peteroznewman
posted this
23 April 2019

- Last edited 23 April 2019
jj77 is correct that the Static Structural to pre-stress a MSUP transient does not cause any initial stress or deformation.

I read the online help as jj77 suggested and learned that a 3 step Transient solution is probably the best option.

For a simple example, say the static load is standard earth gravity (9.8 m/s^2) which causes a displacement of 0.1 m.

Here is what happens in a 2 step solve.

- Step 1 ramps the gravity load on from 0 to 0.001 s with Time Integration Off.
- Step 2 is the beginning of the transient load, with Time Integration On, but the model has been given a 0.1/0.001 = 100 m/s initial velocity at the end of step 1.

This causes as big or bigger problem with the beginning of the transient simulation as the sudden appearance of a gravity load. The problem could be mitigated by ramping on the load over 1 second, which would take a long time to compute, but the model would still have a 0.1 m/s initial velocity at the beginning of step 2. The 2 step solve is actually the method used to create an initial velocity in those problems where this is what is wanted.

The preferred way to accomplish an initial load with zero initial velocity is to have 3 steps.

- Step 1 is the same as above.
- Step 2 also has Time Integration Off and ends at 0.002 s. Since there were no changes in load from step 1 to step 2, nothing moved, which means the velocity at the end of step 2 is zero.
- Step 3 has Time Integration On and begins with the initial deflection and stress, but with a zero velocity.

Attached is an ANSYS 19.2 archive that shows a 3 step transient structural. The Static Structural analysis is just there for comparison and is not required for the Transient Structural to do its initial deformation.