Do fibreglass and carbon fibre mix?

"Let's put carbon fibre in there," says the marketing director. "That stuff's stronger. And I can sell it as a high-end feature."

"Yup, yup," replies the shop foreman. "We can do that. It's a bit pricey though, maybe we could use just a bit of it mixed with the fibreglass."

Fast forward three years, and both men are scratching their heads over why the component- which was, according to the designer, more than strong enough in fibreglass alone- has failed catastrophically even though they added a "better" material.

TL;DR: Mixing different fibres in the same load path can lead to a component being weaker than it would be if only one type of fibre had been used.

Material properties

To understand why this is, we must consider the raw mechanical properties of the individual materials. For this example, we'll use E-glass (the standard grade of fibreglass used for boat construction) and Hexcel UHM 3K carbon fibre (a grade whose properties are typical of the carbons used in boats, but that is of considerably lower quality than aerospace grades). The properties of interest are:

  • Modulus of elasticity (Young's modulus), which describes how stiff a material is. Doubling the Young's modulus means that it'll stretch half as much under a given load.
  • Ultimate tensile strength, which is the stress (force per unit cross-sectional area) that will cause the fibre to fail in tension.
  • Elongation at break, which describes how much (as a percentage of original length) the fibre can stretch when pulled before it breaks. (Exercise for any sudents reading this: You should be able to figure out how to predict elongation at break if you know the Young's modulus and the ultimate tensile strength.)
  E-glass fibre Carbon fibre (Hexcel UHM 3K)  
Density 2.6 1.87 g/cm3
Modulus of elasticity 72 440 GPa
Ultimate tensile strength 3500 3730 MPa
Elongation at break 4.8 0.8 %

(I should insert a certain disclaimer here: These tables are not for design purposes, and you should be seeking project-specific engineering advice if you're actually building a high-tech composite assembly. This blog is not an engineering manual.)

Mixed-fibre example

Consider two identical components, both made from unidirectional fibres and both loaded primarily in tension. One is pure fibreglass; the other is 50% glass and 50% carbon. (We can, for the moment, neglect the epoxy matrix that forms roughly 60% of the volume of the part, as it is the fibres themselves that will bear almost all of the load.)

In the fibreglass part, the tension force is spread over the entire cross-sectional area of the part, leading to a stress (= force / area) on those fibres. If the part was properly designed and built, this stress will be much less than the ultimate tensile strength of the fibreglass, and the load can be safely carried.

In the mixed carbon/fibreglass part, the tension force is not spread uniformly over the entire cross-section. The fibreglass, which has to 'give' a bit to accept a load, wants to stretch, but it's held back by the six-times-stiffer carbon fibre. The carbon, then, takes almost all of the load, leaving the fibreglass nearly unloaded. In essence, the cross-sectional area of the part is reduced to the cross-sectional area of the carbon fibre; the fibreglass can't come into play unless it is allowed to stretch slightly, and the stiff carbon has already taken most of the load before that can happen.

We are applying the same force in both cases, but the stresses are not the same. If the stress in the pure-fibreglass part is a nice uniform $X$, the stress in the mixed-fibre part alternates between nearly $2X$ in the carbon regions and minimal in the glass regions.

As the force increases, the part will elongate. The strain (the elongation relative to no-load condition) reaches 0.8%, and while the fibreglass half is working at about one-sixth of its maximum stress, the stress on the carbon half edges past 3700 MPa. BANG! The carbon components of the mixed-fibre part fail, suddenly transferring all of their load to the fibreglass. In a tiny fraction of a second, the (now grossly overloaded) fibreglass also fails.

Meanwhile, the same force on the pure-fibreglass part is safely distributed, uniformly, over all the fibreglass strands.

The moral of the story? Don't mix materials with radically different mechanical properties in parallel in the same load path. The load won't be distributed evenly; rather, the stiffest material will take almost all of the load until it fails, transferring the load to the next-stiffest material.

Carbon is still pretty awesome

This should not be taken as a slight against carbon fibre. Carbon is an amazing material and, when used properly, produces some of the lightest and stiffest parts available. (Indeed, its incredible stiffness is carbon's single biggest advantage; it is this property, in combination with its light weight, that allows carbon parts to be so much lighter than conventional materials in stiffness-limited applications like masts and wing spars.)

I'm also not intending to knock the idea of using multiple composites in one boat. There are many cases where carbon and fibreglass components can get along very well together- a rudder with a carbon stock and a fibreglass foil, for example, can be a very sturdy and reliable combination. (The key here is that the two materials are handling different types of loads along different load paths; the forces are transferred from one to the other rather than being shared in parallel.)

If you're going to start mixing materials, though, you have to understand the load paths involved. Each material has to be used where its properties will do the most good, and the way loads will be shared between materials must be carefully analyzed; mixing them willy-nilly is likely to lead to unexpected failures like the one described above.




To clarify

Matthew's picture

I have had a few offline questions about this topic. To clarify:

Different materials in series (e.g. apply a load to a fibreglass part, which transfers it to a carbon part): Can be OK.

