foam-fiberglass cloth sandwich for hull

I do tests fairly frequently and there are a few important things to consider - first is a base line for comparative purposes. Usually worked into the tests as a "blind". Next is an accurate and consistently repeatable set of procedures. Unless you can compare apples to apples, data is not especially useful, though sometimes it can be enlightening or offer a surprising result, which naturally requires more tests for confirmation. Lastly is a good working understanding of the physics involved and the actual physical qualities of the materials employed, again, so you have something to compare against.

No offense intended and maybe it's a translation issue, but the test information above is all but meaningless.

 

in hammer test - it was 5kg hammer head with pointed end, sliding inside steel pipe, I think, I hit all panels same way
only 3 m but it was the longest pipe I have
test until some debonding between top ply and styropore
top ply in multilayer sandwich has mor demage to the top ply , I think because it was much stiffer as the another two, but has no typical debonding
(I have no way to check microstructure of styropore)

bending test - car jack
I didn't break any panel but some debonging apear in regular core

[/QUOTE]A reinforcement (ply; glass; ...) in the centre of a sandwich contradicts everything I was ever taught; but I admit that I simply believed and never tested myself.[/QUOTE]

the composite is a very complicated subject, and I think that it will be for long time, try and see process. They just try to characterize the strength of interfacial part of sandwich.

 

I tested some panels, using same procedures ( truck test was for fun )
What I know now is that if I use multi ply , probably I have a stronger and stiffer panel.
I didn't see typical debonding pattern in styropore core in the sandwich with multi ply, and I am ok with this.
And reading some articles about laminates I don't think there is someone who really knows what is going on inside this panels.

 

I nailed panel to the wood frame.
Why I tested these 3 panels?
I'm trying to find, what happen if I perforate the core, and this way I have inside web of epoxy (glue).
Doing this I make some sort of "truss" multi layer panel.
By inserting just extra layers of cloth between layers of foam we have more weak bonding surface.
But if we perforate the core and we put extra layer (can be thin) of glass. The web of epoxy (glue) will be stronger as bond between glass and foam is (foam is the weak point).

some sort of honeycomb fill with foam.

 

composites is not a difficult subject if it is kept simple and straight forward !! it when people start to let there imaginations runaway and start adding things such as plywood as a core . then they get all confused . wood as a core will never work !!
ok now lets rephrase that and say yes it will work BUT !!, wood any kind of wood when it gets damp or is damp before its used as a core will swell and force the layers of glass to gradually delaminate off the wood and so you still have stiffness because of the nature of the ply wood but the glass could be loose . always remember dissimilar materials of any sort are in just about all cases not compatible 100% so failure is imminent at some time for what ever reason !!
my old boat was built 1975 and has no foreign materials in any part of its construction its 100% glass and resin its as sound now as when it was first built .
Cores of different materials like balsa and foam have been in existence for long time and boats built in the 1980s using balsa core are still in day to day commercial use and again as good and as sound as the day we built them .
Workmanship and care and attention to detail during there construction accounts for them not having problems of any sort . they were built for boaties by boaties and boats are our life and always have been . the other biggest problems people have when dealing with composites is simply they do not understand the materials they are using , !there's a great long list of types of glass that can be used in construction but does any one take the time to really study what they think they should or could be using , sure we looked at figures on paper and got the magic formula but do you understand everything about the way its made and why layers run at different angles and is it woven or is it stitched and why these differences ?? same glass just made a different way and can behave not the same .
Cores are a grey area for so many would be builders , what the best ? what's the strongest ? why so many different thicknesses and why so many different densities' ??, when balsa is just balsa and only in very recent history have companies begun to separate densities in to hard ,soft , and something in between . but why ?? no one bothered before so why bother now ?? Mystery ?? and balsa soaks lots resin so must make my boat heavy !!
Foams the list is huge and bewildering unless you know what you looking for and have used before . its only in recent times that manufactures have started to rethink about core shear and peel failures and started producing tough foams and not before time I might add .
And as for core matt it has it place but never ever use in panels that have movement such as hull bottoms its shear is absolutely scary as is with most foams .
Test panels yes but test for a reason , a reason to prove that what you are going to make is going to work how you had hoped or better . how you use and the way you use is also another factor not many understand or even care about but can and does have a great baring on the durability and added strength of what you are doing .

