filling holes in an unstayed carbon mast

[rxcomposite]

"High-Strength Lightweight Carbon Fiber Rods: Their fibers are aligned in a single direction, so they hold up to bending across the fibers but will splinter and separate like string cheese when bent along the length of the fiber"

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

Commercial products such as CFRP will do. Uni fibers laminate will split if alone so it is usually wrapped in +45-45 fabric. Uni Fibers construction when it is fixed in a hole, is very hard to split.

 

[marlinspike]

Commercial products such as CFRP will do. Uni fibers laminate will split if alone so it is usually wrapped in +45-45 fabric. Uni Fibers construction when it is fixed in a hole, is very hard to split.

I think there is basically no way I am laying up something in my garage that is going to be as dense and strong as a commercially manufactured carbon fiber rod, and those are only twenty bucks... just intuitively, I figure I would want the unidirectional fibers aligned lengthwise along the mast, as that is where most of the compression and tension is.

I am not a naval or composites engineer, but I am an engineer with a physics background, and am able to understand the concepts if you guys point me in the right direction. I appreciate all of the advice I am getting on here.

 

[rxcomposite]

I think there is basically no way I am laying up something in my garage that is going to be as dense and strong as a commercially manufactured carbon fiber rod, and those are only twenty bucks... just intuitively, I figure I would want the unidirectional fibers aligned lengthwise along the mast, as that is where most of the compression and tension is.

Commercial CF rods will do. When the 20/50/30 proportions are laid up, the resulting average strength will be much less that the laminate strength of a uni longitudinal strength. I use the Rule of Mixture (ROM) method or the matrix algebra method. Engineer with a Physics background is great. This is basically Mechanical or Civil Engineering formulas.

 

[marlinspike]

It looks like all of the carbon rods have fibers oriented at 0 degrees (lengthwise), so might not have much strength in the direction of forces on the plug... now considering going back to jehardiman's advice of using a metal plug.

 

[fallguy]

Just my builder perspective and not AI. I think it would be rather difficult to put a socket head machine screw into what looks like a #10/12 self tapping screw hole with good success filling the threads of the original tapping screw.

Perhaps you stated somewhere that these are machined holes and I missed it, but they don’t appear as such in the photo.

I’m not dead set against the method proposed by Hardiman, jist pointing out the part that will be easy to do wrong. Unless I did not understand the process. Drilling out a self tapping thread and tapping a machine screw thread seems plausible. Then turning the screw into high strength and fairing over…etc.

And perhaps I missed something?

For me, I realize same fiber alignment seems ideal, but I would prefer fibers in carbon in the repair to run the hole depth or random and not the width. My reasoning(intuition only) is it seems possible pressure on the mast could cause a layered plug to pop apart. I suppose the layer over top is intended to reduce this potential, but I don’t like the idea of any large carbon layer added to the mast. Most of this is intuition about failure modes or adding strength where not wanted…

You have excellent help now with RX contributing. I’d 100% follow his advice and never look back. This means to use commercial CF rod. Then you drill out the threads? and add back the rod with high strength epoxy.

Personally, I see little benefit adding cf laminate to the mast and unless RX told me, I would not because it seems like a mast mod more than a patch, unless the patch is rather small and would not contribute to the mast strength in any way…

I learned here that epoxy and cab is not nearly as hard as I previously thought, but sanding it is misery just the same!

 

[tropostudio]

I did receive some advice from Ted Van Dusen, the engineer who designed and built the spar.

Apparently, the correct method here is to create a custom tapered carbon fiber plug the thickness of the mast, taper the hole to fit, and hammer in the plug with epoxy for a tight fit. Then, sand the surface down to bare carbon around the holes and epoxy on a carbon patch with tapered edges of 1/3rd the spar thickness, using alternating layers of unidirectional carbon cloth going vertical (lengthwise) along the mast, and woven mat.

From the above, it seems Ted van Dusen is suggesting a repair that will address compression and buckling concerns with the tapered, 'hammer-fit' plug; and that tension concerns would be covered by the tapered surface patch. I'd think just adding a compression plug does little to address tension concerns, but maybe that isn't an issue.

@rxcomposite suggested a plug comprised of compression-molded chopped uni to approximate the properties of a filament wound CF mast. Sounds like that could be an easier way to make the 'plug' portion of the repair vs laminating up a sample and cutting a tapered plug from it. You could conceivably use the same taper cutter used for the carbon holes to taper the plug mold.

