Composites Repair

 

In selecting a title for this article, I wanted to convey the perception that composite repair is not that difficult, or at least no more difficult than sheet metal repair. To To be certain, it requires training, practice, and experience to become truly proficient at producing well-crafted, airworthy composite repairs, but then so too does sheet metal repair. The main difference is that composite repair technology most likely like ly represents a new set of skills, processes, and materials to which most technicians have not been exposed.

Should you make the effort to acquire these skills I will let you be the !udge of that, but consider the following" #oeing, $irbus, %ulfstream, &ilatus, 'awker #eechcraft, (assault, #ombardier, )essna, (iamond, )irrus, *to name a few+ all rely on composite materials in primary primar y structural applications. In a paper written by the ational Institute of $viation esearch, it is pro!ected that the use of composites in aircraft construction will quadruple over the next / years.

This rapid growth in aerospace composites will not be without problems. There is and will continue to be a gap between the manufacturers0 *123s+ development and production of these thes e aircraft, and the maintenance industry0s ability to maintain them. This gap has two aspects" numbers and technology. There are simply not enough technicians trained and a nd experienced in composite inspection and repair to meet the anticipated needs. ew inspection and repair technology skills will need to be acquired by b y maintenance organi4ations and personnel. )ompounding these issues is the fact that there is very ver y little standardi4ation of repair processes among the manufacturers. To overcome this gap, many 123s have developed and offer training specific to their types.

5hile this may satisfy the needs of the 123s, it does not lead to any standardi4ation of the composite repair process. #ut upon closer examination, we can place the 123 processes within the framework of a series of sequential tasks, recogni4ing that each 123 may utili4e different procedures for completing the task. I have identified the following tasks in outlining a standard composite repair"

Inspection/damage assessment. 2ach assessment. 2ach 123 provides inspection and damage assessment criteria in their  publications. 6or the most part, they are consistent on inspection criteria" visual inspection of the structures followed by a detailed inspection of known or suspected damage. 1n a glass fiber structure, this may involve paint removal *glass fiber reveals damage well visually+, tap testing, or (T inspections. 7nown 7no wn or suspected damage to carbon fiber structures usually requires (T inspection.

Determine repair authority. 5hat authority. 5hat is damaged, where it is damaged, and how large is the damage are all criteria manufacturers use when providing standard repair data in their publications. 8ou will find much variation among them. Some are very ver y generous and some are very restrictive in what repairs the

 

publications authori4e, but in all cases, there is a point in which you must contact their engineering department for repair support. 8ou need to know where that point is.

Obtain repair data and materials. 5hether materials.  5hether you are using standard repair data from the $339S3 *Structural epair 3anual+ or an 123 engineered repair, you need to fully understand the repair before proceeding. I refer to this as :%etting your head around the repair.; If you do not :see; the finished repair, call the 123 help line for assistance. <se only 123-approved materials within shelf-life limits for accomplishing the repair.

Removal of paint and damaged material. 5hile material.  5hile it may be necessary necessar y to remove paint for assessment purposes, you will now need to remove enough paint to accommodate the repair. I am aware of no n o 123 that allows the use of chemical strippers s trippers for paint removal. epair data will tell you how much damaged material to remove, but generally less is better.

Preparation of laminate surface. 5ith surface. 5ith most 123s, there are two aspects to this task. The mechanical abrasion of the repair area creates a heightened level of molecular activity on the laminate surface, ensuring a stronger chemical bond with the repair materials. The second aspect is the scarfing or stepping down of the damaged laminate to the full depth of damage. This not only effectively reduces the height of the repair, but ensures a better interface between the repair materials and the damaged laminate. 2ach 123 has their preference here, but both b oth require high levels of technician tool skill and practice.

Preparation of repair materials. This materials.  This task contains several processes that will vary among the 123s.

1. The 1.  The 123 will specify wet lay la y or pre-preg materials. 5ith pre-preg, the resin has been incorporated into the fabric and dried and requires a bonding machine for curing. )onsult the $339S3 for the list of approved materials.

2. The 2.  The number of repair plies specified is generally a function of the number of damaged plies.

3. The 3.  The repair ply overlap is how much larger or smaller each ply must be relative to the damage and the previous ply.

. 5hen .  5hen the fabric is woven, the dominant yarns, called warp, run in one direction and are crossed by lesser fill yarns. The 123 designers specify which direction the warp fibers must run relative to the structure. This is referred to as ply orientation */-, =>?-, ->?-, and @/-degree orientations are typical+.

!. Some !.  Some 123s require plies be stacked from smallest to largest, while others require re quire the opposite stack.

 

". If  If the repair requires replacement of damaged core material, consult the $339S3 for approved core ". type and thickness *as well as ribbon direction if honeycomb+.

#aminating. 3ost #aminating.  3ost 123s have restrictive laminating environmental conditions in regards to airborne contaminants and temperature9humidity limitations. 123 publications will include information on correct resin to fabric ratio and avoiding entrapped e ntrapped air *porosity+ in the lamination. 5hile 5 hile these conditions are easiest to maintain by applying the lamination one ply at a time, the repair re pair location may dictate that you pre-stack the plies and apply the lamination as one unit *to maintain ply pl y overlap and orientation+.

$uring. &re-preg $uring.  &re-preg repairs require a schedule of vacuum bagging materials as well as a bonding machine. 5hile you can use this equipment on wet lay repairs, due to their expensive nature, many 123s allow less expensive and less complex curing options. 'eat sources may include heat lamps, heat guns, or hair driers so long as cure temperature is monitored and controlled *recording cure data is also a good practice+. The actual cure parameters are more a function of the resin manufacturer than the aircraft 123 and include minimum9maximum temperature and time requirements as well as temperature ramp rates and scheduled hold periods or dwells.

Inspection. Some Inspection.  Some of the criteria set forth by the 123s include inspecting the repair for proper curing of the resin, complete bonding between the repair and the structure, voids or porosity in the repair, and evidence of correct resin content.

5hile organi4ing 123 procedures into a set s et of related tasks may help in developing a sense of a standard composite repair process, it must be emphasi4ed that strict adherence to the individual 123-approved data, materials, and processes is imperative. If, for example, you find you prefer the method of stacking plies that 123 $ uses more than that of 123 #, you must use 123 #0s method when repairing an 123 # aircraft. There is no substitution of material or procedure allowed without 123 approval.

Aike them or not, composites have earned a place in modern aircraft design and chances are you may end up in a position where you will wi ll be faced with the prospect of inspecting or repairing them. 5ill you be ready

Tim Wright most recently developed and launched aerospace composite programs at Northland Community and Technical College in Thief River Falls, MN, and Wisconsin Indianhead Technical College in Superior, WI. e is currently on the faculty of Wisconsin Indianhead Technical College.