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I'm looking for a simple way (that doesn't require high pressure chambers or state of the art tools) to permanently bind a set of carbon fibre pieces in order to make the frame of a chassis that could be used in a go kart or dune buggy or even a boat? (watercraft is just a bonus, it just needs to support the weight and impacts of a 300kg vehicle, including the driver).

The raw pieces of carbon fibre are as follows:

enter image description here

  • 2 x 10mm square/round tubes (up to 1500mm length) For the outer frame's longitudinal beams (shown in blue)

  • 6 x 10mm-by-5mm strips (up to 600mm length) For the outer frame's lateral beams and cross-diagonal support beams (shown in green)

  • 2 x 10mm rods (up to 800mm length) For a longitudinal reinforcement, also to be sticking out as an axle mounting point (shown in black)

  • 1 6mm thick (very large) sheet of carbon fibre cut to shape and cover the chassis, walls and crevices (not shown)

The methods I'm aware/capable of are:

  • Drilling/tapping screw holes
  • Using epoxies and other chemical adhesives
  • using carbon fibre vinyl as a tightly wrapped joint
  • Using ropes/cables where a long piece protrudes from the body
  • Welding...

That last method I believe is useless in a carbon fibre build, however I have plenty of access to steel materials and the tools to cut/bend/drill them into virtually anything, would this serve any use for my build?

I'm thinking about drilling holes to make mounting points which I can then glue together and seal off with a washer or bung or a locking stopper.

My real questions are (TLDR):

  1. How does drilling holes into carbon fibre affect its structural integrity?

  2. Which bonding agents are ideal for carbon fibre in a structure that needs to withstand impact from many directions (especially up-and-down)?

  3. Is it beneficial to use both steel and carbon fibre together in the same structural component?

  4. Are there any composite material alternatives to Carbon Fibre that are just as, or possibly more applicable to this type of structure? I'm looking for something with a light weight, high tensile stress, resistance to abrasion and of course, a decent price.

Thank you. Please forgive if this question is elementary, I've not been researching composites for long but can't wait to build something.

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    $\begingroup$ Do some research, there are many papers about how to fix composites. But glueing them to a surface may not work if the composite delaminates... How are they used in F1 cars? On aeroplanes? $\endgroup$
    – Solar Mike
    Jun 5, 2021 at 13:52
  • $\begingroup$ How are they used in F1 cars? On aeroplanes? With moulds and ovens that cost six figures $\endgroup$
    – Alfred
    Jun 5, 2021 at 14:42
  • $\begingroup$ Some companies spend 6 figures a second on day to day running costs so moulds & ovens are cheap... $\endgroup$
    – Solar Mike
    Jun 5, 2021 at 14:43
  • $\begingroup$ Exactly, I'm not expecting to replicate the quality of F1 engineering lol, just a clandestine alternative that can still take advantage of most, if not some of the epic qualities of carbon fibre, Teflon, Kevlar, etc. Trying to keep the structural design as simple as possible but the most important thing by far is to use appropriate joints/welds/adhesives. I'm sure my frame pieces will be strong enough to handle a chassis, question is: how feasibly can one bind said pieces in their garage? $\endgroup$
    – Alfred
    Jun 6, 2021 at 7:56
  • $\begingroup$ If it's just a chassis you need no heavier than 300kg your best bet might be welding tohether say 150kg of steel bars... like solar Mike said; when it comes to carbon fibre vehicle components, they're almost always made as a whole bespoke piece. Not sure if you have a mountain of sheckles to buy this equipment and maintain it but you can probably hire a facility with professionals to mould your entire chassis, I imagine it would weigh 10-20kg and cost anywhere between $500-$2000+ $\endgroup$
    – Jalapeno
    Jun 6, 2021 at 9:29

2 Answers 2

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This is Work-in-progress

  1. How does drilling holes into carbon fibre affect its structural integrity?

Drilling holes into carbon fibre has very adverse affects. The main reason is that usually the matrix is some sort of thermosetting resin, and as a result it is relatively brittle. The thing is that the drilling process, will separate fibres from the matrix (delamination). Those cracks are bound to grow if there is any type of dynamic loading/vibration.

So although it is possible to drill holes, in general its not a process that its recommended.

Usually, the best way to have a hole depending on the technology you are using e.g. prepregs or actually winding carbon fibre and then using some type of molding. For example if you are using prepregs, you can drill the holes before curing, place and insert, then cure, and then remove the insert.

