OS-T: 1360 NLSTAT Analysis of Gasket Materials in Contact

This tutorial demonstrates how to carry out nonlinear implicit small displacement analysis in OptiStruct involving gasket materials and contact.

Before you begin, copy the file(s) used in this tutorial to your working directory.
Figure 1 illustrates the structural model used for this tutorial: A 1mm thick cylindrical gasket is sandwiched between two co-axial steel cylindrical tubes. The outer cylinder is subjected to a pressure of 300 MPa on the outer surface as shown. Using symmetry boundary conditions, only a quarter of the geometry has been modeled. The gasket is connected to the inner and outer cylinders using contact.

rd2090_gasket_model
Figure 1. Model and Loading Description

Launch HyperMesh and Set the OptiStruct User Profile

  1. Launch HyperMesh.
    The User Profile dialog opens.
  2. Select OptiStruct and click OK.
    This loads the user profile. It includes the appropriate template, macro menu, and import reader, paring down the functionality of HyperMesh to what is relevant for generating models for OptiStruct.

Open the Model

  1. Click File > Open > Model.
  2. Select the gasket_model.hm file you saved to your working directory.
  3. Click Open.
    The gasket_model.hm database is loaded into the current HyperMesh session, replacing any existing data.

Set Up the Model

Create the Curves for Gasket Material

First, define the loading-unloading curves for the gasket material.
  1. In the Model Browser, right-click and select Create > Curve.
    A new Curve editor window opens.
  2. For Name, enter load-curve.
  3. In the X (closure) and Y (pressure) fields, enter the values shown in the table in step 6.
  4. Close the curve editor window.
  5. In the Curves select table, click Color and select a color from the palette.
  6. For Card Image, select TABLES1 from the drop-down menu.
    For details on pressure-closure definitions of gaskets, refer to the Altair Simulation 2022.1 online help.
    For details on pressure-closure definitions of gaskets, refer to the Altair Simulation 2022.1 online help.
    X
    Y
    0.0
    0.0
    0.005
    200.0
    0.05
    450.0
    0.135
    700.0
    0.22
    820.0
    0.287
    830.0

    Now, unloading curves can be created.

  7. Create the unloading curve named unload-curve1 with the following X-Y data:
    X
    Y
    0.08
    0.0
    0.12
    140.0
    0.135
    700.0
  8. Next, create the second unloading curve named unload-curve2 with the following X-Y data:
    X
    Y
    0.17
    0.0
    0.2
    250.0
    0.22
    820.0
  9. Finally, create the third unloading curve named unload-curve3 with the following X-Y data:
    X
    Y
    0.23
    0.0
    0.265
    360.0
    0.287
    830.0

Create the Elasto-plastic Gasket Material

The membrane behavior of the gasket needs to be defined.
  1. In the Model Browser, right-click and select Create > Material.
  2. For Name, enter gask_membrane.
  3. Click Color and select a color from the color palette.
  4. For Card Image, select MAT1 from the drop-down menu.
  5. For E, enter 2.0E+04 and for NU, enter 0.2.

Next, you will define the nonlinear properties for the gasket material.

  1. Create another material named gask_nonlin.
  2. For Card Image, select MGASK.
  3. Since this is an elasto-plastic gasket material, for gasket behavior leave BEHAV field as 0.
  4. For initial yield pressure, leave the YPRS field blank for the solver to determine it automatically.
  5. For tensile modulus EPL, enter 0.001.
  6. For GPL to specify the shear modulus, enter 2000.
  7. For MGASK_TABLU_NUM, enter 3 to specify the field for # of unloading curves.
  8. For TABLD, select load-curve.
  9. Click table_pencil next to the Data field and select the following:
    TABLU(1)
    unload-curve1
    TABLU(2)
    unload-curve2
    TABLU(3)
    unload-curve3

Create the Gasket Property

  1. In the Model Browser, right-click and select Create > Property.
  2. For Name, enter gasket_prop.
  3. Click Color and select a color from the color palette.
  4. For Card Image, select PGASK from the drop-down menu and click Yes to confirm.
  5. For Material, click Unspecified > Material.
  6. In the Select Material dialog, select gask_nonlin from the list of materials and click OK to complete the selection.
  7. For MID1, select the gask_membrane material.
  8. For STABMT field, select 1 to define some stabilization stiffness.

    OS_1360_01
    Figure 2.
  9. Next, assign this property to the gasket component. Click on the component GASKET in the Model Browser.
  10. For Property, select gasket_prop property.

Assign 8-Noded Gasket Elements

  1. Click on the 3D page from the main menu.
  2. Click the elem types panel and click 2D & 3D.
  3. Click on elems, select by collector type and select the GASKET component.
  4. Toggle hex8 =, and select the CGASK8 element type.
  5. Click update > return.

Review and Adjust the Normals of the Gasket Elements

  1. Click on 2D page from the main menu.
  2. Click on the composites panel.
  3. For comps, select the GASKET component and click display normals.
    The normals of the gasket elements are not in the thickness direction, but in the Z-direction, as shown below.

    rd2090_gasket1
    Figure 3.

