I have a very basic question regarding anisotropic materials elastic constants.
The elastic constants must satisfy the condition of the positive definiteness of the stiffness matrix so that the strain energy is guaranteed to be positive for any strain state. However, are there any extra conditions on these constants ?
In particular, can we have a material with shear stiffness higher than the normal stiffness even if the stiffness matrix is still positive definite ?
If so, is there any physical explanation, on the microlevel or something, for this phenomenon ?
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