Main Help → All Commands → MPM and FEA Boundary Conditions → Load
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.
For FEA calculations, the Load
command applies a force to nodes near a line or to a single node.
Load (dir),(load)
where:
(dir)
is x
or y
(or 1
or 2
, or 3
, or R
or Z
if axisymmetric) to specify the direction of the applied force. To apply a load in any other direction, find the (x,y) components of the load vector and apply them both in two Load
commands.(load)
is the applied force (in force units). It can be specified by a number or by a user-defined function of nodal point position entered as quoted text.You can combine Load
commands with Rotate commands to set load in some direction other than the x
or y
direction.
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
(dir)
is x
, y
, or z
(or 1
, 2
, or 3
, or R
or Z
if axisymmetric) to specify the direction of the applied force. For tractions, (dir)
can alternatively be normal
(or 11) or shear
(or 12) to have stresses normal or shear to the particle domain surface. Normal tractions are positive to apply tensile stress to the domain; shear tractions are positive when they rotate the particle domain in the counter-clockwise direction. The shear option is not allowed for 3D calculations. Normal or shear tractions are most useful for calculations using CPDI. In such simulations, such tractions will remain normal or shear to the particle domain surface as it deforms and rotates.(face)
specifies a surface relative to the particle to be loaded by traction. In 2D, imagine a box around the initial particle. Surfaces 1 to 4 are the four edges of the box in the order bottom, right, top, and left (with normals (0,-1), (1,0), (0,1), and (-1,0)). In 3D, imagine a cube around the initial particle. Surfaces 1 to 4 are same as for 2D (now with normals (0,-1,0), (1,0,0), (0,1,0), and (-1,0,0)), 5 in the bottom z surface (normal = (0,0,-1)), and 6 is the top z surface (normal = (0,0,1)). See note on tractions when particle shape changes.(style)
specifies the style of the applied load or traction. The setting depends two parameters specified by arguments (value)
and (time)
. If either argument is not supplied, they are set to zero. The standard units are force units (for loads) or pressure units (for tractions) for (value)
and alt time units for (time)
(but the units may change depending on the (style) setting).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.