Node-to-Surface Elements Chapter Eight

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements The node-to-surface elements were the workhorse general- contact elements of the 90’s. However, fundamental limitations of node-to-surface technology have caused their rapid abandonment in favor of the newer surface-to-surface elements. Today, these elements are typically used only in highly specialized niche applications: –In 3D, contact between a line (or sharp edge) and a surface. –In 2D, contact between a node (or sharp corner) and a surface.

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements If you have a problem which includes contact at a sharp corner, the surface-to-surface elements, which use the Gauss point as the contact detection point, can experience an over penetration at the corner. In such cases, you can mix the surface-to-surface contact elements with node-to-surface elements. Surface-to-Surface Elements Node-to-Surface Elements used to model the contact at the corner

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements The node-to-surface contact elements are: Contact48 - 2D Node to Surface Contact49 - 3D Node to Surface J I K I J K M L

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements Node-to-surface contact elements are typically used to model point-to-surface contact applications such as two beams contacting (at beam tip) and the corners of snap fit parts. Although these elements can also be used to model surface- to-surface contact by defining multiple elements, this application is now generally obsolete. The location of the contacting area does not have to be known. Large deformation, large relative sliding, and different meshes between contacting surfaces are allowed.

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements Node-to-surface elements, like 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-Surface Elements Because these elements are limited to use with underlying corner-noded elements only, they are plagued by problems that result from faceted contact surfaces. –Corner-noded elements do not have curved element faces. –Curved model geometry must therefore be represented by a series of flat element faces – giving a faceted contact surface. –Faceted contact surfaces are inaccurate, giving “hot spots” of concentrated contact pressure. –Additionally, sliding motion between contact surfaces is very difficult to model successfully, with many convergence problems arising.

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements Contact48 and Contact49 are generated by identifying nodal components which define the target and contact surfaces. Select the desired nodes for the contact and target surfaces and create a nodal component for each surface. –The guidelines for contact and target surface designation are the same as for surface-to-surface elements. J K Contact Node Target Nodes I

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements Specify the element type and real constants and generate the contact elements. Each separate contact area should reference a different real constant set. Main Menu > Preprocessor > Create > Elements > Node to Surf Nodal Component Names

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements General node-to-surface capability is achieved through the use of multiple, overlapping contact elements. By default, one element connects each contact node to every potential target face. The total number of elements generated for a large surface can quickly become undesirably large. 12 CONTAC48 elements Compare to 2 CONTAC171s + 4 TARGE169s

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements You can cut down on the number of elements generated by using the NUMC or RADC features.

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements RADC limits the number of elements generated by defining a radius, centered about the centroid of each target face, within which contact elements will be generated. NUMC sets a hard numerical limit on the number of contact elements that can be generated for each target face. Only those contact nodes that lie within RADC will connect to a target element face.

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements These elements use the penalty method for contact compatibility by default. As an option, a combined penalty and Lagrange multiplier can be used. –You specify a contact stiffness, having units of Force/Length. –Previous discussions about how to determine a contact stiffness apply to these elements as well. –The stiffness can be updated simply by respecifying it in a restart. As with the surf-to-surf elements, this can give you both good convergence behavior, and good accuracy. –For combined penalty plus Lagrange multiplier, also specify a penetration tolerance, having units of length.

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements Both elements also support friction. –Specify a nonzero value for MU and set Keyopt(3) to 1. –A real constant (FACT; the ratio of static to dynamic coefficient of friction) can also be specified. MU = dynamic coefficient of friction. FACT x MU = static coefficient of friction. A tangential stiffness is used for elastic Coulomb friction. –By default, the tangent stiffness equals the 1% of the normal stiffness. –Tangent stiffness cannot be updated in a restart. Neither element provides an offset capability. –Thickness effects of beams and shells cannot be readily modeled with these elements.

Training Manual October 15, 2001 Inventory # Node-to-Surface Elements Postprocessing of contact results items for these elements require the use of ETABLE.

Training Manual October 15, 2001 Inventory # Node-to-surface elements Workshop Please refer to your Workshop Supplement for instructions on: W12. Node-to-Surface Contact