I am working in version 4.3a structural mechanics module with Piezoelectric Material Model. I am a little confused about which material properties are which in the matrix. The input field for the PZT-material properties is 3x6 = 18 numbers. For strain charge form, I expect d33 to be 22.5e-11 [C/N], but it seems it is -9.7e-11 [C/N] for the default PZT-8 material. The documentation says that the input field is in voigt notation "For entering these matrices, the ordering is different from the standard ordering used in COMSOL Multiphysics. Instead, use the following order (Voigt notation), which is the common convention for piezoelectric materials: xx, yy, zz, yz, xz, zy. " What are the 18 numbers then? If I have a 5x5 matrix, I should have only 15 numbers and if I have a 6x6 matrix, I should have 20 numbers. What am I missing here?