IMHO, the best structural integrity is obtained if you are winding the carbon fibre and you are using a metal part for the joint.

enter image description here

Figure 1: Aluminum joint for Carbon fibre tube. source(MadeinChina

  1. Which bonding agents are ideal for carbon fibre in a structure that needs to withstand impact from many directions (especially up-and-down)?

This will probably depend a lot on the matrix you are using for the carbon fibre composite system.

(Also, if you are looking for ideal, then probably you would need to have a six or seven digit account to research what is the best bonding agent for your material).

  1. Is it beneficial to use both steel and carbon fibre together in the same structural component?

Not necessarily steel, but also aluminium or titanium, which are also lightweight. Of course, it can be beneficial if you are using each material to its strengths.

  1. Are there any composite material alternatives to Carbon Fibre that are just as, or possibly more applicable to this type of structure? I'm looking for something with a light weight, high tensile stress, resistance to abrasion and of course, a decent price.

Another option would be glass fibre with a resin system, although the weight and stiffness are significantly lower, however the cost is comparable. There have been even "production" cars like the Lotus Elite which in 1957 pioneered the Glass-Fibre Monocoque chassis (although the idea of the monocoque is different from the chassis you are developing).

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  • $\begingroup$ hi I really like your answer. I'm thinking prepregs are my perfect solution, could you please elaborate a bit on "winding", is it a way to make screw-threads? In metal work we call it tapping. From your answer I can tell drills are a no-no, the winding is achieved by shaping a mould by screwing a hole in the drying mould? Or do you just make the entire rod with a mould that has a screw inside it while it dries/cures? Would this moulding process be possible at home with layered sheets or cheap materials? Would it be more feasible to just go to a workshop and hire them to mould it? $\endgroup$
    – Alfred
    Jun 6, 2021 at 8:10
  • $\begingroup$ @Alfred, apologies for not replying sooner. I completely forgot, because the answer would be quite lengthy. And from what I saw Jollerprutt's answer is at least 90% of what I was intenting to write. So I am upvoting it, and I suggest you take note of it yourself. $\endgroup$
    – NMech
    Jun 10, 2021 at 10:13
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Cool that you're considering CF, it's a fun material.

So first, to answer your questions.

  1. If done right it can be fine, but even then it's the least preferable (in my opinion) way to bond FRPs (Fiber Reinforced Plastics). It's hard to do well without special tools, especially in curved and/or hollow profiles, since you need to support the material around the hole on both the entry and exit side. If not done right you will most likely get de-lamination and open up to a broad range of ways for your structure to fail.
  2. General answer, Epoxy. Try a one-component epoxy adhesive for high impact resistance.
  3. It can be, depends on application. Aluminum is often used due to low weight and price, tough very sensitive to corrosion and galvanically a terrible match for carbon. Titanium works well but is expensive to process.
  4. Wood :) Glassfiber. Carbon is preferable for your structure though. Note: Carbon and glass FRPs are not resistant to abrasion, in fact this is the preferred way to cut them. Because the fibers and matrix (plastic )are brittle. Composite structures often utilize metals and engineering plastics like Acetal to deal with abrasion. If you need abrasion resistance during a crash you could incorporate some aramid fibers (like Kevlar).

About your actual question, how should you bond your structure together? Since you asked for a permanent bond I would recommend you to steer clear from nuts and bolts.

I would suggest you try the following.

  1. Place all your parts in their desired positions, a simple wooden jig with some clamps will make it easier.
  2. Bond the parts together with a non-sagging epoxy adhesive, give it time to cure (harden).
  3. Wrap the joints tightly with carbon fiber weave or NCF soaked in epoxy resin, let cure.

Tools needed. Buckets to mix in. Sticks for stirring. Gloves for protection. Scissors or knife to cut carbon mats.

Important!

  • Always use protective gloves! Contact with uncured epoxy can make you allergic to epoxy. Oil from your skin will interfere with the bonding. Cut (or sanded) glass and carbon fibers are very itchy and annoying ;)
  • Don't mix to much epoxy resin at once! The reaction is exothermic and can cause a thermal runaway that might result in a toxic fire.
  • Always sand down any surfaces where you plan to use epoxy, don't apply epoxy to glossy surfaces.
  • Practice on some scrap pieces, the bonds are permanent.

Regarding your design.

I feel like your structure is severely underdimensioned if the plan is to survive a crash, sorry if I sound harsh.

  • I think you want something closer to 40+ mm tubes for your blue and black members, skip the rods and go for tubes.
  • Maybe 30+ mm tubes for the green members, strips are terrible for all loads except tension.
  • Do not even consider using vinyl wrap for load bearing joints.

Cheers :)

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