Joining skins through foam cores.

This point got raised in another thread, and I thought it would be valuable to get advice and opinions aobut it.

Basically, if you are laying up cloth on the inside of a foam core, and the other side already has the fibreglass applied and cured - is it good practice to make a small hole in the foam so that the two skins are joined at various places ?

In my mind, it would be a bit like riveting two aluminium panels together as well as bening glued to the foam core.

Besides the extra weight, would it make a better, stronger job ?

 

whats the tensile strength of the foam and the area your talking about?

 

I've heard of this being done on occasion, but have yet to see any engineering justification for it.

In the case of a bending stress on the sandwich panel, one skin will be in tension and the other will be in compression. The core should only be transferring the shear stress between the two skins. If the panel is going to fail due to core shear, the solution is a denser core with a higher shear strength. Cross-drilling the core for plugs of resin is unlikely to make much of a difference until they occupy a significant fraction of the panel area, at which point you may as well just use solid fibreglass.

In the case of a point load / impact, the core will undergo compressive stresses as it transfers part of the load to the inner skin. In this case, it is often desirable to have the core fail before the skins, as well-designed foam-core sandwich panels tend to do. Cross-drilling the core would probably offer no advantage here.

When end-grain balsa is used as a core, you sometimes get ridges of resin between the balsa blocks. These seem to be too thin to have much structural effect, but they might help to restrict the movement of any water that gets in.

I doubt it. Switching to a stronger, denser core will probably be of more benefit than any sort of cross-bonding scheme carrying a similar weight penalty. Rivets, as you'll recall, only work when they can hold two parts tightly in contact- a better analogy for a cross-drilled foam core sandwich might be a couple of aluminum sheets held apart by an array of long, thin bolts. Stresses that the foam core would distribute evenly become concentrated on small points.

 

Thanks guys for both answers.

The core will be over 80 gsm and 12mm thickness, so fairly heavy, this would be the sides of a 28 ft yacht. but I take your point about joins not being like rivets, more like metal studs.

The picture I was playing with was substantial 'plugs', like about 1/2" diameter diagonally patterned at about .5 metre over the panel. (see sketch)

My logic suggested that a panel that thick made totally of fibreglass would be incredibly strong (but too heavy) , so just use minimal re-inforcing points.

The points would definitely be non compressable (unlike the rivet analogy), but hey, thats why we use heavy foam. Neither do we want the foam to move inside the glass, or the skins to move relative to each other - so it works in my mind, but in reality ??

 

sometimes stresses can be concentrated in too rigid a structure and make it fail

 

Not a good idea, it will have uneven stress distribution and result in cracks developing around the hard point. This would only be good where you will have fasteners for deck hardware, fittings or other "hard point" equipment mounted, and than I would have several layers of hard material between the skins at the mount locations. The other place this might be a good idea is if you expect some flexing, which means it should be in a line to form a hinge-like structure to allow some flex without the foam being damaged. If the plans do not call for it, I would not try this.

 

I have plans that called for smaller holes 1/8 inch through the core one inch on center in specific areas to increase the sheer. It helps to bond the skins, which are carbon fiber.

The foam is 60 lbs and in specific load areas it called for 80 lbs foam. I was informed of this after the foam was laid up. Speaking with the designer he suggested the holes to increase the sheer in the specific areas and not to mess with removing the foam or adding more foam just the holes.

So the holes through the foam is a method used but the size is much smaller. Im my case we are only speaking of a small area of several feet. The holes were in the foam and filled before glassed.

In your application I can not see the benifit of holes in the foam and laminate on just one side. I think you may be asking for more problems such as bad bond. If the holes are filled then the panel bent the fill will not bend and foam will.

 

I'd be curious to see it tried.

But my bet would be that stress concentrations will result, leading to an earlier and less predictable failure than a plain sandwich panel. The failure modes of a simple sandwich are quite well understood.

 

You'll get print through and bumps

 

Well, as luck would have it, I believe I have answered my own question by visiting a foam and "goo" supplier and looking at real samples of construction. Radical concept - visit the real world - I must remember that

After having a look at some core samples, the concept of having sections of the skins joined by resin "bridges" is not only OK, but in many cases ,the standard.

Cores are often supplied with slits that goes right through the core (there is a light fibreglass cloth on one side like the backing sheet for tiles) - and these slits are either filled with a lighter mixture, or plain resin. If the outside cores are properly engineered, the problem of stress points and print through are totally negated.

As I had hoped, foam is just a convenient lightweight incompressible component, and whether it occurs in the structure in big sheets, or 3 inch squares, it is all the same. Someone mentioned balsa with its grooves that get filled with goo. There is no hard spot or stress concentration to worry about if the outside skins are adequate. In monocoque construction, the outside skins are the sinews, and the core is just the framework.

The reference in another thread by Jimbo
http://www.boatdesign.net/forums/materials/diab-foam-29359-3.html#post303481

to the foam being a "planar I beam" is most apt when the scored foam is used.

On the downside, my initial idea of glassing one side of the foam to get a self fairing panel to finish the laying up on wont work in the wider "unslitted" foam cores - they are just too stiff. I think "themanshed" warned me of this earlier.

However, if I use the scored foam, lightly glass the re-inforced side, I will get all the bend I want with the self fairing capabilites, and will just have to fill the scored "holes" prior to glassing, on the outside of the hull.

I can do it .... I really think I can. test panels are imminent i tel you !!!!

 

I was making some cheap insulated fishboxes using 1" construction foam, the kind with foil on both sides and real lite weight and friable, maybe 2-3 lbs per cubic foot. With just glass on both sides (polyester resin) I could rip it apart too easily, so I tried small holes every 3" all over to form resin 'columns'. That helped but it was still pretty weak, the resin columns being brittle. So then I poked single strands of woven roven through the holes, leaving about 1" on both side of the foam and then laminated the faces of the foam the same as before. It was a vast improvement, the strands of woven roven giving an actual glass to glass tie in through the foam. The compressive, tensile and shear properties were surprising to me and plenty good enough for what was needed. It was a bit tedious to do though and I wouldn't want to do a lot of it.

 

Thats a good testing example.

I wonder if you can get continuous filiments of fibtrglass, like wool on a roll, so you could stitch a running seam through foam.

The fibreglass would get wetted out both sides and through the holes if they were made big enoug.

Spent $600 on foam and goo yesterday.

Cant wait to try this foam stuff out.

 

You seem to be well on the on the way. When have grinding mastered come visit the Man Shed

 

rwatson

Reading your comments in a number of threads regarding your build I recommend you go and volunteer a few days of your time with a local composite builder and learn some of the basics.

You mentioned that you want to build more than one of these boats, this being the case you are wasting time and effort if you do not build moulds.
It would not take a long time or be too expensive to make the hull bottom and two hull side moulds from MDF.

You can buy glass strand, its called gun rowing but you would not try to thread it through the foam as for it to be effective the spacing has to be very close.
You get a small increase in laminate peel strength when you use infusion cores as these cores have 2mm perforations at 20 - 30mm spacing. I have not seen any data quantifying this increase in peel strength but it is evident when you peel off the laminate from a plain core and infusion core.

Good luck with your project
Andrew