Reinforcing fiberglas with steel?

Howdy Guys, new member here so be gentle .

I need to reinforce some bolt holes in some fiberglass and thought I'd pose the problem to the experts.

The 'glass is only about 1/16" thick and the 1/4" bolt holes are close enough to the edge that they need some kind of reinforcing because they've broken out their tops. I can sandwich in some 1/16" thick steel to add strength to the holes but I don't know the best way to bond the steel to the fiberglass (i.e. with resin, epoxy, or ?). Whatever bonding means I go with has to be capable of filling in a few irregularities as deep as 1/8".

Also, do I need to coat, paint, or plate the steel reinforcement before laying it in so as to protect it from rusting? (Note: The reinforced part will not be immersed in water but will be subject to occasional splashing by fresh water only.)

Thanks,

Harvey
Houston, TX

 

Welcome to the forum.

Yep, some steel will work, but there are better materials too. Steel tends to rust, which will bust out of a laminate, particularly if there are penetrations (fasteners), so I'd consider a some G-10 or other inert material, so you don't have issues down the road.

Can you provide a more descriptive idea of what you're doing? Maybe some pictures? Most of us will just "bulk" up the area with more laminate or use a backing plate (such as you've envisioned). Aluminum might be a better choice, but a lot depends on what you're trying to hold up. Would threads in the backing plate be beneficial? What's getting bolted/screwed down, etc., etc., etc.

 

PAR,

Thanks for your reply. Due to the difficulty of describing my project, I've sent you an E-mail.

Harvey

 

What can be said in a email that can't be said here?
Metal reinforcements tend to peel off due to the difference in stiffness and thermal expansion.
Steel is almost the worst material for weight and corrosion. Local reinforcements of the same laminate is the most practical method.

But it depends upon the application, which we can't see.

 

Steel is absolutely the wrong choice. I've learned that the hard way when reinforcing a roll bar with steel tubes and wire mesh. After a few years the once smooth surface started to crack and bumps formed because rust expanded under the grp.

The only metal reinforcement that causes no such problems is perforated stainless steel plate.

 

If you are talking about a backing plate, they only need some bedding compound in between.

 

Guys,

Thanks for your replies so far. I received a very nice e-mail from “PAR” this morning and he assured me that I won’t be tarred and feathered for posting a non-boating question on the BoatDesign forum and he encouraged me to post all of my details so everyone can get a better handle on my project.

You see, Guys, I’m not a boater (red-faced, looking at feet, and kicking at rocks). I’m a pilot (quickly ducking!!!) A Google search yesterday on repairing fiberglass brought up the BoatDesigner forum and that seemed a better-informed place than posting my question on one of the airplane forums and attracting a bunch of pilot’s half-baked opinions! However, the rules for posting on your board stated that the post must be boat-oriented. I figured that my fiberglass reinforcing questions were appropriate to boats and I’d be okay with your forum's rules as long as I didn’t mention I was working on an airplane.

Now that that's not an issue, I’m working on a little two-seat Cessna 150 and its fiberglass nosewheel pant has damage at two of its attachment holes. The pant is predominately held on by the wheel’s axle but is prevented from slipping by an “anti-rotation” bolt at the top. It’s the two anti-rotation boltholes that are damaged. (See picture 1)

The wheelpant is constructed of common (i.e. mid-1970s, non-exotic) fiberglass that is, otherwise, in very good condition. The 'glass is about 1/4" thick at the 7/16 diameter axle holes and is in good shape but the thinner (~ 1/16” thick) 'glass at the 1/4” dia anti-rotation boltholes have broken out at their tops. (Picture 2) I only have room for a 1/16” thick reinforcement and figured that a horseshoe-shaped steel doubler would be stronger than 1/16” more fiberglass. So I made a trial piece using .060” aluminum (Picture 3) that turned out very well. I figured that it wouldn’t take long before the steel started rusting and causing problems (3 minutes at most in the Gulf Coast area!) so I figured that “boat guys” would know what to do.

As it turns out, none of you above thought my plated steel idea was very good and suggested that I consider G-10 or stainless steel. I don’t know what G-10 is (yeah, I know… PILOTS!!!) and my experience with drilling SST in the past has not been favorable.

Sooooooo, now that you know the background story behind my initial post, what are your suggestions?

Harvey

Sniff, sniff… That isn’t hot tar is it???
.
.

 

Go with G-10 epoxied in place, then drilled out and bolted in place. Same as the above.

G-10 by the way is a high density epoxy board (with fillers, ect...) that is very stiff, and easily machinable. It is commercially available in a lot of sizes, shapes, and thicknesses. Because it's an airplane there may be an advantage for regulatory issues if you use what's called FR-4 instead of G-10. It's basically the same thing, but FR-4 is fire retardant.

 

Nice to see someone use the appropriate tools. Are those Clecos holding the pant to the aluminum support on the strut? Not much material to work with and though G-10 was my recommendation, I'm now thinking you just don't have the room, so I'm changing my mind and recommend the whole of the fastener hold on area, be reinforced with carbon fiber. How much room above the pant do you have, before risk of crush is a real potential? In other words, can you increase the "meat" around the fastener holes, with some portion of the strut assembly coming down and bashing it to bits?

In fact, since the existing laminate seems pretty thin, I'd grind away a good portion of it, maybe even a couple of "paths" angled down away from the fastener locations, so I could lay some 2" strips of carbon tape, to extend the reinforcement down into the main body of the paint. This would introduce a good bit more "fairing" to the project, but the strains would be borne by a lot more area. I'll assume the pant is polyester, so use epoxy as the resin system.

