If I assume your compliance matrix is defined in a conventional way, S12 being negative denotes the normal Poisson's effect. -nu/E.

Thank you very much!

Can I understand it in this way? The S12 has the same physical meaning of C12, means E11(strain) to F22(stress).

For an othrohombic crystal, S12 = 1/(((C13C13-C11C23)/C13)*((C13C22-C12C23)/(C33C12-C13C23))+C12-C11C23/C13). Thus, the positive or negative values of S12 depends on the specific Cij.

The physcial meaning of negatvie value Sij is in fact the physical meaning of Cij. The C12 is positive, thus S12 denotes a normal and positive Poisson`s ratio.

I wonder if I understand it correctly ? Thank you very much again!

Wenbin Yu

They are different.

When you apply a unit load in the second direction F22, S12 the corresponding normal strain in the first direction E11.

When you apply a unit strain in the second direction E22, C12 the corresponding normal stress in the first direction F11.

Thank you very much!