Motorsport Composite Fundamentals: Repairing Carbon Fibre Parts

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Repairing Carbon Fibre Parts

08.05

00:00	While repairing carbon fiber parts is largely the same as the fiberglass parts that we covered in the last module, there are some subtle differences.
00:07	So, in this module, we'll look specifically at carbon fiber, and that'll also give us a chance to cover a part with a slightly different construction than a standard laminate.
00:16	More specifically, something with a core material.
00:18	The example part we'll be looking at is a carbon fiber splitter with a foam core from our GT86 Endurance race car.
00:24	Race car splitters naturally live in a fairly abusive environment just inches from the ground.
00:29	Being on the leading edge of a car, they also tend to cop the flying debris, or in some cases, the brunt of contact from other cars.
00:37	After multiple years of use, our splitter has numerous small chips in the outer skin, as well as some more significant damage through to the core material.
00:45	So, the plan is to tidy it up and restore some of the structural integrity.
00:49	While we won't be able to make it look like new, and the repairs will be evident under close inspection, it should be generally unnoticeable from a few meters away, and more functionally, we should be able to get the surface back to its smooth profile for the all-important airflow.
01:03	As always with composite repairs, we first need to remove all the damaged material before we can start fresh with a repair.
01:09	We'll first mark out the area around the major damage that we want to remove.
01:13	Any delamination of carbon to carbon, or carbon to foam, as well as damaged foam needs to be removed.
01:20	Thinking ahead, we'll be bonding in a new piece of foam.
01:23	So, if we cut out a simple shape with straight edges, this will make our lives a lot easier.
01:28	Before we start cutting, PPE is essential here, including eye, hearing, and respiratory protection, as well as some nitrile gloves to prevent skin irritation, but rip away easily if they get caught up in rotating power tools.
01:41	On that note, we'll be using a Dremel die grinder with a tungsten carbide perma-grit cut -off disc that'll hold up well to the abrasive carbon.
01:47	We can use this to accurately cut out the damaged carbon and foam, and then use a blade and some coarse 80 grit sandpaper to deburr and tidy up the edges, keying the carbon around the area while we're at it.
01:59	After this, we need to thoroughly clean out the dust and debris with compressed air and a rag with some wax and grease remover or isopropyl alcohol.
02:07	We're going to use an epoxy adhesive from Wirth called Powerbond, which is designed specifically for automotive body repairs.
02:14	The black color also helps hide the repair behind the carbon weave.
02:18	We'll use this to fill the area of missing foam in the smaller repairs.
02:22	While being heavier than the foam, it's such a small amount that we won't pay too much of a weight penalty.
02:27	Similar to our method of fixing surface voids, we're going to first use some flash tape to create a dam underneath the void.
02:34	This will hold the adhesive filler in place while it cures and reduce the amount of shaping we need to do afterwards.
02:40	This is a two-part adhesive and while mixing nozzles are available, I'm taking the simple approach and mixing appropriate amounts of each part on a piece of scrap metal with a popsicle stick.
02:50	Once mixed thoroughly, it can then be teased into the damaged areas to fill them.
02:54	We just need to be sure to force out any big air pockets that will cause voids and use enough filler to come proud of the surface so we can sand it back to flush soon.
03:03	While this starts to dry, we can move on to the large area of missing foam.
03:07	For a big repair like this, we're going to need to bond in more foam to achieve the same structure at a relatively low weight.
03:14	We have some PVC foam on hand that'll work perfectly for this, although PMI foam is another option.
03:20	As long as it's relatively lightweight at around 70 to 100 kg per cubic meter, compatible with the epoxy resin that we're using and has a fine cell structure so we can shape it nicely to fit, it'll work well.
03:33	This is initially cut with a hacksaw to roughly fill the area, leaving plenty of excess material that can be sanded away.
03:39	Due to the shape of the splitter and the thickness of our foam, I'm having to use some adhesive to bond two parts together.
03:46	Then it's a lot of back and forth trimming and sanding the piece to fit nicely in place with as small gaps as possible.
03:52	Once we're happy with the shape, we can then apply adhesive to the surfaces of the splitter and then clamp our new piece of foam securely in place.
