When importing solid models that have thin parts, it is often better and simpler to analyze them using plate elements. Autodesk® Simulation can be used to convert thin CAD solid models to plate elements. A plate element is drawn at the midplane of the part. Entire assemblies or individual parts in assemblies can be converted to plate elements. An assembly where plate elements can be used for one of the parts is shown in Figure 4.1. This model is a manifold connected to two flanges. The manifold can be modeled with plate elements and the flanges with brick elements. This model, Manifold Assembly.step, is located in the
"Chapter 4 Example Model\Input File" folder in the class directory or in the copy of the solutions folders on your computer. We will use an absolute mesh size of 0.75" for all parts.
Figure 4.1: Assembly to be Modeled Partly with Plate Elements
Chapter
4
"Start: All Programs:
Autodesk: Autodesk Algor Simulation 2012: Autodesk Simulation 2012"
Press the Windows "Start" button and access the "All Programs" pull-out menu. Select the "Autodesk" folder and then the "Autodesk Algor Simulation 2012" pull-out menu. Choose the "Autodesk Simulation 2012 software"
command.
"Getting Started: Launch:
Open"
Click on the "Open" button in the Launch panel.
Alternatively you select “Open” from the quick access toolbar or Application Menu.
"STEP (*.stp, *.ste, *.step)" Select the "STEP (*.stp, *.ste, *.step)" option in the CAD Files section of the "Files of type:" drop-down box.
"Manifold Assembly.step" Select the file "Manifold Assembly.step " in the
"Chapter 4 Example Models\Input File" directory.
"Open" Press the "Open" button.
"Use STEP file units"
"OK"
A "Select Length Units" dialog will appear. Choose "Use STEP file units" from the pull-down menu and click the
"OK" button.
"Linear: Static Stress with Linear Material Models"
"OK"
A dialog will appear asking you to choose the analysis type for the model. From the pull-out menu, choose "Linear:
Static Stress with Linear Material Models" and press the
"OK" button.
"Mesh: Mesh: 3D Mesh Settings"
Select the "Mesh" tab. Click on the "3D Mesh Settings"
button in the "Mesh" panel.
“Options…” Press the "Options…" button.
Mouse
"Absolute mesh size"
Access the "Type" pull-down menu and select the
"Absolute mesh size" option.
0.75 Enter "0.75" in the "Size" field.
"OK" Press the "OK" button to exit the options dialog.
"OK" Press the "OK" button to exit the model mesh settings dialog.
Mouse Right-click on the "CAD Mesh Options" heading for Part 1 in the tree view.
"Part…" Select the "Part…" command. We will now be able to specify the mesh settings for only the manifold.
"Midplane" Select the "Midplane" radio button.
"Options…" Press the "Options…" button.
Mouse
"Absolute mesh size"
Access the "Type" pull-down menu and select the
"Absolute mesh size" option.
0.75 Enter "0.75" in the "Size" field.
"Midplane"
Select the "Midplane" icon. The dialog shown in Figure 4.2 will appear with the options specific to a midplane mesh. We do not need to change any of the default options but they are discussed below the figure.
Figure 4.2: Part Mesh Settings Dialog with the Midplane Icon Active Thickness control section
User-specified maximum thickness: By default the midplane mesh process will search for surface pairs that are within an automatically calculated or a user-specified distance from each other. We will call this distance the maximum thickness. When these outer surface pairs are found, the location of the midplane of the region is determined and the mesh is placed at this location.
The program default is to use the automatically calculated maximum thickness, which is determined as a function of the initial surface mesh. If this maximum thickness value is smaller than the part thickness at any region, that region will be missing from the resultant midplane mesh. In such cases, you can enable the "User-specified maximum thickness" option and enter a value greater than the maximum thickness of the part or parts to be midplane meshed. This option may also be used to intentionally exclude thicker regions of a part from the midplane mesh.
Maximum allowed thickness variation: If this checkbox is activated, the midplane mesh process will only convert the mesh on a part to a midplane if the difference between the maximum thickness and the minimum thickness in the part is less than the value specified in this field.
Use junction method: If this checkbox is activated, a chordal axis transform (CAT) algorithm will be used to generate the midplane mesh. This may result in a better approximation of the midplane for models containing complex geometries such as junctions and intersections.
"OK" Press the "OK" button to accept the default settings and to close the mesh options dialog.
"OK" Press the "OK" button to accept these parameters for the manifold part and to close the mesh settings dialog.
"Mesh: Mesh: Generate 3D
Mesh" Select the "Generate 3D Mesh" button in the Mesh panel.
All three parts will be surface meshed. The midplane mesh engine will convert the surface mesh into a midplane mesh for the manifold part. The flanges remain as solid objects.
By analyzing the manifold we can see that the manifold has no thickness. You can also see that the nodes where the manifold meets the flanges have been matched so that the loads will be transferred between the parts. If mesh sizes between plate and solid parts are significantly different, smart bonding may at times be used to connect the components without matching the nodal locations. This is also dependent upon other mesh settings, such as the "Use virtual imprinting" option that was previously discussed.
It is important to note that not all combinations of midplane and solid models are acceptable.
Figure 4.3 shows an acceptable configuration and an unacceptable configuration.
Figure 4.3: Midplane and Solid Combinations
The configuration on the left is acceptable because the midplane of the gusset will be in contact with the top of the plate. The configuration on the right will not work because the midplane of the plate will not be in contact with the bottom of the gusset. Therefore the nodes on the parts will not be matched. The midplane mesh can be extended in the plane of the elements at the edges in order to match the parts. The nodes cannot be stretched out of the plane, nor can the node of the solid mesh be moved in order to match the parts.
Mouse Right-click on the "Element Definition" heading for Part 1 in the tree view.
"Edit Element Definition…" Select the "Edit Element Definition…" command. The dialog shown in Figure 4.4 will appear.
Figure 4.4: Element Definition Dialog for Plate Elements
By default, unique thickness values will automatically be assigned to each individual plate element within a midplane-meshed part. This can be verified by checking the model and inquiring on the element information for selected plate elements within the Results environment.
If the "Use mid-plane mesh thickness" option is deactivated, the average thickness for the part will be calculated and displayed in the thickness column. This thickness will then be applied to all elements within the part. The user may also specify the thickness on a per-surface basis, if desired. Choosing the "Surface-based" option from the "Properties" pull-down list will expand the table to list each surface of the midplane-meshed part. The thickness, normal point coordinates and other data can then be entered for each surface.
Mouse
Deactivate the "Use mid-plane mesh thickness" checkbox.
The average thickness calculated for the part will be displayed in the "Thickness" column.
Mouse Activate the "Use mid-plane mesh thickness" checkbox.
"OK" Press the "OK" button.
"File: Save" Click on the "Save” button in the quick access toolbar.
We will continue developing this example model later on in this chapter.