A prestressed modal analysis was run on the same model, using WB 8.1 and WB 9.0. The results differed significantly:

f1 18.232 Hz 30.839
f2 19.331 31.080
f3 30.747 32.584

Why is there such a difference?

A program change was introduced in 9.0, due to the resolution of Defect 22811. In ANSYS 8.1, SOLID182, 183, 185, 186, and 187 included pressure load stiffness in modal analyses. Starting in ANSYS 9.0, pressure load stiffness was removed from all cases of modal analyses. This change is responsible for the difference.

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