The subscripts in piezoelectric constitutive equations have very important meanings.
They describe the conditions under which the material property data was measured.
For example, the subscript
on the compliance matrix
means that the compliance data was measured under
at least a constant, and preferably a zero, electric field.
Likewise, the subscript
on the permittivity matrix
means that the permittivity data was measured under
at least a constant, and preferably a zero, stress field.