Strongest materials for cored panel re-enforcements

[BigBeakie]

For installing hardware on decks & cabintop, I wanted to test which techniques and materials would provide the strongest attachments of stanchion bases, padeyes, sailtracks, winches, clutches, turning blocks, fairleads etc.

I had some surprises as I worked through the options. Clearly, an oversize rigid backing plate coupled with solid " columns" around the through bolts, would be the best as it leverages the tear out strength of foam cored panels, which is enormous. But I wanted to mount some hardware where backing plate was not an option. What technique is the strongest?

I tested the pull out strength of 4 different options as per the following test method. I used 50mm thick Diab H80 Duflex panel material with 2mm fiberglass skins both sides, from ATL Composites. I used 316 stainless M8 x 50mm ring eyeballs which were pulled using a 1.5 ton block & tackle with a 1 ton crane scale to measure the pull out force in kgf.

The M8 eyebolts were screwed into decored 25 mm diameter holes, leaving the bottom glass skin intact and the bolts were screwed in full depth until they bottomed out on the bottom skin. The holes were filled with :

A. Epoxy resin only
B. Epoxy resin toughened with Cabosil ( fumed silica)
C. Epoxy resin mixed with chopped e-glass fibers about 10 mm long in a high solids ratio
D. Epoxy resin wetted out layers of 600gsm double bias e-glass cloth in a high solids ratio

I expected the Cabosil mix to be stronger than the resin only, and I expected the DB cloth to be stronger than the chopped fibres.

The pull out failure of the Cabosil mix was 220 kgf & failure mode was the threads failed while the resin alone was 450 kgf.and the failure mode was the foam core cracking but threads did not fail. Other tests on foam core strength gave about 800 kgf for foam tear out failure.

Both the glass samples maxed out the crane scale at 1,000 kgf with no sign of any thread failure whatsoever. Interestingly, the foam didn't fail either. Not sure why.

Hope that may be of some interest or help to someone.

[Benz]

Standard practice when tapping blind into core is to cut it out and replace with G10, glued in and fiberglassed over. Not to discount your efforts, but why reinvent the wheel?

[Allied39]

Well I personally commend your effort, knowledge is power.

I saw someone recently recommend " industry standard" while giving bad advice.

Industry standard generally= cheapest, fastest, most profitable way generally unskilled labour (especially post covid ) to do things.

I recently saw a 2 year old boat with a bulkhead making a what would have eventually been a successful attempt to exit the vessel through the hull.

Rarely do my standards and industry standards intersect.

And of course there's also the fact that knowing only one way to do things is....well....limiting at best.

I am currently playing about with a project which looks like it will have three cores installed in different areas, balsa, foam and nida core.

The ability to apply different techniques when required to achieve a higher standard is a strength......most definitely not a weakness.

[Simi 60]

Originally Posted by Benz

Standard practice when tapping blind into core is to cut it out and replace with G10, glued in and fiberglassed over. Not to discount your efforts, but why reinvent the wheel?

Before G10 was around we'd shred glass offcuts into strands and make a hairy glue mix
Why reinvent the wheel using G10?

Not everyone everywhere has access to G10

[BigBeakie]

If I could get G10 here in Thailand, I may have used that. We made our own G10 backing plates in a clamp press, and use that wherever possible, and also as a tapping plate.

In effect, the decored holes with multiple layers of DB is G10, but 45mm thick! Our holes are decored 10mm back from the edge of the 25mm/1" hole saw cut out ( making a 45mm diameter plug) so that the top glass skin of the Duflex composite panel adds to the bonding integrity of the glue surface area.

[Benz]

Originally Posted by BigBeakie

If I could get G10 here in Thailand, I may have used that. We made our own G10 backing plates in a clamp press, and use that wherever possible, and also as a tapping plate.

In effect, the decored holes with multiple layers of DB is G10, but 45mm thick! Our holes are decored 10mm back from the edge of the 25mm/1" hole saw cut out ( making a 45mm diameter plug) so that the top glass skin of the Duflex composite panel adds to the bonding integrity of the glue surface area.

I'm glad it works, but you can't think that any filler will be as dense as G10 (or the similar FRP plate material available from ATL, which surely you can source if you can get Duflex). Still, with enough depth for the screws to tap into.....

[BigBeakie]

That's true. At some point there will be a crossover between material density and thread depth.

Alas there are several ATL products that are not imported into Thailand. You have to be a bit resourceful to attempt boatbuilding over here