 

You don't seem to have a direction...

if you want more shear strength, use a higher density foam.

If you want more compressive strength for impact resistance, use a higher density foam or another type of core such as balsa or poly core or wood or foaming epoxy resin etc the choices are limitless..

If you want more bending strength, add more fibres to the outside laminates...

If you want stiffer panel, use thicker core...

It doesn't have to be any more complicated than that. What you are doing is using more materials and resin and you will not achieve as good strength to weight to stiffness ratios as you could by using the above reasoning.

 

My direction? Yes I have.
I have access to one type of foam, which I can use, and I am trying to find solution without buying extra materials
every article about foam core tell us that there is problem with debonding.
So I'm trying to find way to make the debonding problem minimum.
And I think 3D web inside core is a good option.
unfortunately I lost link about 3D web sandwich panels which I have from my friend, who gave me this info( sooner or later I will find it).
They tested 3D web sandwich panels and conclusion was (from my notes):
1/panels designed to overcome delamination problems typically encountered in traditional sandwich panels
2/ increased the compressive shear modulus and compressive strength significantly
3/ decreasing the thickness of the panel, increases the buckling load of the through thickness web resulting in the increase of the compressive strength of the panel significantly
4/ density of web increases the compression strength
5/ prediction model, as based on beam elements is simpler

if what they say is true maybe I can build good panel without thickness increase

 

Sure its true... there is many references available on truss core type structures... but trying to manufacture them yourself will generally cost more and create quality issues, not to mension increase weight.
Do you have a mail service where you are? Can you not just mail order the product you need?
Or is the real question, they are too expensive...?

 

The choice of materials in your cored structure suggests, not a complete understanding of the concepts or the material property realities and essentially creates a self defeating assembly, in this regard.

I'm reminded of a fellow that wanted to build a hollow wooden mast and called to ask, if using the abundance of 1/8" plywood he had on hand was a good idea. Of course I said no, but he insisted, did some no control tests (like above) and laminated a wooden mast from several layers of this plywood. The result was heavy (very) and it did break on his third trip out in the boat, when a gust came up and spanked him.

Good luck with your assumptions.

 

I'm in Europe now. I can order the foam direct from China or some distributors here.
I have source of free foam - 5mm and is perforated, so it will be great if I may use it. After receiving the resin I will make real test with epoxy and foam, after that I will know more about weight and how the panel works (I hope).

 

who said, that I'll be building plywood boat.
I tested "plywood panels" because I'm waiting for the resin.
Regarding "no control tests". It is true I don't have lab to do control test, but I don't have "control" shop to build the boat, with process control system. I have nice shop but it is not NASA.
Talking about plywood mast, I had one, I made my homework, reading some books.
I made some calculation with my friend. Finding right epoxy took some time but the mast still is ok.
My carbon bike is not, was made in control plant. They say , I used it to hard, this is why.

 

I rest my case . . .

 

thank you for your posts

 

I can tell you that pvc or other structural foams of around 80kg/m^3 absorb 300grams of resin per side per square meter. So if you have 2 layers of foam glued together, it will take 600grams of resin in the joint, 3 layers you now have 1200grams extra... if you put glass cloth in there, you have weight of cloth plus equal weight of resin in addition to above. It adds up very quickly... then you have no strength increase either unless you scarf the sheets up ajd make shear ties with the fibreglass - like the webs in cardboard. .. lots of work, you will wish you bought the correct core in the first place...

 

the foam is free, this is the reason, why I'm trying find if I may use it
plan was : build with plywood/epoxy - I don't like debonding foam
when I have free foam, maybe I can use foam/epoxy panels with small risk of debonding
I should have the resin soon so I know more about weight, and "no control" test results
regards