Rigid body to Rigid body contact
The Simulation Show
jmk
Dear Andy,
2.4 Constraints and Rigid Bodies
Avoid the use of *CONSTRAINED_NODE_SET unless nodes in the node set are coincident. To rigidly connect two or more deformable, noncoincident nodes, use *CONSTRAINED_NODAL_RIGID_BODY instead of *CONSTRAINED_NODE_SET so that nonphysical resistance to rotation is not imposed. A rigid body can be defined in LS-DYNA in several ways:
• A part that references *MAT_RIGID is a rigid body.
• A set of nodes referenced by *CONSTRAINED_NODAL_RIGID_BODY forms a rigid body.
• A node or set of nodes referenced by *CONSTRAINED_EXTRA_NODES is added to or becomes a rigid body.
Rigid bodies are subject to certain rules of modeling to which deformable bodies are not. For example:
• With few exceptions, *CONSTRAINED_OPTION cannot be applied to any element or part that is rigid or to any node that is included in a rigid body. The exceptions are * CONSTRAINED_RIGID_BODIES, * CONSTRAINED_JOINT, and * CONSTRAINED_LAGRANGE_IN_SOLID (if penalty-based coupling is used).
• Constraint-based contact algorithms, predominately used in LS-DYNA in tied (not tiebreak) contact types, cannot be used on rigid bodies.
• Prescribed motion cannot be applied to more than one node of a rigid body. The preferred method of prescribing motion to a rigid body is via *BOUNDARY_PRESCRIBED_MOTION_RIGID in which the motion is defined with respect to the center-of-mass of the rigid body.
• Though nodal single-point-constraints (*BOUNDARY_SPC) on nodes of rigid bodies will internally be converted by LS-DYNA into an equivalent set of constraints on the rigid body’s center-of-mass, the preferred method of constraining rigid body motion is via Card 2 of *MAT_RIGID, or if the rigid body is defined as a nodal rigid body, via *CONSTRAINED_NODAL_RIGID_BODY_SPC. *BOUNDARY_PRESCRIBED_MOTION_RIGID would be another acceptable means of constraining a rigid body (velocity or displacement could be prescribed as zero). The mesh of rigid bodies should generally be no coarser than that of a deformable body, unless the rigid body’s motion is fully constrained and/or prescribed, or unless the mass properties are specified directly via *PART_INERTIA. Because mass is lumped at the nodes, a coarse mesh will often give highly inaccurate inertia values.
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Sincerely,jmk
Components for which deformation is negligible and stress is unimportant may be modeled as rigid bodies using *MAT_RIGID or *CONSTRAINED_NODAL_RIGID_BODY. The elastic constants defined in *MAT_RIGID are used for contact stiffness calculations. Thus the constants should be reasonable (properties of steel are often used).
Though there are several contact types in LS-DYNA which are applicable specifically to rigid bodies (RIGID appears in the contact name), these types are seldom used. Any of the penalty-based contacts applicable to deformable bodies may also be used with rigid bodies, and in fact, are generally preferred over the RIGID contact types. Rigid bodies and deformable materials may be included in the same penalty-based contact definition. Constraints and constraint-based contacts may not be used for rigid bodies.
Rigid bodies should have a reasonably fine mesh so as to capture the true geometry of the rigid part. An overly coarse mesh may result in contact instability. Another meshing guideline is that the node spacing on the contact surface of a rigid body should be no coarser than the mesh of any deformable part which comes into contact with the rigid body. This promotes proper distribution of contact forces. As there are no stress or strain calculations for a rigid body, mesh refinement of a rigid body has little effect on cpu requirements. In short, the user should not try to economize in the meshing of rigid bodies.
*CONTACT_ENTITY is an altogether different way of defining an analytic, rigid contact surface which interacts with nodes of deformable bodies. For more information
Sincerely,
James M. Kennedy
KBS2 Inc.
November 5, 2022
jmk
Dear Andy,
2.4 Constraints and Rigid Bodies
Avoid the use of *CONSTRAINED_NODE_SET unless nodes in the node set are coincident. To rigidly connect two or more deformable, noncoincident nodes, use *CONSTRAINED_NODAL_RIGID_BODY instead of *CONSTRAINED_NODE_SET so that nonphysical resistance to rotation is not imposed. A rigid body can be defined in LS-DYNA in several ways:
• A part that references *MAT_RIGID is a rigid body.
• A set of nodes referenced by *CONSTRAINED_NODAL_RIGID_BODY forms a rigid body.
• A node or set of nodes referenced by *CONSTRAINED_EXTRA_NODES is added to or becomes a rigid body.
Rigid bodies are subject to certain rules of modeling to which deformable bodies are not. For example:
• With few exceptions, *CONSTRAINED_OPTION cannot be applied to any element or part that is rigid or to any node that is included in a rigid body. The exceptions are * CONSTRAINED_RIGID_BODIES, * CONSTRAINED_JOINT, and * CONSTRAINED_LAGRANGE_IN_SOLID (if penalty-based coupling is used).
• Constraint-based contact algorithms, predominately used in LS-DYNA in tied (not tiebreak) contact types, cannot be used on rigid bodies.
• Prescribed motion cannot be applied to more than one node of a rigid body. The preferred method of prescribing motion to a rigid body is via *BOUNDARY_PRESCRIBED_MOTION_RIGID in which the motion is defined with respect to the center-of-mass of the rigid body.
• Though nodal single-point-constraints (*BOUNDARY_SPC) on nodes of rigid bodies will internally be converted by LS-DYNA into an equivalent set of constraints on the rigid body’s center-of-mass, the preferred method of constraining rigid body motion is via Card 2 of *MAT_RIGID, or if the rigid body is defined as a nodal rigid body, via *CONSTRAINED_NODAL_RIGID_BODY_SPC. *BOUNDARY_PRESCRIBED_MOTION_RIGID would be another acceptable means of constraining a rigid body (velocity or displacement could be prescribed as zero). The mesh of rigid bodies should generally be no coarser than that of a deformable body, unless the rigid body’s motion is fully constrained and/or prescribed, or unless the mass properties are specified directly via *PART_INERTIA. Because mass is lumped at the nodes, a coarse mesh will often give highly inaccurate inertia values.
Sincerely,
James M. Kennedy
KBS2 Inc.
Novermber 5, 2022