Don't apply concentrated force load to surfaces

It’s definitely the most common mistake made by new users (especially those who don’t have experience with Abaqus) so let me explain this in detail.

TL;DR: Never apply a concentrated force load to a surface unless you are sure that you know how it works and you have a good reason to do it (it’s rarely needed though).

Why ? Let me show you. Here’s a simple cantilever beam. Assume that we want the top surface to be loaded with a force of 200 N. Let’s use the concentrated force load for that:

Now let’s check the results:

This is true scale deformation. And the maximum stress is around 3.6e4 MPa. A bit too much for a 10x10x100 mm steel cantilever beam loaded with 200 N. It should be around 0.14 mm and 60 MPa according to hand calcs (recommended whenever possible).

So what happened ? Let’s check the total reaction force (also recommended). It happens to be 117200 N instead of 200 N. Why this value ? If you divide it by 200, you will get 586. How many nodes are there on the surface of the beam where load was applied ? 593… Does it ring a bell ? Let’s check the input file. There we can see:

*Cload
Internal_Selection-1_Concentrated_Force-1, 2, -200

The Internal_Selection-1_Concentrated_Force-1 entry is a name of an internal node set. And *Cload means that each node in that node set receives the specified force.

Now if we use the surface traction load instead:

we will get the following results:

  • max displacement: 0.143 mm
  • max stress: 60.05 MPa
  • total reaction force: 198.9683 N

Much better, right ? And it’s in agreement with hand calcs. Case solved :slight_smile:

P.S. Surface traction also uses the *Cload keyword but it divides the load to apply it uniformly to all nodes:

*Cload
1413, 2, -5.05281853E-001
1342, 2, -5.03713604E-001
1414, 2, -5.04257172E-001
1392, 2, -5.04347942E-001
1391, 2, -5.04430560E-001
1316, 2, -5.04075864E-001
1214, 2, -5.01590558E-001
1309, 2, -5.02944832E-001
1273, 2, -5.01578748E-001
1104, 2, -5.02055236E-001
1103, 2, -2.50253643E-001
...

The first entry is the node number, followed by a degree of freedom and force magnitude. If you sum it all up, it should give 200 N.

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