Boundary Load

Use a to apply tractions or pressure on boundaries.


	This section is only present in the Layered Shell interface, where it is described in the documentation for the Boundary Load node.

Force

Select the — , , , , or for 2D components, . Then enter values or expressions based on the selection and the space dimension.

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For , the traction components are given explicitly.

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For , COMSOL Multiphysics divides the total force by the area of the boundaries where the load is active. Then the force is applied in the same way as for a . When working with curved boundaries or local coordinate systems, use this option carefully, as the result is not always intuitive.

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For , a scalar input is given, and the orientation of the load is given by the normal to the boundary. The pressure is positive when directed toward the solid. In a geometrically nonlinear analysis, the current surface normal and area are used.

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For , enter the and with respect to a point. Select the — , , or . For , the coordinates of the application point is the centroid of the selected boundaries. For , select a geometrical point in the section . For , enter the global coordinates of the , xa.


	When using the Resultant option, it is formally possible to select a nonplanar boundary, but the traction distribution may then be unpredictable.


	After selecting a Load type, the Load list normally only contains User defined. When combining with another physics interface that can provide this type of load, it is also possible to choose a predefined load from this list.

Table 4-6: Boundary load types.
Load Type	variable	SI unit	Geometric Entity Level	Space Dimension (components)
Force per unit area	FA	N/m2	boundaries	3D (x, y, z) 2D (x, y) 1D (x) 2D axisymmetric (r, z) 1D axisymmetric (r)
Force per unit length	FL	N/m	boundaries	2D (x, y)
Total force	Ftot	N	boundaries	3D (x, y, z) 2D (x, y) 1D (x) 2D axisymmetric (r, z) 1D axisymmetric (r)
Pressure	p	Pa	boundaries	3D 2D 2D axisymmetric
Resultant	F, M	N, N m	boundaries	3D (x, y, z) 2D (x, y)

Traction Field

This section is only shown when has been selected as . The distribution of the tractions over the boundaries is controlled by the settings here.

Select a — or .

When is selected, the traction field approximately matches the stress distribution over a beam cross section.

For , you can write expressions for the traction distributions. Enter expressions for the six dimensionless vector-valued traction distribution functions, 1, 2, 3, 4, 5, and 6. Usually, the local coordinates of the loaded region would be used for this purpose, but there is no limitation on the form of the functions. The built-in variables for the local coordinates are named <physics_tag>.<load_tag>.x2 and <phys_tag>.<load_tag>.x3, for example solid.bndl1.x2. The default value is the distribution that is used when the option is selected.

The local coordinates of the boundary, x2 and x3, are defined in the following way:

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The origin is at the centroid of the boundary.

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The axis directions coincide with the principal axes of the boundary.

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x2 is the axis around which the principal area moment of inertia is larger.

When is selected, you can also select a — , , or . This function acts as a multiplier to the traction distribution functions. When is selected, the weight function w = 1. For , enter a radius rw of a circle, outside of which no load is applied (w = 0). Inside the circle, w = 1. For , enter a weighting expression. Usually, the built-in local coordinates of the loaded region would be used for this purpose, but there is no limitation on the form of the function. A time- or parameter-dependent function can, for example, be used to model a moving load.

The traction distribution functions have the property that the total traction field is

where the coefficients ci are chosen so that the given force and moment resultants are obtained.

Symmetry

This section is only shown when has been selected as . If the resultant force is applied in a symmetry or antisymmetry plane, this fact needs to be taken into account when creating the corresponding traction field.

Select a symmetry type — , , or . When one of the types of symmetry is chosen, specify the symmetry plane by entering a nsym, and a , xsym, located in the plane.

When specifying a resultant load in a symmetry plane, you still provide the full value of force and moment, as if there were no symmetry in the model. Any components of the given load that do not fulfill the selected type of symmetry will be discarded.

Linear Buckling

To display this section, click the button (

) and select in the dialog box.

If you are performing a linear buckling analysis with a combination of live and dead loads, select the check box to indicate that the load contributions from this node are constant. The default is that a load is proportional to the load factor.


	For more information about live and dead loads, see Buckling Analysis.

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You can add the Phase subnode to specify the phase of this load in a frequency domain analysis.

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You can specify this load to be a Harmonic Perturbation in a frequency domain analysis.

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You can assign this load to a load group. See Load Cases in the Structural Mechanics Modeling chapter.

Location in User Interface

Context Menus

Ribbon

Physics tab with or selected:

Physics tab with selected: