Why is a prestressed modal analysis giving different results in ANSYS at 11.0? The SHELL181 model is of a curved pipe prestressed by internal pressure. In ANSYS 10.0 the first frequency was 199 Hz. At 11.0 the first frequency is 184 Hz. The un-prestressed first frequency is 184 Hz.
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ANSYS 11.0 correctly accounts for the follower nature of a pressure load and does not show any stiffening. In 11.0 we began including "pressure load stiffness" automatically in a case like this one. "Pressure load stiffness" accounts for the fact that pressure is a follower load. Because it is a follower load, pressure does not produce any stress stiffening and 11.0 is correctly showing that. (See Elements Manual Chapter 2, Table 2.9 for elements that utilize pressure stiffness.) |
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