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# Shear Modulus G12 of a Unidirectional Composite

## Shear Modulus G12 of a Unidirectional Composite

(OP)
I believe I am getting myself confused and seeking clarification.

Lets say I have a unidirectional carbon fiber laminate. From testing, I have E11 and the E22 properties. I am looking to get G12, say for an FEA model. Since I have a composite material it's not isotropi, so the isotropic formula for G = E/2(1+poissons) is not valid. I have to use the Huber's equation of the orthotropic case: G12 = sqrt(E1*E2)/2*(1+sqrt(poissons12*poissons21)).... Is that correct?

OR is there another way to get G12 of a transversely orthopedic material that I am missing??

### RE: Shear Modulus G12 of a Unidirectional Composite

No, you cannot use a formula. You need test data or prior knowledge.

And stop posting similar questions in different forums. Use only one forum - this one is the best for composites related queries.

### RE: Shear Modulus G12 of a Unidirectional Composite

A very rough estimate (a guess) can be found from micromechanics, e.g. www1.gantep.edu.tr which seems to have a few options for different formulas to try (just google shear modulus micromechanics for far more). The charts of theoretical vs. measured that are included by assist. prof. Ahmet Erklig (not sure what Turkish university but the alternative formulas seem pretty standard) show how inaccurate this is (very lucky to be within a factor of two and often wrong by a factor of ten) plus the basic materials data needed is hard to find and often consists of undocumented guesses. Use at your peril. Better by far to get a measurement or try using a measurement from manufacturer's data for a similar material (try Hexcel's datasheets which are largely online).

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