## General Mechanical

#### strange case

• Miguel
Subscriber

Two analysis of the same system: same geometry, same materials, same connections, same mesh, same settings, same boundary conditions,... just one difference:

case 1: point-displacements are enforced where forces are supposed to be applied in order to make convergence easier. Once solved (successfully) reaction forces for the imposed displacements are obtained (503N and 550N respectively)

case 2: these forces are applied in the same points instead of the enforced displacements and the system is also solved successfully.

Surprising outcome: results (deformations, stresses,etc.) are completely different in each case! How can that be?

Thanks

• Daniel Shaw
Ansys Employee

Are the displacement patterns identical or do the imposed displacements prevent some motion that is not prevented by the applied forces?  For example, is a surface free to rotate with the applied forces but restrained from rotating with the applied displacements.  If so, the stiffenesses are different.

• peteroznewman
Subscriber

Daniel made a good point about the difference between a displacement on an area vs a force on that same area.  However, Miguel said the entity was a point.  So Miguel, please show details about the enforced displacement.

• Miguel
Subscriber

Yes, it is a scoped point but Daniel´s hint was absolutely right and showed me the problem:

the force is applied to the point in one step and then withdrawn in the next one (step1: 0 to 500N, then step2: 500 to 0N) but I did the same with the point displacement without realizing that being a non-linear situation the system remains in a deformed state before the force has come back to 0 in step2 while with the "returning" enforced displacement, this one obligues the point to go back to its original position, thus forcing the structure beyond its "natural" final position. In fact, results after step1 were coincidents in both approaches, only in step2 the results diverged. With this explanation, now everything seems to fit in so I feel confident this is what happened. Would you also agree with it?

Thanks a lot to both of you for your help.