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
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.