal surface treatments, vacuum bag or reacted force pressurisation and little or no environmental control. Staff multiskilling and rotation influence the currency of experience and hence the quality and performance of adhesive bonds [28]. The requirement for environmental controls, the attention to bonding procedure detail and the need for staff training and supervision are of particular concern. If the use of training measures can be combined with regular monitoring, then any deviation in quality of repairs or bonding operations being undertaken can be identified. At one RAAF repair depot, the ongoing review of wedge test data enabled deviation in standard practices or degradation in application equipment to be identified and remedied [29]. The use of quality control tools can also aid in continued monitoring of processes to improve reliability [30,31].

Depot-level repairs are conducted with facilities and staff skills that vary considerably. Some depots have almost factory-level facilities and high level of staff skill. Other depots are capable of only low-level bonded repairs and are little removed from a field repair capability.

Laboratory experiments are designed to establish knowledge and principles. It is easy to overlook important detail from factory or field experience since most laboratories are held to close environmental tolerances and do not resemble the workshop environment.

1.4.3 Constraints for On-Aircraft Repairs

On-aircraft repairs impose additional constraints on processes and procedures. The considerations include: accessibility of the area, limitations in the use of corrosive chemicals, adequacy of environmental controls and constraints on the tools for pressurisation and heating of the bond during cure. Safety, health and environmental issues are more demanding for on-aircraft bonding since it is harder to control, contain and clean-up hazardous chemicals. Constraints on the use of electrical power on fuelled aircraft, or those with inadequately purged fuel tanks, can restrict the range of treatment and bonding methods available. The surrounding aircraft structure imposes constraints on the choice of surface preparation, heating arrangements and pressurisation tools.

3.1.3.3.1 PS-ANM-25-20 “HEWABIs for composite structures”

To show compliance with §25.571(a), the applicant must show, among other things, that catastrophic failure due to accidental damage will be avoided throughout the operational life of the airplane. The applicant is required to consider possible damage scenarios when evaluating accidental damage that could result in catastrophic failure. One of these damage scenarios the applicant should assess is accidental damage caused by HEWABI events. HEWABI events (e.g., impacts by service vehicles) are impacts that are spread over a large area and convey sufficient energy to cause potentially catastrophic structural damage. While the damage caused by a HEWABI event is typically readily visible in metallic structure, such damage may leave little or no external indications in composite structure. To ensure that any potentially catastrophic damage resulting from a HEWABI event is detected and repaired, applicants must provide appropriate conditional inspection instructions, or other procedures, to be implemented at the occurrence of such impact events as required per § 25.571(a)(3).

3.1.3.3.2 PS-AIR-20-130-01 “Bonded repair size limits”

This policy reviews the regulatory basis and establishes the guidance in setting size limits for bonded repair to critical composite (monolithic and sandwich structures) and metallic structure. Bonded repair of critical structure must first be constrained to the sizes allowed by substantiating design data. Source documents, such as a structural repair manual, may define repair size limitations based on the limits of the substantiating design data generated by the DAH for repair purposes. This policy informs ACO engineers and designees that due to inspection limitations, bonded repair must be further limited to a maximum size whereby limit load residual strength can be demonstrated with a complete or partial failure of the bond within the repair or base structure arresting design features. This policy is not intended for minor repairs.

3.1.3.3.3 PS-ACE100-2-18-1999 “Policy on Acceptability of Temperature Differential between Wet Glass Transition Temperature (T_gwet) and Maximum Operating Temperature (MOT) for Epoxy Matrix Composite Structure”

This policy is for general aviation aircraft, but can be applied to other products. It states that in general, a minimum of 50°F differential will be used as a guideline determining the acceptability of the temperature differential between T_gwet and MOT for epoxy matrix composite structure. This guideline can be superseded by additional data showing sufficient structural capability when the T_g temperature differential is not met.

3.1.3.3.4 PS-ACE100-2005-10038 “Bonded joints and structures – Technical issues and certification considerations”

This policy is for general aviation aircraft, but can be applied to other products. The purposes of this policy statement include: (1) to review the critical safety/technical issues, (2) to highlight some of the successful engineering practices employed in the industry, and (3) to present regulatory requirements and certification considerations pertinent to bonded structures. One key topic in this policy statement relates to the glass transition temperature. The guideline for epoxy matrix composite structure is for the T_gwet to be 50°F greater than the MOT. The analogous guideline for adhesive materials is for the T_gwet to be 30°F greater than the MOT.

3.1.3.3.5 AIR100-2010-120-003 “Acceptance of composite specification and design values developed using the NCAMP process”

This policy provides clarification on the acceptability of material specifications and allowables developed by the National Center for Advanced Materials performance (NCAMP) for Composite Materials. Material specifications developed following the NCAMP standard operation procedures are compliant with § 2x.603. Applicants who wish to use associated NCAMP databases and material allowables need to validate the applicability of that data to their project.

3.1.3.3.6 AC 21-26A “Quality system for the manufacture of composite structures” and AC 21-31A “Quality control for the manufacture of non-metallic compartment interior components”

A quality system established for manufacturing composites should be similar to any other quality system established to meet the requirements of § 21.137. These ACs address areas of the quality system that may require expansion to adequate accommodate the manufacture composites, as compared to metals.

The FAA sponsors research on composite materials to evaluate new technologies and ensure appropriate standards are set. FAA research is published through the William J. Hughes Technical Center Library, at http://www.faa.gov/about/office_org/headquarters_offices/ang/offices/tc/library/. These reports often provide valuable background information that supports published guidance or training.

In addition to FAA guidance, numerous industry groups publish useful documentation on best practices and some material data. The FAA is the primary funding agent and provides leadership for CMH-17. CMH-17 is a volunteer organization that creates, publishes and maintains proven, reliable engineering information and standards, subjected to thorough technical review, to support the development and use of composite materials and structures. CMH-17 provides useful guidelines for the characterization of composite materials used in structural applications with some emphasis on aerospace needs. The data documented in CMH-17 provide a statistical basis in material properties that are most useful in controlling stable materials and processes and providing basic design properties. CMH-17 also provides technical guidance on M&P control, design, analysis, testing structural substantiation, and maintenance.

The ATA/IATA/SAE Commercial Aircraft Composite Repair Committee (CACRC) is another industry organization that supports composite use in aviation. The charter of the CACRC is to develop and improve maintenance, inspection and repair of commercial aircraft composite structure and components. CACRC task groups include Repair Materials, Repair Techniques, Procedures, Analytical Repair Techniques, Design, Inspection, Training and Airline Inspection & Repair Conditions. They publish documents on practices such as machining of composite materials and heat application for thermosetting resin curing, as well as material specifications and training recommendations.

ASTM develops and maintains test standards for composite materials. Committee D30 often meets jointly with CMH-17.