How can EA change with respect to rotation?

[rhig2000]

Does it make sense that EA would change with respect to rotation in some situations?

For the simple example, EA does not change. EA = 8.2401×107 lbf, according to all of the following:

• Classical Stiffness Matrix (1,1)
• The extension stiffness EA
• Timoshenko Stiffness Matrix (1,1)
• Classical Stiffness Matrix at the Shear Center (1,1), and
• Computed using EA = 1/S′11 with S′11 = S11 − xT2 * S16 + xT3 * S15

However, I have other cross-sections where they differ slightly and not so slightly. For example:

• 5.3099302845E+07 per Classical Stiffness Matrix (1,1)
• 5.3099284723E+07 per The extension stiffness EA
• 5.3099302847E+07 per Timoshenko Stiffness Matrix (1,1)
• 5.3099302845E+07 per Classical Stiffness Matrix at the Shear Center (1,1), and
• 4.9537539669E+07 computed using EA = 1/S′11 with S′11 = S11 − xT2 * S16 + xT3 * S15

I'm not concerned about the slight differences within VABS output, but the last one computed using EA = 1/S′11 represents a significant difference (7%). Is this to be expected or must I have an error in my computation somewhere?