Correct way to apply directional forces on surfaces in ANSYS MAPDL — SFE vs F

[AltoAuto]

Summary of the problem
We are trying to understand how to correctly apply a force on a surface (face) in a specific direction (like +X) in ANSYS MAPDL, and whether the standard approach is to use:

SFE (surface load on element faces) or F (nodal forces on nodes on the face).

###Our assumptions & confusion
In theory, a force on a surface should be a traction (force/area), which is what SFE does by applying pressure on element faces.
However, SFE in MAPDL only seems to apply loads normal to the element face using PRES, GUI packages like ANSYS Workbench allow you to apply a force and pressure directly on a face in any XYZ direction (not just normal), and they do not show artificial stress spikes or discontinuities near corners, even on coarse meshes.

This raises questions:
Are GUI systems like Workbench internally converting that to SFE with some directional support we’re not aware of?
Or do they simply spread the load as distributed F nodal forces, carefully balanced to approximate the traction?
Or is there another element formulation or hidden mechanism?

We’re looking for the most physically rigorous yet practical approach used by experienced MAPDL analysts or by ANSYS itself under the hood. Any insight (or example input scripts) would be greatly appreciated!

[germa89]

Interesting question.

Long answer.

You are right, SFE only applies pressure (PRES) on element faces. From my understanding of FEM, at the end of the day, every load is transferred to nodes. Hence this pressure will be also transferred to the nodes.

A non-fully-normal force should be also transferred to the nodes. So in theory, using F is the way.
However, you need to calculate yourself the values of normal force and tangential force at each node, and depending on the surface geometry it might be complex (rotate the coordinate system might be something that helps with this). Additionally, if the mesh is too coarse, you might miss some effects (like a warping arising from the tangential force).

I would say there is also an option of using contact elements. You could define some SURF elements on top of the surface you want to apply, define a frictional contact and apply a displacement/force on them. But I guess that controlling the force applied on the original surface is going to be complex.

I would probably go for the easy answer:

Easy answer

Take Ansys Workbench, apply a force in the desired direction in a surface. Export the ds.dat file and see what MAPDL commands Workbench is using.

[mikerife]

Hi @AltoAuto @germa89
MAPDL has two ways to address this:

1 You can define a coordinate system where one axis, say the X, is in the desired direction. Then use the NROTAT command to rotate the node coordinate system of the set of nodes, that you want to apply the force to, to match the local coordinate system. Then the F,nodeid,FX,value will apply the force in the local X direction.

2 Overlay a mesh of surface effect elements on the set of solid element faces. Then use SFE to apply the pressure to the surface effect element [SEE]. These SEEs, like Surf154, have multiple face designations. Like two in plane directions to apply a traction. Or an face which ends up being a pressure orientated in a user defined direction. As an example Surf154 has 5 total 'faces' and the first 3 have two options depending of the element keyopt setting.

You should find that Mechanical is using surface effect elements when writing the MAPDL input file.

mike

[AltoAuto]

@mikerife @germa89 Really thank you guys for the thoughtful responses!