suitable Face for my core of the sandwich

[melixman2016]

Hi All,
I have to do a four point flexural test for sandwich
I have to find a suitable Face for my core of the sandwich.
I am to choose one of these three choices:
first selection: 2 layers 0°/0°
second choice: 2 layers 0°/90°
third choice: 2 layers -45°/45°
I am for first choice(0°/0°), because Faces can only be loaded under normal stress!
am I right?

 

[RPstress]

A four point bend test is often to test the facing material (unlike ASTM C393 which is to test the core, though C393 has 4-pt options). If so the test facing should be similar to the actual facings of interest. If this is for specimens to be tested under ASTM C393 then the skins should be strong enough to apply failure loads to the core and all 45 deg may be too weak unless the skins are made thicker. If your material is woven then there is almost no difference between options 1 and 2. As with your previous query we must again ask what you are testing for; what is the point of your testing? Note: often a 0/45-type layup of cloth is used, e.g. 0/45/s, 0/45/0 or 0/+45/-45/90/s if one is feeling fussy. (Often with cloth one does not bother with +- and 90 specification, leaving only 0 and 45 with details about what is actually laid up covered by drawing notes.)

Your query about facings being loaded under normal stress makes little sense to me; yes, the facings are under only longitudinal load. The loads across the facings are purely from Poisson's effects.

 

[melixman2016]

Thank you for your important answer.
we do this test to measure deflection and bending strength of sandwich.
my material is not woven, but is a UD-fabric (I don't know what it is called in English, see Attachement).
how will it look like when I work withClassical Lamination Theory and calculate modulus of elasticity(E[sub]1[/sub]) for each face layer?
thank you again for your Help and Time

 

[berkshire]

Melixman.
UD stands for UniDirectional This is a fabric where the majority of the threads run in one direction.
B.E.

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

 

[Compositepro]

Thus the term UD fabric.
I would say that choice one or two would work but I would go with 0/90 because all zeros tend to split very easily and this could introduce an unexpected failure mode.

 

[RPstress]

No idea where I got the idea these were woven plies; apologies for any confusion. I would still tend to a more quasi-isotropic layup if possible. A layup without 45 deg plies is more likely to be vulnerable. However, of your stated options, option 2 is least offensive to my eye. However, the all-0 deg option will be strongest if a little inpractical in the real world.

 

[melixman2016]

Thank you very much
Your advice was correct.
I did the experiments yesterday.
I have produced all variants, 0°/0° is the best variant
45°/+45° is the weakest.