    So, adjusting the normals needs to be in thickness direction.

  4. Display only the GASKET component.
  5. Click on by nodes on bottom face and select the GASKET component.
  6. For choosing the face nodes, click on nodes and select three nodes on a face of any gasket element in the thickness direction and click adjust normals.
    The normals are now adjusted to be in thickness direction of gasket, as shown below.

    rd2090_gasket2
    Figure 4.
  7. Click return to go back to the main menu.

Define Contact between the Cylinders and Gasket

Now the contact surface for the bottom surface of the top cylinder needs to be defined.
  1. Hide the GASKET component and display only the SOLID1 component.
  2. In the Model Browser, right-click and select Create > Set Segment.
  3. For Name, enter SOLID1_bottom.
  4. Click Color and select a color from the color palette.
  5. For Card Image, select SURF from the drop-down menu.
  6. Click on Elements and on the yellow Elements panel.
  7. Under the modeling window, select add solid faces from the selection menu.
  8. Click elems >> displayed.
  9. Click on face nodes, select the three nodes on the bottom surface (i.e. surface contacting the gasket, as shown below) and click add.

    rd2090_solid1
    Figure 5.
  10. Click return.
  11. Next, hide the SOLID1 component and display only the SOLID2 component.
  12. Create the set segment SOLID2_top for the top surface of the SOLID2 component contacting the gasket.
  13. Similarly, repeat the steps and create GASKET_top and GASKET_bottom segments for the top and bottom surfaces of the GASKET component, respectively.

Now, an interface between the top cylinder and gasket are created.

  1. In the Model Browser, right-click and select Create > Contact.
  2. For Name, enter SOLID1_GASKET.
  3. Click Color and select a color from the color palette.
  4. For Card Image, select CONTACT from the drop-down menu.
  5. For Main Entity IDs, select the SOLID1_bottom surface.
  6. For Secondary Entity IDs, select the GASKET_top surface.
  7. For TYPE, select STICK from the drop-down menu.

    OS_1360_02
    Figure 6.

    Next, an interface between the bottom cylinder and gasket are created.

  8. In the Model Browser, right-click and select Create > Contact.
  9. For Name, enter SOLID2_GASKET.
  10. Click Color and select a color from the color palette.
  11. For Card Image, select CONTACT from the drop-down menu.
  12. For Main Entity IDs, select the SOLID2_top surface.
  13. For Secondary Entity IDs, select the GASKET_bottom surface.
  14. For TYPE, select STICK from the drop-down menu.
  15. Click review to review the interface.

    OS_1360_03
    Figure 7.

Define Nonlinear Implicit Parameters

  1. In the Model Browser, right-click and select Create > Load Step Inputs.
  2. For Name, enter NLPARM.
  3. For Config type, select Nonlinear Parameters.
    The default for Type is NLPARM.
  4. For NINC, enter 1.


    Figure 8. NLPARM Definition
  5. Use the values shown in Figure 8.
    For details on the nonlinear implicit parameters, refer to the online help.

Create NLSTAT Load Step

  1. In the Model Browser, right-click and select Create > Load Step.
  2. For Name, enter NLSTAT.
  3. Click Color and select a color from the color palette.
  4. Click Analysis type and select Nonlinear static from the drop-down menu.
  5. For SPC, select SPC from the list of load collectors.
  6. For LOAD, select LOAD from the list of load collectors.
  7. For NLPARM, select NLPARM from the list of load step inputs.

    OS_1360_04
    Figure 9.

Define Output Control Parameters

  1. From the Analysis page, select control cards.
  2. Click on GLOBAL_OUTPUT_REQUEST.
  3. Below CONTF, DISPLACEMENT, STRAIN and STRESS, set Option to Yes.
  4. Click return twice to go to the main menu.

Submit the Job

  1. From the Analysis page, click the OptiStruct panel.

    OS_1000_13_17
    Figure 10. Accessing the OptiStruct Panel
  2. Click save as.
  3. In the Save As dialog, specify location to write the OptiStruct model file and enter gasket_complete for filename.
    For OptiStruct input decks, .fem is the recommended extension.
  4. Click Save.
    The input file field displays the filename and location specified in the Save As dialog.
  5. Set the export options toggle to all.
  6. Set the run options toggle to analysis.
  7. Set the memory options toggle to memory default.
  8. Click OptiStruct to launch the OptiStruct job.
If the job is successful, new results files should be in the directory where the gasket_complete.fem was written. The gasket_complete.out file is a good place to look for error messages that could help debug the input deck if any errors are present.

View the Results

In HyperView, plot the displacement and contact pressure contours at the end of the analysis.

rd2090_displacement
Figure 11. Contour of Displacements in Cylinders and Gasket Subject to Loading

rd2090_gasket_thick
Figure 12. Contour of Gasket Thickness Direction Pressure

rd2090_contact_pressure
Figure 13. Contour of Contact Pressure