The McMaster CF rod will be pultruded CF (100% uni) - probably very high fiber volume. Would a taper-fit plug out of this work well in compression here? Take a short bit of that rod with fibers oriented horizontally. Apply a lot of pressure. It'll fracture. Is the tapered hole in the existing mast constraining such a plug enough to make a good compression fix? My gut says no. Others?

 

[fallguy]

The fibers can run the other way. The build effort and epoxy bond is more vital. If you drive the plug in and all the epoxy is forced out; the weakness is not the cf rod. If the bond is marginalized; it is far worse.

If the epoxy bond strength is X and you force almost all of it out; the bond strength drops greatly to X/2 or worse and quickly. So, workmanship is vital. A bond margin of 1mm would be my goal.

This reminds me of the titanium rings on the sub that failed. Not only did they smear on the bonding agent, but nothing they did appeared to maintain the bond margin, unless it was designed in a tapered cut. For this work; it would be easy for OP to hammer tap the 1/2” plug to flush and think he was good, but not so.

Maybe rx has some tricks for bond margin, but I would oversize the plug length and determine the plug depth where the plug is 2mm smaller than the hole on the outside edge and push only to that line.

 

[marlinspike]

Maybe rx has some tricks for bond margin, but I would oversize the plug length and determine the plug depth where the plug is 2mm smaller than the hole on the outside edge and push only to that line.

My concern with that is that the compressability of the 2mm of epoxy prevents the plug from carrying a load. If the plug is tapered and hammered in with a patch on the outside, I'd imagine it would in technically carry compressive loads even if not bonded at all.

 

[marlinspike]

Just heard again from Ted Van Dusen- he said "vertical" means 0 degrees, e.g. along the length of the mast is the direction 75% of the fibers go, but using a plug made from a McMaster Carr rod with the fibers facing radially outward from the center of the mast should be fine, with a patch on the outside holding the plug in and adding additional support.

 

[fallguy]

My concern with that is that the compressability of the 2mm of epoxy prevents the plug from carrying a load. If the plug is tapered and hammered in with a patch on the outside, I'd imagine it would in technically carry compressive loads even if not bonded at all.

See what RX suggests. I don’t think you want no bond or a smashed in tight plug, but that is just my intuition.

 

[tropostudio]

I'm going to disagree with you a bit here, @fallguy. According to OP, Ted van Dusen recommends as first half of the fix that a tapered CF plug with properties similar to the mast section would be hammer-driven with epoxy into a corresponding tapered hole. I infer that's about making a tight press fit designed to take up compression loads with bthe epoxy filling any minuscule voids beteen the plug and mast. It ensures there is not an annulus of material with a much lower E than either the CF plug or CF mast that would make the plug inconsequential as a compression fix. The tapered surface patch would help retain the plug, take up some of the load in compression, and carry tension across the holes, spreading it into the existing mast via sufficient secondary bonding area in shear. Without the surface patch, tension could make the tapered plug less effective.

 

[tropostudio]

Sounds like @marlinspike has a fix approved by the mast desgner/builder that saves the hassle of laminating a custom plug! Good luck with the repairs.

 

[rxcomposite]

The goal is to mitigate the loss of strength in the open drilled holes. The best approach is to plug them with a similar material of the comparable modulus and CTE so as not to stress it further. I suggested fabricating a plug made of short fibers. The fibers would the align along the fibers of the original lamination. @marlinspike does not want that and prefers $20 pultruded carbon rods. That will do except that then the fibers are at 90 degree to the original fiber alighment. No problem. It is in shear and the plug has nowhere to go (like a rivet), thus it is basically in compression.

I think the industrial adhesive 3M 2216, is the same "goey stuff" that was used in Titan. It is a very strong, aerospace grade but not strong enough to withstand that depth/pressure. Plus the method of application was wrong. We used to vacuum the joint for void free sealing. The data sheet specifies a certain amount of thickness. Ordinary low viscosity high strength epoxy will do for a tight fit plug.

 

[Fellies]

If thats the route, why dont you just buy some uni and vac bag it in a stack in your garage and then shape it, tapered it slightly and bond them in. I think your biggest hurdle would be finding the correct glue to allow for the required tensions on the mast so the plug stays true

Either way it really seems like its overkill. The screwes are the strength and using new ones with counter sink with a tiny bit of silica and mf to make sure it blends nicely would anyways give u a nice surface over you could just spray.

Just my opinion.

 

[fallguy]

Would this work for plug building? For $100 and shipping; if it can work…

I’ll not go on arguing about bonding, but it seems important to bond some.

High-Strength Lightweight Carbon Fiber Sheet 6" x 6" x 1/2"
McMaster-Carr https://www.mcmaster.com/product/8181K111