Do Carbon Foils Fatigue Over Time?

As I was comparing foils the other day I noticed that some are clearly stiffer than others. This shouldn’t be news to anyone. However, I’m wondering if there is, or should be, any noticeable fatigue in foil stiffness over time?

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From my knowledge of carbon through bikes - carbon fatigue is generally through micro cracks in the resin holding it together (which can occur through repeated flex), delamination of the carbon layers, or breaking of the actual carbon fibers. Carbon can obviously crack through impact, but the question was focused on fatigue. I supposed repeated flex of a mast or wing could eventually fatigue it, but I personally haven’t seen or felt it.

The leverage and construction complexity of a foil box is definitely enough to fatigue it though.

Interesting I just finished gathering data on 3 foils all of the exact same model:

900 area
935mm span
11mm thickness.
Room for deflection: 8mm until fuse touches.

The first foil with approximately 500 miles of use on it laid on a flat surface with weights added to it deflected the full 8mm at:
35lbs.

The second foil that is newer, minimal use, deflected the full distance at:
55lbs.

The third foil, brand new, never ridden, deflected the full 8mm at:
75 lbs.

So the question I’m digging at here is: should the be perceived as significant fatigue being observed in foil stiffness? Or, should this be considered a significant variance in manufacturing?

I’ll be able to test the new 900 and can check it for fatigue after a few rides but I’m a little in shock over such a large variance.

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Jumping in with a couple observations:

Mast mounts: Universal mounts like NoLimitz and Cedrus that don’t fill the entire fuse pocket - I suspect because the tortional load is concentrated on the smaller surface area of the ‘prongs’ it stretches out the pocket faster. Antidotally, the Lift 170 I had on my NL seemed to have more wiggle in the connection over time.

Masts: I would suspect carbon masts have more flex the longer they are used, especially at longer lengths. I rode a new mast and an old mast on the same set up and maybe its confirmation bias but I could swear the new mast was a lot stiffer than the old one. If anyone has a new and old version of the same model/size mast would be super curious to see numbers like @Bwalnut is doing.

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Room for deflection: 8mm until fuse touches.

Could you describe the set-up, I’m having trouble visualizing where you would deflect a foil by 8mm and hit the fuse.

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flexing out the camber.

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Fatigue with composites is quite complicated. A well designed composite structure shouldn’t suffer from stiffness changes due to fatigue. But if the loadpath results in a state of stress that is resin-dominated (more stress in the resin than fibers), then you can experience reduction in strength and stiffness. This is why we’ve always had an aluminum mount. Monocoque carbon masts put significant stress in the resin at the board interface due to what is called “inter-laminar tension.” Aluminum is much stiffer and stronger than epoxy. The long answer is discussed extensively on our blog, actually as it relates to the Titan failure: https://projectcedrus.com/general/thoughts-on-titan/

@Tanner.0 wear and tolerance stackup at the fuselage interface is a different subject entirely. Most of our adapters are designed from OEM supplied CAD and fit the same. The load transfer from the adapter to the fuselage is going to occur at the fore and aft ends of the pocket, near the corners (where it’s stiffer) and the attachment screws. The middle of the pocket does not contribute much at all, which is why we remove that area of the adapter. But most of the slop in those connections is actually due to an over-constrained design. You can’t have a proper tapered fit that bottoms out on 3 surfaces. You can only pick two. But a lot of interfaces try for a fit where the sides bear out and the bottom touches. As the sides wear due to abrasion, you can’t go any deeper because it’s bottomed out, so you have slop. A properly designed tapered joint, or mortise and tenon, should not bottom out at the bottom. But again this is unrelated to the original question regarding fatigue.

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So really need a Tuttle box but it’s only one position.
What’s the closest design on the market to what you’d consider an ideal adjustable design from all the adapters you see fit your mast, best mast to fuselage.

I find carbon wears at the mast fuse connection. I like carbon masts as you can sand down the top of the mast 1mm or so and get it tight again. On no limits and Cedrus have to tape it. If you get a little wiggle the wear happens much faster so try to keep everything tight.

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Some thoughts from the board and foil builders for causes of this fatigue:

“Micro tears in fibres and resin matrix bonds failing little bits at a time”

“Possibly the materials weren’t fully saturated with epoxy.”

Real world feedback and thoughts formulated today:

My original 900 became difficult to get on foil as the miles stacked up. I assumed it was a tail issue but now assume it’s related to foil flex.

I tested the new 900 today and it jumped out of the water. Zero water start issues when tested with a new, as well as old tail that I thought was trouble.

Follow up deflection testing is needed and I confirmed that the 55lbs tested foil only had 3-4 sessions on it. As such, I should only need a handful of days on the new foil to confirm if there is an active level of deterioration happening in these.

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“Possibly the materials weren’t fully saturated with epoxy.”
Excuse my amateurism but is that even possible with prepreg carbon?
Aren’t all foil brands building with prepreg? Partially for this reason?

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The construction practices happening at various factories are beyond my knowledge.

actually very few brands are using prepreg. But even with prepreg, yes it is certainly possible. Without adequate pressures during molding, you can get poor consolidation and wetout. The rheology of the resin is very complex, and if you don’t follow the proper temperature/pressure/time cycle (all resin specific), you can suffer from all sorts of issues with prepreg.

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Theoretically if things were done incorrectly:

I assume a slow and steady degradation would be expected until the performance loss became too great to ignore?

Or should I expect an immediate issue, noticed on day one?

It’s possible that the core is breaking down over time rather than the laminate

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Interesting. Is this because the laminate isn’t stiff enough to protect the core? Or would this be because the core was soft to start?

Sounds like the same issue as a surfboard losing its “pop”.

My guess is that the shear forces on the core material itself or at the interface between the laminate and the core are leading to more flex in the wing over time.

In my experience carbon degradation and fatigue is a product of flex. Any kind of movement is going to lead to degradation. This is why this idea of “good flex” is nonsense. It inevitably leads to failure

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The core takes the shear stress between the top and bottom laminate and maintains the thickness - if the core starts to break down and soften or crack, the skins can move relative to one another adding to flex. My guess is that if your wings are changing over time, it’s probably in the area where the fuselage is blended with the wing since there are corners (leading to stress concentrations) and the laminate is out of column as it wraps into the fuselage area. I’ve seen Lift wings that weep water at the TE/Fuse join, indicating that the bonding adhesive and/or core is starting to fail due to repeated flex (probably torsional).

Would be interesting to check your wings in torsion, mount the new and old ones to your mast and see if there’s a difference when you load up one side of the wing

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