Composite material properties 1

[crisb]

It seems common that composite material properties are provided for Ea (longitudinal modulus), Eb (transverse modulus), Gab (in plane shear modulus), and nu ab (major poissons ratio).

However (for an elastic orthotropic material) the program requires data for Ec, Gac, and Gbc, and optionally for nu ac and nu bc.

Also sometimes I find that the value of major poissons ratio provided is incompatible (too high) with the other constants and the program does not accept it.

Does anyone have any comments please on values to use for the other constants?

 

[GBor]

All of these properties can be approximated from the properties of the constituent materials. I'm not near my books right now and the equations are pretty long, so I'm not walking around with them in my head.

As for Poisson's being too high, is it >0.5? If not, what software are you using. I know 0.5 is a limit for some.

 

[crisb]

GBor: Many thanks for your comments. I am modelling a composite plate with outer woven layers, then some unidirectional layers and an inner foam core. I am using Adina with mutiple layer shell elements and elastic othotropic materal models.
The program requires material data for Ea Eb Ec, nuab nuac nubc, Gab Gac Gbc. Manufacturers data typically only gives values for Ea Eb nuab and Gab. I was wondering how other people set values for the missing properties.
The program states that nuab must be less than sqrt(Eb/Ea). For the material in question this sets an upper limit on nuab of 0.25, whereas the manufacturers data gives a value of 0.4.
As a further aside Nafems benchmark example 1 has E1=1e5 E2=5e3, and nu12=0.4, however sqrt(5e3/1e5)=0.22.

 

[SWComposites]

As a rough estimate for uni tape:

nu21 = (E2/E1)*nu12
E3 = E2
G13 = G12
G23 = 0.67*G12
nu13 = nu12
nu23 = 0.45

As a rough estimate for fabric:

nu21 = (E2/E1)*nu12
E3 = E2tape ~ 1.2 Msi
G13 = G12
G23 = 0.67*G12
nu13 = 0.30
nu23 = 0.45

"nuab must be less than sqrt(Eb/Ea)" this does not make sense. Limits on nuij can be found in the text book by RM Jones, "Mechanics of Composite Materials"

SW

 

[GBor]

You may also want to look at the properties for the resin system. If you can imagine, the out-of-plane properties are primarily the resin properties, which are usually fairly isotropic, so E13 = Eresin, G13=G23=Gresin, etc.

I think I originally misunderstood your question. You are looking for properties of the laminate (the entire stack), not the lamina (single layer). As SWComposites has above...much shorter than the original equations I thought you were looking for

 

[crisb]

Thankyou for the above comments which are very helpful.
The Poisson's ratio limit appears to be shown by Jones as nu12<sqrt(E1/E2) equation 2.51. Assuming that this is correct it appears that the program may be reporting the limit incorrectly (as reciprocal) and there could be a software bug which I need to ask support about. The same (incorrect?) limit of nu12<sqrt(E2/E1) seems to be reported in the following reference.

http://pdf.aiaa.org/jaPreview/AIAAJ/1968/PVJAPRE4974.pdf

 

[SWComposites]

Some codes and papers define nu12 as the minor poisson's ratio, rather than the major PR. And the code documentation may not be clear. Leads to all sorts of confusion. You may have to contact Adina tech support.

 

[crisb]

SWComposites: Confirmed ... yes, yes, yes and yes!