Modeling null materials and rigid bodies for LS-DYNA

You can model null materials and rigid bodies in the LS-DYNA environment using the following material types:

• *MAT_NULL (*MAT_009)

• *MAT_RIGID (*MAT_020)

Modeling null materials

You can use the *MAT_NULL material to bypass some calculations during the solution.

• For beam and shell elements, LS-DYNA ignores elements that use the *MAT_NULL material. However, LS-DYNA computes the mass of these beam or shell elements and adds that mass to the nodal points that define the elements' connectivity.Note: For beam elements, if the specified mass density value is less than 1.e-11, then LS-DYNA ignores the mass.

• For solid and thick shell elements, LS-DYNA considers equations of state without computing deviatoric stresses. In LS-DYNA, you use an *EOS_ keyword to specify the properties and parameters for the type of equation. Currently, you cannot directly specify an *EOS_ keyword in Pre/Post. Instead, you can:Create a User Defined Text modeling object that contains the *EOS_ syntax. You can then use options in the Solution dialog box to insert that object into the appropriate place in the keyword file.Export the keyword file from Pre/Post. Then, use a text editor to add the *EOS_ syntax in the appropriate location in the file before you solve.

With the *MAT_NULL material, LS-DYNA only uses the values for the Young's Modulus and Poisson's Ratio for computing the contact stiffness.

The *MAT_NULL material is useful when you want to model contact surfaces with shell elements that are not part of the physical structure but that are necessary to define the regions of contact. You can also use the *MAT_NULL material to model gasses and liquids, such as air.

Modeling rigid bodies

You can use the *MAT_RIGID material to designate a portion of your model as a rigid body for the solution. For example, you may want to designate a component of your model as a rigid body when its deformation is negligible and the associated stress values are unimportant to the overall solution. When you solve a solution that contains a *MAT_RIGID keyword, LS-DYNA computes the mass, inertia tensor, and center of gravity based on the mass density specified for the *MAT_RIGID keyword.

Defining null materials and rigid bodies

Select the Use Pre/Post Material check box in the following physical property table dialog boxes to define a null material or a rigid body:

• PART (beam)

• PART (shell)

• PART(solid)

• PART(thick shell)

You can then select the *MAT_RIGID or *MAT_NULL options from the Material Type list in the appropriate physical property table dialog box.

You cannot directly specify values for all the required material properties for the *MAT_RIGID and *MAT_NULL keywords in Pre/Post. Use the Pre/Post Material option to specify a material that has the appropriate values for the properties listed in the table below. Then, when you export your solution, the software extracts the values from the associated materials and uses them to define the variables for the *MAT_RIGID and *MAT_NULL keywords in the LS-DYNA keyword file.

Material property in Pre/Post	Variable for *MAT_RIGID and *MAT_NULL keywords
Mass Density (RHO)	RO
Young's Modulus (E)	E
Poisson's Ratio (NU)	PR

When you export the solution, the software writes out the *MAT_RIGID and *MAT_NULL keywords with a unique material ID. It does not use the material ID of the associated Pre/Post material that contains the property information. Additionally, it does not write out the associated Pre/Post material to the keyword file.

Importing null and rigid materials

You can import the *MAT_RIGID and *MAT_NULL keywords into Pre/Post. The software imports those keywords as an isotropic material (*MAT_ELASTIC keyword). In the appropriate physical property table dialog box, the software selects the Use Pre/Post Material check box and displays the appropriate option in the Material Type list.

Where do I find it?

Application	Pre/Post
Prerequisite	A Simulation file as the work part and displayed partLS-DYNA as the specified solver
Command Finder	Physical Properties

Look up more details

Specifying non-laminate materials for LS-DYNA models

Creating LS-DYNA materials in Pre/Post

Defining material properties for LS-DYNA materials

Laminate materials for LS-DYNA

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Modeling null materials and rigid bodies for LS-DYNA , Simcenter 3D 2021.1 Series

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