04:00	It's worth mentioning that there are a lot of different approaches that could have been taken here with a never-ending list of possible adhesives.
04:07	Some people choose to use a general automotive body filler to bond in the foam.
04:10	Others might mix epoxy and glass bubbles to the consistency of peanut butter for an easily sandable filler.
04:17	Regardless of the approach, it needs to be left for a suitable amount of time to dry and cure, which is usually overnight.
04:23	We can then remove the flash tape and clamps and begin to sand our filler repairs back to match the surface of the splitter.
04:29	As always, the correct PPE is critical when sanding.
04:33	Using 400 grit sandpaper on a DA machine makes quick work of shaping the repairs.
04:37	Again, it's important they're flush with the original surface, if not slightly sub flush, so we can add a few layers of carbon and still match the surface without a bulge or a divot.
04:48	After the area is cleaned again, we can prepare for some hand lamination.
04:52	For these repairs, we'll be using 200 gsm 3k 2x2 twill and build up two layers over each repair.
04:59	Although the best option here would be to use the same fabric as was used to make the part, so we can match the surface blight of the part and create the most undetectable repair.
05:08	Stabilizing the fabric with a spray adhesive first allows us to handle it without so much fraying, and we can easily cut some pieces to appropriate sizes.
05:17	Put simply, we want the first layer to be a larger patch and then follow this with a smaller patch.
05:22	Both should easily cover the area of repair, but this will stagger the change in thickness for a more seamless repair.
05:28	With the reinforcement prepared, we can then start to mix up the resin.
05:32	For this, we'll be using West Systems 105 epoxy with 205 hardener.
05:36	These should be mixed on scales at 5 to 1, so for every 100 grams of resin, this will mean 20 grams of hardener and then they are mixed well.
05:45	It's also possible to add a small amount of black pigment to the resin to hide the repair, particularly if we're using a very light reinforcement over a white foam, but that shouldn't be an issue for us here.
05:55	With the resin mixed, we can then use a paintbrush to wet the repair surface and begin hand laminating.
06:00	The approach i'm using here is to wet out the reinforcement remotely on a clean piece of scrap material like melamine, and then transfer these to the part, which helps reduce excess resin on the part itself.
06:12	With the layers in place on the part and consolidated, another helpful trick is to use some release film and pull this over the repair, securing it with masking tape.
06:20	This helps hold down the reinforcement and match the repair to the rest of the surface of the part and minimize the amount of sanding that we need to do.
06:27	Now, we need to leave the repairs to cure.
06:30	The amount of time really depends on the product you're using and the ambient temperature.
06:34	It could be 4, 6, 12 or 24 hours or anywhere in between, but usually overnight is a safe bet.
06:40	Once cured, it's back to sanding with 320 and 400 grit to blend in the repair.
06:45	What's really important here is paying close attention to the color of the sanding dust.
06:49	If it's white, we're sanding resin, which is good.
06:52	Once it starts turning a darker gray, this indicates we're cutting into the carbon, which we want to avoid as much as possible.
06:59	Depending on the part, it's not always possible to avoid this completely.
07:02	What I'd recommend is sanding until we're happy with the surface shape with 400 grit, and this will also key the surface for bonding.
07:09	And then we can brush another layer or two of epoxy over the repair and sand this back, working up to a higher grit, wet sanding and polishing as required to match the surface finish of the part.
07:19	We might even choose to clear coat the parts from here, but again, it's just going to depend on what we're working on.
07:25	The key points to remember here are that when repairing any composite parts, it's critical to remove all the damaged material first.
07:32	Small cracks and holes can be filled with repair resins, fillers or adhesives compatible with the construction of the original part.
07:39	Epoxies are usually a safe approach for carbon fiber parts.
07:43	Larger repairs missing core materials will need to be filled with the same core material, whether that be foam or honeycomb structure, and this will be bonded in place.
07:51	Once these are sanded back to the desired shape, we can then patch over the repair with carbon and the resin of our choice, profiling again to match the desired surface and potentially coating for a seamless finish.