force-displacement diagram of a sandwich

[melixman2016]

Hi,
that's force-displacement diagram of a sandwich,
Sandwich consists of pulp foam and flax laminate
The aim of the sandwich is to absorb energy, which can later be used in a crush box.
How do the results look like?
Are there any remarks?
I also have pictures for each sample, I can upload it here if someone needed to see it.
Thank you

 

[SWComposites]

Looks like a lot of scatter (variability).

 

[RPstress]

What is the test? That looks like a factor of three on the initial failure. Is the initial failure always the same (i.e., whar is breaking first)?

 

[melixman2016]

thanks for the answer
The test is 3 point flexure test
in all samples, this kind of failure has occurred:
core failure
adhesive failure
usually fails glue first.
the adhesive used is polyurethane in pasty form.
would it be better if I used glue in liquid form?
core consists of pulp and Faces of flax fibre laminate (see attachment)

During the experiment.

End of the experiment

 

[RPstress]

Is the 'pulp foam' a wood-based product? A liquid adhesive would be a bit stronger (but not much) and would not adhere as well as a paste to a course foam. Also the foam seems weaker than the adhesive; details of the glue failure are important: does the glue foam joint leave a layer of pulp on the glue, does it fail the glue attachment to the skin, or is a bit of glue left on both surfaces? Glue attachment to the core depends on how fine the surface texture of the 'pulp' is; a coarse foam should attach better to paste than to liquid glue. Polyurethane is not the best glue but is not too bad. Flax is pretty strong for a natural fiber and is not a bad choice (do watch out for invasion by moisture which will weaken it considerably; like all the natural fibers flax is very vulnerable to moisture and may be weakened by a factor of three or even more).

 

[berkshire]

One thing not mentioned here ,what is the goal of these tests , are you just seeing how far you can go with this material, or are you coming up short on a value you want to achieve?
B.E.

You are judged not by what you know, but by what you can do.

 

[melixman2016]

"pulp foam" consists of a carton of eggs mixed with water, CMC(Carboxymethyl cellulose) and NaHCO3 and then leave to dry in a microwave oven.
Adhesive bond is left on both surfaces, but mostly on the skin.
Flax has been in the laboratory for 2 years.
can you please name some glues that are good for sandwiches?
I have noticed that the surface of the core is very dry and quickly absorbs moisture. and glue do not uniformly spread on the surface of the core.
it's a good idea that I vary a little bit the viscosity of the paste polyurethane?

The goal of these tests is to determine the failure modes of this new sandwich,and evaluate the different failure modes and their causes.

 

[Composite Repairman]

melixman2016

For more streght bond between core and skin try to apply epoxy resins such as EA9396, EY3804 or Epocast 50A.
For example, they are used to repair disbond between skin and core of composite material parts on aircrafts Boeing and Embraer.
EA9396 and EY3804 usually are used to repair of exterior parts, but Epocast 50A for interior parts and it has good properties of resistance to flame iaw FAR 25.853a.
They are room temperature cured, as well you can accelerate curing process using heat sources.

ATACS 4103/5103 and DP100 are fast curing adhesives and using to bond different materials.

Anyway find in google and check datasheets of materials for properties and applying before use them.

Best regards
CRM

 

[RPstress]

You're presumably interested in cheap energy absorption for your eventual 'crush box'. It will help to have stuff that has a high strain to failure. It may help if it's quite weak to ensure it absorbs energy as it breaks. Your core being crushed up egg boxes may be appropriate for compression. Your skin is a bit brittle in tension but that doesn't seem to be involved in the failures except for some local bending which doesn't seem to involve brittle failure. Your core-skin glue could maybe use a thicker high-strain option. EvoStik impact adhesive or similar springs to mind. Judging by your pics the core could do with some flexible glue content to increase shear properties (and energy absorption on shear failure).

 

[melixman2016]

Thank you very much for the names of the adhesives, I searched for them in Google. these are real good adhesives
Thank you for your suggestion for adhesive.
yes, the costs in this project are also important, so we try out cheap materials.
Thank you very much for pointing out that the surface layer is brittle.
this is another photo, where I have marked is core shear yield strength F[sub]yield[/sub] and facing yield stress

 

[RPstress]

I'd missed the info that you're mixing some cheap 'glue' (the CMC) in the core. Maybe a little more, but you've presumably done work to try to optimise CMC content. Is cost the main driver or is weight a little bit important too? The sigma_yield strengths look a bit low to me for flax; is the flax in a PU matrix (maybe it's impregnated by the glue for attachment to the core)? (This may well affect viscosity chosen for the glue.) Any idea what the skin flax/matrix ratio is? Those Fyield core shear strengths look ok to me. That's an innovative natural core. Just watch out for moisture ingress.

 

[melixman2016]

Chemists have optimized the mixture by means of Design of experiment:

Mixture is:
- 200 g paper fibres, 800 ml water (ratio 1:4)
- 9 % binder (CMC), based on the dry matter pulp = 18 g
- 3 % raising agent (NaHCO3), based on the dry matter pulp = 6 g
- Microwave drying process

yes, the weight is also important in this work.
Epoxy resin is used as a matrix for the Skin.
The Fiber volume ratio is 45%.

 

[melixman2016]

I just happened to notice something now.
the support bars and loading bar are not equal in size!
that will lead to different pressures on both sides.

 

[Kwan]

It may be that the scatter is a result of the lines of adhesive I see on the facesheets. It's likely that the scatted could be reduced if you consistently apply the adhesive (control spacing, etc). Just my observation.

 

[melixman2016]

you're right,
my supervisor advised me not to put many adhesives on the core, he said that they are evenly distributed under pressure on their own.
the problem is the core is too dry and not quite flat