Node-to-Node Elements Chapter Seven

Training Manual October 15, 2001 Inventory # Node-to-Node Elements Although the node-to-node contact elements are the oldest contact elements in the ANSYS program, the development work continues. More features have recently been added together with a new element, CONTA178, introduced in 5.7. These are the simplest and least expensive (in terms of solution CPU) contact elements available. When modeling conditions warrant their use, they can be an effective tool for modeling a variety of contact situations.

Training Manual October 15, 2001 Inventory # The most commonly used node-to-node elements are: CONTAC52 and CONTA178 can also be used for 2D, by constraining all UZ DOFs. Of the three node-to-node contact elements, CONTA178s offer the most extensive capability: –More contact algorithms –Exact compatibility of contact constraint (‘zero” penetration) –More options for contact behaviors –More flexibility for defining contact normals... Node-to-Node Elements I J CONTAC12 - 2D Gap CONTAC52 - 3D Gap I J CONTA D Gap (with damping) I J

Training Manual October 15, 2001 Inventory # Node-to-Node Elements Node-to-node contact elements can be used to model point- to-point contact. –A pipe whip model where the contact point is always known is an example of point-to-point contact. These elements can also be used to model surface-to-surface contact problems if: –The amount of relative sliding deformation is negligible –The deflections (rotations) of the two surfaces remain small. –An interference fit problem is an example of a surface-to-surface contact problem where the use of gap elements may be sufficient, if the above conditions are met. –Note: if the nodes on the two surfaces do not line up, the gap direction may also be specified explicitly.

Training Manual October 15, 2001 Inventory # Node-to-Node Elements Node-to-node elements transmit forces at the nodes. –Compare to surf-to-surf, which transmit pressures at Gauss points. This feature limits their use to lower-order (corner-noded) elements only. –This is due to the uneven nature of the kinematically consistent reaction forces at the nodes of midside-noded elements: 1/2 1/6 2/3 -1/12 1/3 1/4 -1/12

Training Manual October 15, 2001 Inventory # Node-to-Node Elements Node-to-node contact elements can be created in three ways: 1.Direct generation 2.Creating elements at coincident (or nearly coincident) nodes. Select the nodes on the surfaces Set element attributes for the node-to-node elements Then create elements using: Preprocessor -> Create -> Elements -> At Coincid Nd Note: This feature requires that the separation between surfaces must be less than the tolerance of coincidence, and must also be less than the distance between nodes on each surface.

Training Manual October 15, 2001 Inventory # Node-to-Node Elements 3.Creating elements at offset nodes: –Convenient for non-coincident nodes Preprocessor -> Create -> Elements -> Offset Nodes … Specify the distance and the offset direction. Example: contact between two concentric pipes separated by a gap. Use a cylindrical coordinate system centered in O and set DX equal to .

Training Manual October 15, 2001 Inventory # Node-to-Node Elements Orientation of Gap Interface CONTAC52 and CONTA178 require a node separation to automatically orient the element along the I-J axis, –Take care to ensure that the I-J direction is perpendicular to the contact surfaces. –Otherwise, an undesired tangential component of response will arise.

Training Manual October 15, 2001 Inventory # Node-to-Node Elements KEYOPT(5) for CONTA178 provides additional options to explicitly specify the element orientation. Contact normal aligned along the average of the direction cosines of the X-axis of the nodal coordinates on both nodes I and J. For this option, rotate the contact element nodal coordinate system to point to the surface normal of the solid model: Preprocessor > Move/Modify > Rotate Node CS - To Surf Normal > On Area Contact normal based real constants NX, NY, NZ. Contact normal aligned with the element coordinate system.

Training Manual October 15, 2001 Inventory # Node-to-Node Elements Contact Compatibility CONTA178 offers several different contact algorithms: –Pure Lagrange multiplier method Near zero penetration and slip, no contact stiffness required More DOFs, chattering becomes an issue Avoid when using the PCG solver –Augmented Lagrange method, requires FKN and TOLN –Pure penalty method –Lagrange multiplier on normal & penalty on tangent CONTAC12 and CONTAC52 are limited to the penalty method –You specify a contact stiffness, having units of Force/Length. –Refer to the Appendix in chapter 3 about how to determine a good value for the contact stiffness.

Training Manual October 15, 2001 Inventory # Node-to-Node Elements Friction and Contact Behavior All three elements support friction, by specifying a nonzero value for MU. CONTA178 supports the same types of contact behavior as the surface-to- surface elements: –Standard – Rough – No Separation (sliding) – Bonded – No Separation (always) – Bonded Contact (always) – Bonded Contact (initial contact) CONTAC12 and CONTAC52 are limited to standard friction behavior. Initial Interference CONTA178 allows an initial interference to be ramped. CONTAC12 and CONTAC52 support an initial gap or interference, but it cannot be ramped. Postprocessing Postprocessing of contact results items for these elements requires the use of ETABLE operations.

Training Manual October 15, 2001 Inventory # Node-to-node elements Workshop Please refer to your Workshop Supplement for instructions on: W11. Node-to-Node Contact