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Load and Traction Commands

The Load command applies loads to nodes in FEA calculations or applies loads to particles in MPM calculations. The Traction command applies tractions to surfaces associated with particles in MPM calculations.

Loads in FEA Calculations

For FEA calculations, the Load command applies a force to nodes near a line or to a single node.

Load (dir),(load)

where:

You can combine Load commands with Rotate commands to set load in some direction other than the x or y direction.

Load and Traction in MPM Calculations

For MPM calculations, the Load command applies a force to all particles in the current ParticleBC block:

Load (dir),(style),(value),<(time)>

The Traction command applies tractions to surfaces associated with all particles in the current ParticleBC block:

Traction (dir),(face),(style),(value),<(time)>

where

By default, the loads are specified as load per particle. Alternatively, you can use a LoadType command to specify the total load applied to the current ParticleBC block. Tractions are always in pressure units and not affected by use of LoadType command.

The difference between loads and tractions are that loads are applied at the center of the particle and tractions are applied on the surface. The load method distributes forces to nodes around the particle, which tends to cause artifacts in the stress for the loaded particle. The traction method applies the forces to nodes near the particle surfaces instead, which seems to have fewer artifacts.

Another difference between loads and tractions is how they are affected by particle shape changes. During MPM calculations, particle shape distorts and rotates due to deformation. The traction will account for the current particle shape and therefore apply a true stress. The traction direction is set in parameter #1. If is x, y, z, R, or Z, the traction will remain in that direction. If it is n or t, the traction direction will rotate with the particle. An ExactTractions command can change the way traction loads are found from deformed particle edges.