# How to perform 3 point bending test by using force instead of displacement in ABAQUS?

I know you can apply displacement to a rigid body (cylindrical rod) to perform a 3 point bending test in ABAQUS. Is it possible to apply force to the rod (e.g. rod with super high young's modulus so it wont deform). I am trying to apply topology optimisation on a structure, so the displacement will change when material is removed at every iteration.

• How are force applied and displacement linked? Jan 6, 2019 at 7:22
• I believe stress and strain relation?
– Kai
Jan 6, 2019 at 19:24

If the model is a three dimensional object, restrain exactly 6 degrees of freedom, so it can not translate or rotate, but it can deform in any way without creating reaction forces at the restraints.

If the elements in the model have rotation degrees of freedom, this is easy: you just restrain all 6 degrees of freedom at one grid point.

This link shows some examples of how to do this for models that only have translation degrees of freedom - i.e. almost all solid elements in FE modelling software.

If you then apply self-equilibrating loads, there will be no reactions at the constrained degrees of freedom. You should check the reactions are very small compared with the applied loads (they probably won't be exactly zero, because of numerical rounding errors) to make sure the model is working correctly.

For a two-dimensional model use the same basic idea, but you only need to restrain 3 degrees of freedom to prevent translation and rotation in the plane of the model.

There is a generic trick for converting displacement constraints into forces. This works in a wide range of applications including in this case. Though there are other methods too, and you dont really need this in abacus.

Make something else constrain your object then put a simple spring connection between the actual constraint. Make this spring very very stiff. This then acts as if the thing was moved by displacement was a force, now all forces get solved.