Talk Composites - The Forum for Advanced Composites

Best for Screw Retention Composite Fiber

By thelongride - 10/8/2020 4:42:35 AM

Hello, so I'm working with nomex honeycomb panels with composite fiber on either side. I have a series of screws entering one side of the laminate, but not exiting the other. My question is what composite fiber would be best at retaining the screws (like most resistant to a normal force)? Previously I've been skinning the honeycomb entirely with carbon. But the screw retention feels a bit weak.

The application here is honeycomb cored skis.

A few options:

And I'm guessing these are not the way to go, but perhaps?:

I'd put down carbon, but from past experience it doesn't provide much support to holding screws in the laminate

By thelongride - 10/9/2020 11:08:27 PM

Lester Populaire - 10/9/2020 1:07:28 PM
thelongride - 10/9/2020 2:47:14 AM
Lester Populaire - 10/8/2020 8:06:50 PM
thelongride - 10/8/2020 5:06:23 PM
Lester Populaire - 10/8/2020 9:08:32 AM
I built a lot of skis with balsa cores where you run into the same issues. I would recommend machining a pocket and fill it with glass and then some birch plywood. With the top skin this will form a micro sandwich that will securely hold the screw long term.
If you only have a single layer it will wiggle lose over time and ripp out.

And if you don't want to go that route i would recommend a 0.8mm or more thick titanal layer locally. Aluminium is much better for this application than composites as it's much tougher.

Lester, this is slightly unrelated. But on your balsa skis did you use vertically laminated strips or endgrain? I've heard vertically laminated simply doesn't hold up to shear forces, but I've been curious to hear from someone with direct experience.

I used cores from Bcomp which are engineered specifically for the application. 

I used a lot of end grain balsa back in the days for kiteboards which has extremely good mechanical properties but can be a real pain in the ass to machine to thin thickness. I would not recommend using flat grain balsa and not just because of shear strength, but mainly because of peel strength and low stiffness in the out of plane axis which will promote face wrinkling at lower stress levels.
End grain balsa will absorb more resin and this is the only reason why the shear strength is better over flat grain as both directions are 45° off axis to the load direction for this specific load case.

I hope this gives you a bit of an overview. Keep us in the loop with your skis. I am really curious. Are you building competition touring skis?
I am currently in the process of building a pair of super light freetourers. target is around 1kg per ski with full size base and edges and sidewalls for a ski 1850 in length and 90mm waist. See how that goes...

Yes, they're for skimo racing. Built a few with with paulownia cores, but thought I'd branch out. The micro-sandwich is a great idea.

If I were I to use flat grain balsa as the core for this binding reinforcement micro sandwich, as opposed to birch ply, would you foresee the same issues with peel strength?

Good luck with the tourers, if you drop the base and edge thickness it's easy to hit 1kg Wink

No peel strength is really important in the area around the screws. Isn't there a 700g weight limit on touring race skis? should be fairly easy to hit i feel without making big compromises structurally.

I'm not too much of a weight weenie and i just like to try a bunch of ideas i had on this ski, but i'm actually planning to play around with added weights on tip and tail and see how the swing weight changes the skiability.

It's 750g for binding ski combination, Most race bindings are around 115g, about 10g for mounting screws. So a ski around 630g would be perfect.

Cool idea with the swing weight, I could see that making for a much smoother run in straighter lines