Carbon fiber panel repair for rust

Zeke · 06-07-2002, 08:38 PM

What is anyone's opinion of using epoxy resin and carbon fiber over sandblasted metal to repair or strengthen body panels? Especially in tight quarters and compound shapes.

chibone_914 · 06-07-2002, 09:19 PM

Hi Milt,

I should say why not? Carbon fibre appears it would be more resistant to being punched through, thus justifying it's additional cost.

URY914 · URY914's Garage

I'm not so sure it would work as the bond between the resin and the metal is the determing factor. The people to ask would be www.fibreglast.com or www.fgci.com

Zeke · 06-08-2002, 01:53 PM

As far as bond is concerned, apparently the factory used glass duck tails and turbo tails over metal frames bonded to metal engine cover. This from a recent thread. I know from experience that resin bonds well to perfectly prepared metal, i.e. sandblasted.

Expense? Some of these replacement panels aren't cheap. I just wondered about some areas adjacent to longitudinals or rear bulkhead or front floor pan/ tunnel that are ususally under carpet.

Carbon Joe · 06-08-2002, 05:37 PM

Zeke914,
How's your car coming since last we talked? As far as bonding and structural strength you can't go wrong with a good epoxy and carbon fiber. If its penetration your worried about use carbon/kevlar cloth. If you need any help with materials and resins, just let me know.

Zeke · 06-09-2002, 09:23 AM

It's coming along, thanks. Just returned from Long Beach Swap. Bought some POR 15. The seller there has a demo piece of rusted metal that he laminated with POR 15 and fine glass cloth. I say this is the way to go on small rust repair, especially in tight spots where cutting out and welding back is difficult.

A pint cost me $21, it's not cheap.

Special K · 06-09-2002, 12:54 PM

Bonding is not the problem with using carbon graphite over bare metal, it'll bond just great. The problem is that the carbon graphite will corrode the steel (dissimilar metal corrosion) fairly quickly, which will weaken the bond. You could however have at least one ply of fiberglass between the steel and the carbon cloth for a barrier, that way you could still have the light weight, and strength of the carbon, without the corrosion problem.

P.S.- Carbon Graphite will even corrode titanium, so carbon steel wouldn't put up much of a fight.

Zeke · 06-09-2002, 09:31 PM

Very interesting, Special K. What kind of metal, if any, do you think is used in Champ cars, IRL and F1 cars where bolts are attached, or are the suspension pick up ponts and engine mounts simply bonded to the monocoque? Metal must come in contact with the carbon somewhere in the process.

Frankly, for our purposes, a hybrid of carbon and glass fiber, using the glass fiber layer as an isloator, should be plenty strong enough.

Special K · 06-10-2002, 08:40 AM

Well, as far as the Formula and other race cars, I can't say for sure but in all probability they're stainless steel fasterers. There has to be some sort of barrier like maybe a thick gel coat layer, fiberglass, rubber grommet, etc., because even stainless steal will corrode next to graphite. On the F-18 SuperHornet, and the AV8B Harrier (Both mostly titanium or aluminum substructures with Carbon Graphite outer skins) we use a hightemp liquid rubber adhesive between any disimilar metals i.e.- aluminum/titanium, titanium/steel, and Carbon Graphite mating with anything including the fasteners that hold it to the structure. I'd recommend a fiberglass layer between the carbon fiber and the metal structure, layed-up at the same time to ensure a good bond between "all" the layers. You'll end up with a very strong repair/bond with none of the problems associated with carbon coming in contact with bare metal. Here's another little tip. Get the contour and size of the repair as close as you possibly can to the desired end result, because cured carbon graphite is some VERY tough stuff, and eats sanding disks for lunch. We use diamond impregnated die grinder cutters, and they work great. If you do have to do any sanding/grinding after the initial repair, coat the area with a thin layer of resin to seal it.

P.S.- Use Epoxy resin, not polyester. Do your lay-up, let it set at room temp for 1 hour, then use a heatlamp at 190F +- 10F for 1 hour. Super durable repair!

Special K · 06-10-2002, 09:23 AM

Oh yeah, if you really want it strong, apply 2 layers of Carbon fiber cloth, with the second ply at a 45 degree bias to the first (3 layers all together-fiberglass, carbon, carbon- all laid-up and cured at the same time.)

Zeke · 06-10-2002, 03:50 PM

Well, I think this is all excellent information regarding the possibility of making rust repairs on the 914. Many of the areas that rust are very complex (like the rear suspension console), but they retain enough of the contour that you could lay up against an unseen side of the weakness, finish off the seen side where the repairs shows and undercoat the the other side. I am going to give this a try in a couple of areas that are small that I haven't attacked yet. (I tend to work from the major repairs toward the minor ones last.)

Dave at Pelican Parts · Dave at Pelican Parts's Garage

Just don't use this to repair structural areas of the car. The suspension console, for one.

The carbon/kevlar stuff is great in tension, but not so good in compression. Pretty lousy in bending, too. And a lot of the loads in the structural areas of the car are compression and bending loads.

If you don't know how the loads are transmitted through an area of the car, don't use FG or composites to repair it. Use metal. If you know the loads are in tension only, or if it's not a load-bearing area, then the FG/etc. is OK.

I personally would prefer to have metal repairs, myself...

--DD

Special K · 06-11-2002, 08:58 AM

Zeke914, I totally agree with Dave on the suspension console repair. I thought that you were talking about 'body panel' repairs, in which case composite materials would work just great. The F-18 E/FF (not to be confused with the E/F model) does sport a fully composite forward fuselage, including the inner structural components, but the structural components are molded in an Autoclave at high pressure and temperature, not simply "laid up". I'm not an Engineer, just a guy the assembles the planes and does the composite repairs. There is however another guy on this board (his name slips my mind) that 'is' an Aeronautical Engineer at Lockheed in Texas. If he happens to see your post, maybe he could give his 'professional' opinion on the best use of composite (Graphite, Kevlar, FG) materials.

Zeke · 06-11-2002, 05:08 PM

No, I won't attemp any repairs to a load bearing component like the suspension console. That was a bad example on my part. I was thinking of complex areas in the trunk, etc.

Dave, I sure everyone would PREFER metal repairs. This thought is about alternatinve methods for small areas that would otherwise be a major undertaking to remove the affected area (as in where three body panels intersect) in an otherwise sound car.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

Cool. I did say "prefer" as well.

I was just thrown by your example...

--DD

retro74 · 06-23-2002, 03:59 PM

My car is up on blocks and I'm cutting out the trunk panel this week. The discussion on carbon fiber is interesting and I have had some experience with this in Aerospace and bicycle frame design, but not engineering experience. The super strong stuff has been high modulus carbon fiber and there have been a zillion ways to use the stuff...

My tech question here is what about "pre-preg." This stuff is shipped in a refrigerated truck and has a shelf life. The carbon fiber is pre-coated and the stuff begins to cure when the sheets are exposed to air temperature. It is commpnly layed-out over honeycombs to ge the desired thickness.

i would impagine that an aerospace company could have some of this stuff "off spec" due to shelf life, end rolls, etc. and you could save a lot of maney that way.

Just my 2 cents.

Joe