Load Cases
For a Stationary or Frequency Domain study, you can define load cases and constraint cases. Any load or constraint can be assigned to a load or constraint group, and then be used conditionally.
For most load types, the load case acts as a simple multiplier, but some cases need a more detailed discussion:
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A Prescribed Displacement or Prescribed Displacement/Rotation node can be assigned both a constraint group and a load group. You can use the constraint group to switch on and off the whole constraint. The load group acts as a multiplier to any nonzero prescribed values of displacement and rotations, and will not have any effect unless the constraint is active.
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When a load case multiplier is used for Thermal Expansion, the multiplier is applied not to the actual temperature, but to the difference between the temperature and the strain free reference temperature. The temperature difference, and thus the thermal strain, is proportional to the load case multiplier.
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Since Thermal Expansion nodes are exclusive (only the last one given gives a contribution for a certain domain), you cannot switch between different Thermal Expansion nodes only by assigning them to different load cases.
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Hygroscopic Swelling behaves analogously to Thermal Expansion.
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For External Stress, the given stress values are multiplied by the load case multiplier.
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A Spring Foundation or Thin Elastic Layer node can be assigned a constraint group, which you can use to switch the whole boundary condition on and off. If there is also a Predeformation subnode, then you can assign a load group to that subnode. The prescribed predeformation is then multiplied by the load case multiplier. Predeformation nodes are exclusive, you cannot switch between them by assigning them to different load cases.
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A Rigid Connector can be assigned both a constraint group and a load group. You can use the constraint group to switch off the prescribed displacements and rotations. The load group acts as a multiplier to nonzero prescribed values of displacement and rotations.
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If you have added Applied Force or Applied Moment nodes under a Rigid Connector, you can assign individual load groups to these nodes.
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If you have added Applied Force or Applied Moment nodes under a Rigid Domain, you can assign individual load groups to these nodes.
Any expression that acts purely as a load, that is, contributes only to the right-hand side of the system of equations, can be part of the load case handling. This is true even if, for example, the corresponding feature does not have a setting for load cases, or if it is a contribution you have created using equation based modeling.
To do this, you can modify any such expression expr to if(group.<lgName>,group.<lgName>*expr). Here, <lgName> is the parameter name you have chosen in the settings for the load group.
In most cases, the expression group.<lgName>*expr is sufficient. The only reason to use the longer version with the if() statement if you want to avoid that expr is evaluated for load cases in which it is not used.
For example, if you have a boundary load which partially is always active, and partially is conditional, you can write 20[MPa]+group.lg1*10[MPa] in the input field for the pressure.
In particular, this approach is useful for features that override each other, like Thermal Expansion, since you can then accommodate several load cases in a single node in the Model Builder.
In the COMSOL Multiphysics Reference Manual:
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About Load Cases
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Defining Load Groups and Constraint Groups
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Load Group
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Constraint Group
For an example about how to set up expressions for controlling position and distribution of loads using load cases, see Tapered Cantilever with Two Load Cases: Application Library path COMSOL_Multiphysics/Structural_Mechanics/tapered_cantilever.