In static stress analysis with linear material models, there are four different categories of loading: nodal, edge, surface and element. The loading options are listed below. Loading options for thermal analysis will be covered in Chapter 8, "Steady-State Heat Transfer" and in Chapter 9, "Transient Heat Transfer."
Nodal loading:
• Nodal Force: Will apply a force along any vector to the selected nodes.
• Nodal Lumped Mass: Will apply the effects of a mass concentrated at the point of attachment. For the mass moment of inertia (rotational inertia) about a given axis to have effect, the element type must support rotational DOFs.
• Nodal Moment: Will apply a moment about any vector to the selected nodes. The nodes must be on an element type that supports rotational DOFs.
• Nodal Prescribed Displacement: Applies a displacement along any vector to the node.
• Nodal Temperatures: Applies a temperature to the selected node for use in a thermal stress analysis. Temperature data can also be mapped from a thermal analysis.
• Nodal Voltages: Applies a voltage to the selected nodes for use in a piezoelectric analysis. Voltage data can also be mapped from an electrostatic analysis.
Edge loading:
• Edge Force: Applies nodal forces to the nodes on an edge so that the magnitude is evenly distributed over the length.
• Edge Prescribed Displacement: Applies nodal displacement boundaries to each node on the edge.
Surface loading:
• Surface Force: Evenly distributes a force over the surface.
• Surface Variable Load: Applies a pressure that will vary with position according to a mathematical function. Refer to the example at the end of this chapter.
• Surface Pressure/Traction: Applies pressures either normal to a surface or along a specified direction.
• Surface Hydrostatic Pressure: For brick elements, this applies a hydrostatic pressure which is zero at a specified Y-elevation and increases linearly in the -Y direction. The model must be oriented with the +Y direction being vertically upward. For plate elements, the model and hydrostatic pressure may have any orientation (see the "Hydrostatic Pressure Loads" heading under the "Loading Options" section of Chapter 4).
• Surface Prescribed Displacement: Applies nodal displacement boundaries to each node on the surface.
• Surface Temperature: Applies nodal temperatures to each node on the surface.
• Surface Voltage: Applies nodal voltages to each node on the surface.
Element loading:
• Accelerations/gravity: Will apply an acceleration load to the entire model. The parts must have a mass density defined.
• Centrifugal loads: Will apply a centrifugal load perpendicular to a global axis to the entire model. The parts must have a mass density defined.
For additional details about the application and definitions of any of these preceding loads, refer to Appendix C.
NOTE: Nodal loads may be defined at a remote point not on the model geometry and attached to a set of model nodes, edges, or surfaces using automatically generated line elements.
Refer to the help files for additional information regarding the application of remote loads and constraints.
Loads applied to nodes, edges or surfaces can be copied and duplicated on other nodes, edges, or surfaces. This can be done by clicking on the load and right-clicking in the display area.
Select the "Copy" command. Right-click on the node, edge or surface where you want to duplicate the load, and select the "Paste" command
Load Cases
When applying most loads, there is a "Load Case / Load Curve" field. This field will control which load case the load is applied in. Each load case will be analyzed separately.
For example, if you want to see the effect of a 100 pound force applied in the X direction and a separate 500 pound force applied in the Y direction, you can place these forces in load case 1 and load case 2. If you also want to see the combined effect, you can copy these forces and apply them both in load case 3. When the analysis is performed, there will be three sets of results in the Results environment. You can toggle through the load cases using the "Next"
and "Previous" options in the "Load Case Options" panel in the "Results Contours" tab.
Certain loads need load case multipliers in order to be applied. For instance, if you apply a pressure or a surface force, you need to assign a value in the "Pressure" column of the
"Load Case Multipliers" table in the "Multipliers" tab of the "Analysis Parameters"
dialog. This dialog is accessed either by selecting the "Parameters" button in the "Model Setup" panel in the "Setup" tab or by right-clicking on "Analysis Type" heading in the tree view and selecting the "Edit Analysis Parameters …" command. This multiplier is a global multiplier for all of your pressures and surface forces in your model. If you entered 1,000 psi for the pressure and put a load case multiplier for pressure of 2.0, your actual pressure in the model for that load case will be 2000 psi. The value in the "Index" column refers to the load case number
There are seven multipliers in the "Analysis Parameters" dialog for a static stress analysis:
Pressure: This multiplier will multiply all pressures, tractions, surface forces, surface variable loads and beam distributed loads.
Accel/Gravity: This multiplier will multiply the acceleration loads defined under the
"Accel/Gravity" tab.
Rotation: This multiplier will multiply the rotation rate specified under the
"Centrifugal" tab.
Angular Accel: This multiplier will multiply the angular acceleration specified under the "Centrifugal" tab.
Boundary: This multiplier will multiply the magnitude of all displacement boundary elements.
Thermal: This multiplier does NOT directly multiply the magnitude of the temperature applied to each of the nodes in a model. Rather, it multiplies the thermal load as defined by the equation:
Thermal Load = (Coefficient of Thermal Expansion) * (Nodal Temperature – Stress Free Reference Temperature) So, the difference between the nodal temperature and the stress-free reference temperature (defined in the "Element Definition" dialog) is being multiplied. Therefore, a thermal multiplier of 2 will result in exactly double the stress relative to a thermal multiplier of 1, even for parts with non-zero stress free reference temperatures.
Voltage: This multiplier will multiply the magnitude of the voltage applied to each of the nodes in a model.
You can combine these multipliers in any order and can turn off loads for different load cases by entering a zero for that column. Refer to Figure 2.9 for an example.
Figure 2.9: Analysis Parameters Dialog
The load case multipliers shown in Figure 2.7 would be used to model the following situations:
1. Only surface applied forces, pressures, and/or traction loads applied.
2. No load except for gravity.
3. No load except for thermal loads.
4. All loads listed in preceding items 1 through 3.
5. 1.5 times the surface applied forces, pressures, and/or traction loads combined with gravity and 1.25 times the thermal load.