I wanted to talk a little bit about repair because the topic always comes up in any discussion about composites – especially among those that have little to no experience with them. Aren’t they fragile? How do you fix them when they break or get damaged somehow?
I wanted to allay some fears out there that composites are difficult to fix or that they are never as good as new if you have to fix them. In fact, sometimes the fix is to make them better than they were either designed or constructed. You might get stuck with something that shouldn’t have passed inspection at the factory and you have to fix it.
Fear not. Anyone with decent carpentry skills and attention to detail will make a repair to a composite part at least as good as if not better than the original. This is true in most cases of recreational equipment, sports equipment, some of your bath fixtures at home, canoes, kayaks, surfboards, even light composite aircraft. All of these can be repaired fairly easily by the do-it-yourselfer.
It all starts with first finding out what’s damaged and how extensive the damage is. If it looks like not too much damage at the surface, you might want to look a little deeper. When composites break, all sorts of things break. Sometimes you get delaminations of layers, sometimes fibers break, and sometimes just the matrix plastic holding the fibers together cracks and breaks up.
Your repair is going to start with finding out which of these things happened to your composite and how extensive the damage is. Does it go all the way through the part? How big is the damage area with relation to the part? A really close inspection of the damaged area and the material surrounding it is in order here. You are looking for hidden cracks in the matrix plastic, any signs of fiber breakage or spalling, or potential delaminations that might be hidden under something like an impact area.
Once you have assessed the damage to your composite, the next thing to assess is how much of the damaged material needs to be removed so that you can clean up the damaged area to get to the region of competent material – composite that is not damaged or compromised in some way. This will let you know what supplies you need to buy when you go down to your local hardware store. Even Home Depot carries composite repair materials – mostly fiberglass, but they have a fair amount of epoxy resin and potentially even some carbon fiber cloth for sale on the shelves. If you can’t find what you need there are several resources on line to get supplies and raw materials. Amazon is a good place to start.
Now that you have what you need to make your repair, your carpentry skills will now be put to the test. Very carefully remove the damaged material until you are down to competent composite. Now we need to provide a surface to stick the new fabric to that will bridge any gaps and cover more than just the hole that you made in your composite.
From here on the process is very similar to hand layup where you put down some resin with a paint brush, stick some cloth to it and roll out as much of the resin as you can. Then – as they say – rinse
and repeat, adding layers of cloth and resin until your part is fixed.
This is the process that is followed in aerospace for repairs of very high-end aircraft when they get damaged. Considerable research has gone into their repair techniques to ensure that when the airplane is repaired it is actually as good as new.
Considerable research has also gone into and continues to go into automating repair. The process I laid out here is very labor intensive, and therefore very expensive. So, the Air Force has upgraded the depots that do maintenance on composite aircraft so that the depot personnel can take pictures of the damage and send those pictures to engineers working CAD systems where they engineer a repair and send it back to CNC repair machines on the repair floor of the depot. This, of course, is still in its infancy, but in a very short span of years it will become the norm for aircraft repair. Boeing is even looking into much the same things for the 787 Dreamliner.
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