Different materials in parallel (e.g. some carbon and some fibreglass are mixed to share the same load): Not OK. The stiffer material (the carbon) will take most of the load, fail, and suddenly transfer all of its load to the less stiff material (the fibreglass).



This is very useful information. I'm finishing up a complex build of a John Shuttleworth catamaran and have been constantly thinking about information like this. What do you think about using carbon fibers to make connections, like bulkheads and beams connected to the hull?

Also, where are you located?

Willie Pack

Where to use carbon

Matthew's picture

Hi Willie,
Carbon really only makes sense in weight-critical or stiffness-critical situations. That applies to spars, poles, etc. and possibly to rudders, gangplanks, lifting foils..... but if you start using it for hull structure, you rapidly get into a spiral of spending more and more money for ever-smaller gains. I think you're unlikely to save any appreciable amount of weight by using carbon for bulkhead and beam joints, unless the rest of the surrounding structure is also carbon.
In any case, Shuttleworth engineering is (rightly) well-respected in the sailing catamaran field, and you're unlikely to go far wrong by sticking with the laminate specifications on John's plans.

We're based on the outskirts of North America's freshwater sailing capital of Kingston, Ontario.

Carbon fiber added to old fiberglass hull

I would like you to give me your advice regarding a boat that I am considering buying. It is a beautiful looking Alajeula 38, and the entire boat appears to have been well maintained throughout its lifetime. However, it was built in 1976 of fiberglass, and originally the factory build had a very heavy, thick, strong hull. The current owner of this sailboat appears to have been obsessed with a full restoration and modernisation, with new engine, carbon spars, sails, etc. even to the point of shaving back the hull half an inch all over, adding a 5mm foam insulation/core, and recovering the whole thing with carbon fiber instead of fiberglass. The hull was then faired an it looks smooth and great. The rationale for this massive undertaking being to lighten the hull while also making it stiffer and just as strong as before. Is that possible? I mean, the boat looks glorious and I am very tempted to buy, but not sure if the hull is safe. Can a 7mm thick carbon fiber hull be added to a fiberglass hull in that way?

RE: Carbon fiber added to old fiberglass hull

Matthew's picture

Is it possible? Yes.
Does it make any kind of sense? No.
The Alajuela 38 is a traditional, deep, full-keel double ender. They were built with a thick, heavy hull because this hull form does not penalize weight in the hull at all. There's no incentive to make such a hull lightly built, but there's a huge incentive to make it resistant to groundings and impacts. (That said, carbon spars will certainly improve her performance.)
Carbon spars will undoubtedly improve her performance, but I can't see any possible way that replacing a fair bit of the hull with carbon sandwich could conceivably be a good idea in such a boat. That's not to say it's necessarily unsafe - you'd need a vessel-specific engineering assessment to determine that - but it certainly doesn't do anything good to the boat's value.

fiber glass carbon fiber mix

i am looking at mixing carbon fiber to my production of fiber glass flagpoles, this is to make the poles lighter and stiffer ,in you opinion would this be possible and would i encounter any problems in doing so ??


Matthew's picture

Well, if you have both fibreglass and carbon in parallel in the same load path, then you're in the situation I described in the article - the carbon takes all the load, the fibreglass is just adding weight, until something fails. Better to go all-fibreglass or all-carbon.

Automotive Uses

This is great information and easy to understand. I think you already answered my question but I'm going to ask for the conformation. I wanted to purchase some carbon fiber panels for my car. The shop has offered me a carbon fiber/fiberglass mix which reduces the cost considerably. It's not something I have ever considered before. Obviously there is load placed on the panels when the vehicle is travelling (forward movement and, g-forces through turns) but is this really enough load to be concerned about for this particular application?

Cosmetic carbon fibre

Matthew's picture

Hi Chris,
What you're looking at there is most likely a fibreglass panel with a single, thin carbon outer layer for cosmetic purposes. Thus, you get the great look of carbon weave with the cost of fibreglass. This is a fairly common technique and, as long as it's not a highly stressed part, poses no problems.

Most body panels are not load

Most body panels are not load bearing. CF is a great solution for this as it's stiffness resists wind deformation (which leads to lower aerodynamic drag co-efficient).
Personally I wouldn't even bother having a glass component to the panel at all. A course Carbon weave matting as the base layer followed by a fine weave as the dress layer should be all you need.
Keep in mind though, that if you have an impact on that panel it will shatter due to it's lack of flexibility.
Fiberglass is a lot more flexible and will withstand small impacts better than carbon fiber, though it is a lot heavier (still lighter than steel).

CF Superstructure

Hi Chris,

I am about to undertake a boatbuilding exercise.
I am planning to build the hull out of tri-axial Glass over a Marine Ply core (it's a one off project).
I was considering using Carbon Fiber or woven Carbon Fiber/Kevlar over a foam core for the superstructure, the idea being to keep the Boat's center of Gravity low.
Now I realize that the hull structure is going to flex inwards due to hydro-static pressure, is this transferred compressive force going to cause problems for me using a carbon composite superstructure?

Add new comment