If you think simply bulking up the anti rotation holes a bit will do, you can skip the carbon tape strips, but the area decidedly needs more material. The way I see it, there's significant torsional loading on the pant, tweaking it sideways, plus vibration form airflow and unpaved runways, so my logic is to kick it's butt, rather than have to do this again a few years.

 

PAR,

Yes, they're Clecos and they're holding the currently-installed horseshoe-shaped aluminum "trial-fit" doubler to the wheel pant. My plan was to replace the Clecos with rivets after I fabricated the final reinforcement piece and epoxied or fiberglassed it in place. The thought behind my horseshoe design was to not only replace the broken-out original fiberglass sections but to also distribute the stress load over a greater part of the fiberglass wheelpant than original. If I did use aluminum, I would have it anodized to protect it from corrosion.

To answer your question, look at my picture #2. The shiny silver area with the hole through it is the steel nose strut and just below the hole is a flat spot. That flat spot is the top of the cast aluminum nosewheel fork. (The strut and fork are permanently pressed together to form one part.) The flat spot on top of the fork is where the steering arm assembly sits and the hole in the strut is for a securing bolt for the steering arms. That assembly overhangs the flat area about six inches on each side and is flush with the top of the flat spot. That would be the limiting factor as to how far upwards I can reinforce the anti-rotation boltholes.

I have a modest machine shop in my hangar so I CAN work SST if I have to. However, I would think that a .063in thick piece of 6061T6 or 7075 (both are hard aluminum) would definitely be stronger than the flimsy original fiberglass anti-rotation bolt ears.

Harvey

 

Welcome to the Non- boat section - just a little joke.
We get a fair number of not quite boat questions, about materials/ techniques which might still be applicable.
Unless someone is a total jerk, no one minds, or they just go to the next topic.

Can you tell us if the pant is actually made with fiberglass? Or does this look like it is just molded plastic? Either way you can make the same fix, but having molded plastic might result in a less good bond, depending on the type of plastic.

Typical fixes like this involve sanding down the broken off segments to taper the laminate. Put a backing board (whatever will make an appropriate surface to get a good bolted interface) with something to allow it to be removed when the cure is finished. Packing tape for mailing packages works well with epoxy.
Sand off the outside finish for a ways, so you can get a good bond over a larger area. Lay small patches in the tapered area gradually getting larger to fill the taper. Then add two plys over the larger area you took paint off, to be sure and add back the strength.
Trim the upper edge, sand out the surface to make it fair, and repaint.

It is obvious you have room for one or more plies on the inside. You should have more than the original strength.

I didn't say but you need to use epoxy to get a good bond and good strength.

It is possible to use the aluminum doubler shown. The Gougeon brothers West system tech line suggests coating the pant with epoxy, sanding the surface of the aluminum, coating with epoxy, then "wet" sanding thru the epoxy, to insure the fresh surface is always protected from air before bonding. You should get a very strong bond. You still might want to do the above fix to restore the looks of the pant and give a little more strength.

You would probably get away with the aluminum doubler.

There is one suggestion, you will probably get a half dozen more.

 

Not wanting to start off on the wrong track, I just went out in the hangar and checked and lo and behold, it might be plastic! Nowhere could I find any evidence of any woven cloth under the surface. I looked on the inside (which has been painted with overspray from the outside color coats but has not been "finished") and also looked at the edges where the anti-rotation bolthole "ears" or "tabs" have broken off. (The edges were clean with no obvious fraying).

While I don't know if my pant is the same but Cessna has been using a PVC-type of plastic for non-structural components such as wingtips and interior panels for many years, including my plane's year model.

So, I guess this now changes everything?

Harvey

 

No you can still do the same thing, but the taper sanded area for making a reinforcing laminate now has much lighter strength to be bonded to.

My elaborate fix might be a waste of time.
If you want to retain the look of the original ears you might still do it, and the strength would be greater, but you probably have plenty with the aluminum doubler.

The only thing about aluminum, is that it doesn't do as well in fatigue (continuous vibration in this case) as you might expect. Of course, since we don't know the vibration enviornment any guess at the life is just a guess.

You have a choice of fixing it with unknowns or just go buying a new pant.
I'd fix it. If the fix fails the tire will just wear it away, quickly. My GUESS is that it wouldn't be dangerous, but strange things happen with aircraft.

Good luck

 

It appears that the axle bolt provides most of the "attachment strength" while the anti-rotation bolt provides some additional strength but is mostly there to keep the wheel pant from trying to pivot around the axle.

Harvey

 

I'm under the same assumption Harvey, the size and location of the anti rotation pin seems to suggest load is borne on the fasteners.

Your aluminum backing plate should do (1/16"?) and this might just be enough to offer additional support, once the broken up laminate/plastic is repaired.

Take a grinder to the broken areas, looking for fibers. Better yet, you can just take the broken piece and burn it with a torch. The resin will burn off, but the fibers will remain.

Log onto westsystem.com and download the free user's guides about bonding, aluminum and plastics treatments (carmalization). I personally think it's polyester resin and mat (a 'glass fabric), but it wouldn't be the first time I was wrong. In either case the repair will be similar with possibly some procedural differences. Epoxy is really the only good option for this type of work, other than mailing the pants to someone to have them do the repair.