can I do a 12mm thick foam core out of 2 x 6mm core?

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#1

Hello,

It may be trivial, but…

I have a 6mm PVC foam core (density =101kg/m3, from Diab). There are some places in my construction there I know I have to use thicker core.
So can I just bond 2 such pieces of foam, to get a proper 12 mm thick foam?
(I mean, if that would behave (from mechanical point of view[macroscale])as a one homogeneous 12mm foam ). Would a thing like this be reliable? Can a delamination occur between them?

And if so, what kind of glue should that be? something flexible I think.

Thanks

#2

Shouldn’t be an issue. Unless you are doing something really extreme in terms of bending with it you should be able to just use thickened epoxy

#3

It can actually be more superior to bond two together in certain scenarios. You can use film adhesive, wet adhesive or plies in between

#4

Thanks for answers,

the first scenario is that would be sandwich construction for car body (it would be monocoque) I use Ansys FEA software for that composite analysis. Now it shows that for that car body there are small stresses in comparison to yield strength of that sandwich and the strains are little there also.

The second scenario is that I would like to use another doubled 6mm PVC foam as a core for “sandwich” in places where suspension is mounted. I used several failure criteria (Tsai Wu , Tsai Hill, maximum stress) they say I cannot add any more load otherwise the construction there fails.
But maximum strain criterion there ,says that the structure there is middle deflected.

So the summary (a report): strains are not big anywhere, stresses are big or small depending on place

So what would you say?

#5

And are these wet adhesives ,film adhesives could be as flexible as the PVC foam,

Diab PVC foam: density 100kg/m3
Elastic Modulus E = 92 MPa
Shear Modulus G= 40 MPa

Thanks,

#6

For the body, Im sure that will be fine given your description, particularly in standard loading methods. Sharp point puncture impact may be a different scenario of course.

Have a look at the two photos, one rear crash structure i poached off online, the other aluminimum honeycomb is one we used for an FSAE impact attenuator at one stage (yes slightly different application but similar principle). Cant find pics of a few other samples I have.

Just to confirm what you’re saying for the suspension mounting points: With max strain criterion, there is a large deflection in the middle of the mounting point so you need more stiffness? Certainly adding two cores with say 2 layers carbon in between will enhance stiffness there, or of course additional external plies as additional plies at that location should have a greater effect.

Ive used film adhesives mainly for this but no reason why you couldnt do it wet either.

#7

Just to confirm what you’re saying for the suspension mounting points: With max strain criterion, there is a large deflection in the middle of the mounting point so you need more stiffness? Certainly adding two cores with say 2 layers carbon in between will enhance stiffness there, or of course additional external plies as additional plies at that location should have a greater effect.

You can see what happens to actual 12 mm foam core: (core failure criterion)

http://s7834.chomikuj.pl/ChomikImage.aspx?e=duIdNw_8czMP_HwbJugXAFD_lSzWTy-ReMzKVjnvfELBchGY_yNF2CdI1wo0e31ZJE9vKN7-_x1Ha9atbgych3Sxn-6CwP_5feIpBlK8f0s&pv=2

with zoom out:
http://s7717.chomikuj.pl/ChomikImage.aspx?e=0yX4Ty5RrXy6NRfAhWab-IqyiqWVLLW-D_qoGqKLf3a5ueUaN-9RgGsUjCBoBTqYOa2tW0xeK0pDRlr5jAwy2EgpVzkd5dAsok7AvGJEFWU&pv=2
These colorful squares in the first picture are suspension mounting points

As much as I understand “core failure” criterion - it is associated with stresses only

And I checked the emax (maximum strain) criterion again: it shows that strain is very little (from 0 to 1 scale) it is 0.02.

So the scenario is : no strain (so no deformation) and big stresses it says to me that because there is no deformation so the tiny ply of any kind of glue will not damage itself and the cores where the glue is between them, isnt it?

Thanks

#8

Ok, so there are very big stresses currently but very small strain? Out of curiosity, is the core modelled as being very stiff or have you stiffness coming from lots of plies?

You are right I think, as long as the adhesive between the two plies is good, this should be fine.

Some help with failure modes:
http://www.hexcel.com/Resources/DataSheets/Brochure-Data-Sheets/Honeycomb_Sandwich_Design_Technology.pdf

Also, pic attached for interest of some tests I did many moons ago on fastener design for 10mm Nomex with 3 layers plain weave carbon (wet lay-up)for suspension pick up points. Just some simple tests for proving the concept of fasteners, ended up using more plies that were autoclaved pre-preg.

#9

hello,

I’m really sorry for making a mistake in data provided.

There are in fact two scenarios: One shown in these photos (in previous post) show core failure mode (cf) where it is nearly to be damaged, there strains and stresses (emax [maximum strain criterion]=0.09-0.14), tsai wu criterion (0.10-0.21), value =“1” means damage
all with weighting factor (safety factor) =2 (it is proper safety factor).

conclusion: there I can use double 6mm pvc core (because of low emax), -true /false?

The second secenario is : core failure in another place is 0.62 and emax =0.42 ,tsai wu =0.94 – this what you ve said general difference between emax and tsai depends on stiffness, like e = s/E, where e stands for epsilon and s for sigma.

Also checked (for that second scenario just for core, )
e1=-0.002 (core is compressed at that point) - I conclude glue to bending or even to buckling (when glue much stiffer than foam), true /false?
shear strain e12 (for core) approx =-00.2) - I conclude glue to be sheared when much stiffer then foam True /false?
e2 =approx. 0 - nothing bad can happen!

General opinion:
When glue will be:
-as much sticky as particles (microscale point of view ) of foam themselves
-as thin so no visible in cross section, or just like it
-having equal or less than core (itself) stifnesess E (Young modulus) an G (Kirchhoffmodulus)

When it would be just fine!