Kevlar re-inforced body parts

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07-04-02, 10:45 AM

#26

dgeesaman

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Quote:

Originally posted by Trexthe3rd
Now we are having a more intelligent conversation. Sorry about my laptop's keyboard sticking, it's getting replaced soon.
Like I said in the initial post, I am planning on applying the laminate on FG. This was suggested by a friend in the raceboat business. Urethane body parts are fine the way they are due to the high degree of material flexibility.
By creating a Kevlar/FG/Kevlar sandwich, I want to improve the chances of survival from your typical minor damages. Since the material is so light, there will be very little significant weight penalty. The 2 types of damages I have seen is usually due to the road debris or sudden contact due to the smaller road clearance.
The road debris will essentially create a dent/hole on the body surface, and this should be reduced to scratchs (due to the surface laminate) instead of chunks being missing.
The bigger damage would stem from hitting bumps, curbs, ramps etc. This usually results in cracked body parts (or as you said, mass breakage). The sandwich lamination should minimize the chances of this type of damage. Granted, if there is a severe impact type accident things are going to break, regardless of how strong the laminates are.
I have access to and will be testing three possible laminates (before I put the laminate on the actual bumper). The first being Kevlar, the second is Carbon Kevlar and the third is layered CF and Kevlar. The test will be rigged to simulate the two situations mentioned.

OK, things are getting much clearer. Frankly, you mentioned a lot of this in earlier posts but it wasn't laid out clearly.

So the idea is to improve on carbon fiber, and expect two changes:
1) You're saying (I don't own any FG body panels) that FG is prone to chipping and nicking and protecting it with Kevlar may help keep the FG more intact from flying debris.
2) FG is very prone to rubbing wear, so that a body panel that rubs against pavement or a wheel will get ground down in a big hurry. Putting Kevlar there will greatly slow the process.

I think item one shows some promise (much more below), but only against very small flying objects. Item two shows great promise, provided you don't care how crappy it looks as it wears.

Here is my take on the two ideas:
- Kevlar (or any other high-tenacity fiber) will do best protecting when it's least constrained. The tighter your epoxy or bond holds the fibers, the more scissor-like the interactions. The bottom line - it will look like hell from the moment you rub something - you'll have fibers frayed all over the area, and the epoxy will probably be completely micro-cracked like the glass in a shattered windshield. It will have that very obvious white appearance.

Basically, in both the abrasive and impact situations the Kevlar and its epoxy will eat the energy. The FG will still see a lot of the impact, but it won't be free to fall off. That's the mode of failure I see.

About the weight savings, it all comes down to how much epoxy you use. If you want a nice smooth (low drag) finish, you'll have to layer the epoxy as thick as the Kevlar cloth is. If you press it too much I'm worried the fibers may be more prone to cutting into themselves. I'm guessing the weight savings will be determined by experimentation during fabrication.

In the case of pavement rubbing, the external Kevlar might help a lot. But I think a lot depends on the direction of the rubbing. If you're rubbing across the flat face, it should be pretty resistant. If you've got a vertical panel rubbing the road along its edge, it will eat right through the FG like before and the Kevlar fibers will just hang there like a bad haircut.

I question the use of FG on RX-7s at all. Boat hulls use Kevlar on FG because boat hulls are a critical load-bearing structure. The water provides for a well distributed loading. The impact of waves is immense, but well distributed over the hull. Race cars (F1) use CF and FG because the incredible downforces put huge loads on the body panels. Again, this load is well distributed. Sails use Kevlar and PBO because of their extremely high modulus and strength, and sails evenly distributed loads. In other words, the panels are key structural components and they must be very stiff. This stiffness is acceptable since none of these applications sees point loading.

This car (and most any other production sports car) doesn't accumulate aerodynamic panel load to justify a stiff and strong panel. (Hell, the Cd is like .28, right?) About the only place it might help is in a rear wing. In addition, the big damage problems are point loads that cause chipping. The material choice is mostly determined by what holds paint best and is compliant enough to not chip from rocks. This is the opposite situation. Stiff composites are brittle composites, and they will always chip and crack under the slightest abuse. FG is very poor in this regard, and Kevlar is only going to make it stiffer.

I think the chipping / flying debris issue will benefit if the Kevlar is used with a very compliant epoxy. This way, the epoxy will give without cracking much and the Kevlar fibers will prevent the object from digging into the FG. However, the more impact energy it absorbs the nastier it will look afterwards. Maybe you can find a way to repair it so that a FG panel can last a lot longer.

I suggest a trip to the local ice hockey or roller hockey rink. Players are using (myself included) CF, FG, and Kevlar-reinforced sticks and blades. These things see impact damage you wouldn't believe. You might get some ideas here or buy a few samples to do testing on.

Dave

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07-05-02, 03:50 PM

#27