Transition between unidirectional field strength and flux density

When the spatial extent of a structure is substantially smaller in the distinguished thickness direction compared to the remaining dominating dimensions, simplifications in this respect are reasonable. Therefore, either the planar stress or the planar strain assumption may be employed for a such thin, shell-like structure. When it is imconstrained in the thickness direction, the first assumption needs to be utilized  

Remark 4-5. Normal and shear stresses in the thickness direction are assumed to be negligible small. 

Considering materials with either unidirectional electric field strength or unidirectional electric flux density aligned with the polarization direction, as described in the previous subsection, different choices concerning the relative orientation of the thickness direction need to be discussed. 

Since no assumptions are made here concerning the piezoelectrically coupled shear behavior in the plane spanning between the 3- and ei-direction, the 

---