Relationship between bulk modulus and elastic constant

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SUMMARY

The relationship between bulk modulus (B) and elastic constants (Cij) is defined as B = 1/3 (C11 + 2*C12) for polycrystalline, isotropic cubic crystals. In cases where materials exhibit lower symmetry, such as orthorhombic structures, the relationship becomes more complex, requiring additional independent elastic constants. The Voigt and Reuss averages provide alternative formulations for calculating bulk modulus from elastic constants, specifically 9B = (C11 + C22 + C33) + 2*(C12 + C13 + C23) for the Voigt average and 1/B = (S11 + S22 + S33) + 2*(S12 + S13 + S23) for the Reuss average. The foundational concepts are detailed in R. Hill's paper, "The Elastic Behaviour of a Crystalline Aggregate."

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I am getting the bulk modulus from fitting the equation of states and calculating the elastic constant. It seems that there is some relationship between bulk modulus B and elastic constant Cij, B=Cij/9,where i,j run from 1 to 3.
But my result is far away from this, say B=50 from fitting EOS and B=300 from elastic constant. Is this resonable or B=Cij/9 cann't be applied to some material(structure)?

Thanks a lot!
 
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The relationship between the bulk modulus and the elastic constants is:

B = 1/3 (C11 + 2*C12) for polycrystalline, isotropic cubic crystals

If the material has lower symmetry like a orthorhombic material it would have more independent elastic constants. Remember the concept of the bulk modulus was developed for polycrystalline material only. Not that single crystals don't resist volumetric distortion - of course they do. This resistance is going to be system specific and to give further details I need more info.

Regards

Modey3
 
To add, if the system is not cubic, there are two possibilities for the polycrystalline Bulk Modulus from the elastic constants.

Voight Average:

9B = ( c11 + c22 + c33 ) + 2*(c12 + c13 + c23)

Reuss Average

1/B = ( s11 + s22 + s33 ) + 2*(s12 + s13 + s23);

Details in the paper

"The Elastic Behaviour of a Crystalline Aggregate", by R.Hill.

Proceedings of the Physical Society of London, Section A. Vol 65, No 5. May 1952,pp349-354.
 

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