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By John Crawford

Consulting Analyst

Demystifying Contact Elements

If you analyze enough problems,

chances are good that sooner or

later youll run across one that

requires the use of contact

elements. Contact elements are

used to simulate how one or

more surfaces interact with eachother. For most analysts, our first exposure to contact

elements can be a little confusing because of the

variety of elements and the multitude of special

features that are available.

We have to determine which contact elements

are appropriate for our problem, resolve any

convergence problems that might arise during

solution, and check the results for reasonable and

accurate answers. Lets see if we can clear up some of

the mysteries that surround the use of contact

elements. Well begin by talking about the elements

themselves.

Node-to-Node Elements

In the early days of finite element analysis, there was

one type of contact element: the node-to-node variety.

The early versions of node-to-node contact elements

were CONTAC12 (2-D) and CONTAC52 (3-D). More

recently, CONTA178 (2-D and 3-D) was introduced to

encompass the capabilities of both of these elements

and also introduce some new features, such as

additional contact algorithms. Node-to-node contact

elements are simple and solve relatively quickly. Their

basic function is to monitor the movement of one nodewith respect to another node. When the gap between

these nodes closes, the contact element allows load

to transfer from one node to the other. What does this

really mean and how does ANSYS know when the

nodes are touching?

Remember that an analysis is made up of one or

more load steps, and each load step has one or more

substeps. Within each substep there can be several

nested layers of equilibrium iterations. The precise

number and manner in which they are nested is

dependent on the solver, how many nonlinear features

are being used and several other things. Contact

analyses are nonlinear and therefore require their own

equilibrium iteration loop. At the end of each contact

equilibrium iteration, ANSYS checks to see if thestatus of each contact element has changed. It also

calculates a convergence value (usually force

equilibrium) and compares it to the convergence

criteria. If the element status has not changed and the

convergence criteria has been met, ANSYS

determines that the solution for this iteration has

converged and moves on to the next outer iteration

loop, the next substep or the next load step, or stops

solving altogether if the analysis is now complete.

If at this point youre a little confused, dont

worry. The critical ideas to remember from this are

the following:

Contact analyses are nonlinear in nature

ANSYS performs a special equilibrium

iteration loop when doing a contact analysis

Contact elements have a status that

indicates if they are open, closed, sliding, etc.

ANSYS checks the element status and the

convergence criteria at the end of each

contact equilibrium iteration to determine if

equilibrium has been achieved

Part 1 of 2:

What they are, how they work and when to use them.

Tech File

33

www.ansys.com ANSYS Solutions | Summer 2004


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34

Tech File

These characteristics are true for all types of contact

elements. While they may seem a little primitive when

compared with the newer contact elements, node-to-

node contact elements have a lot going for them.

Theyve been around long enough to have had their

bugs worked out many years ago, and their extensive

use over several decades means that there is a vast

experience base to draw upon when setting up and

debugging an analysis. CONTAC12 and CONTAC52

can have nodes that are either coincident or non-coin-

cident. While the majority of applications involve using

non-coincident nodes, coincident nodes can be use-

ful for certain analyses. If coincident nodes are used,

the orientation of the contact surface that exists

between the two nodes must be defined. The initial

condition gap or interference can be provided by the

user as being either positive (gap) or negative (interfer-

ence), or automatically calculated from the relative

positions of the nodes.

Node-to-node contact is also available in

COMBIN40. COMBIN40 is a rather unique element

because it also includes a spring-slider, a damper

(which works in parallel with the spring-slider) and

a mass at each node. Any of these features can be

used alone or simultaneously with any or all of the

other features.

While node-to-node contact elements are very

useful, there are some limitations that must be kept in

mind when using them. One limitation is that the

orientation of the gap is not updated when large

deflection analyses are performed. Another limitation

is that these elements do not account for moment

equilibrium. This does not present a problem when a

line drawn between the nodes is normal to the contact

surface because in this instance the moments are

zero, but care should be taken in each analysis to

recognize whether this is the case or not. If not, it is

important to consider what effect this might have onthe results. It is the responsibility of the analyst to

recognize whether this condition is present and

whether it introduces an unacceptable error that

invalidates the usefulness of the analysis.

Node-to-node elements can always be generated

manually, and, depending on the model, you can often

use the EINTF command to make them as well.

Node-to-Surface Elements

The next evolution in contact elements was the

introduction of node-to-surface contact elements,

such as CONTAC26 (2-D), CONTAC48 (2-D),

CONTAC49 (3-D), and the recent addition of

CONTA175 (2-D and 3-D). The major enhancement

offered by node-to-surface contact elements is that

they allow a node to contact anywhere along an edge

(in 2-D) or a surface (in 3-D). Rather than a node being

confined to contacting a specific node, a node can

contact the edge of a certain element. This has

significant benefits when objects translate or rotate

relative to each other. Node-to-surface contactelements are capable of simulating large relative

movements with accuracy.

Because CONTA175 includes all the capabilities

of the other node-to-surface contact elements and

has other features that these elements do not have,

CONTA175 will replace the other node-to-surface

elements in future versions of ANSYS. Beginning in

ANSYS 8.1, CONTAC26, CONTAC48 and CONTAC49

will be undocumented, and they will eventually be

removed from ANSYS.

There are several ways to generate node-to-

surface contact elements. They can be made

manually, but this becomes impractical when making

more than a few elements. GCGEN and ESURF are

commands that are frequently used to generate

node-to-surface contact elements, with GCGEN being

the easiest and quickest way to make CONTAC48 and

CONTAC49 node to surface contact elements, while

ESURF is used to make CONTA175 node to surface

elements. To use GCGEN, you make two components,

one that contains the nodes from one of the contact

surfaces, and another that contains the elements from

the other contact surfaces, and then use GCGEN to

automatically generate node-to-surface contact ele-

ments between every node and every element that arein these components. To use ESURF, you select the

elements that the CONTA175 elements will be

attached to and their nodes that are on the surface

you wish to place the contact elements onto, making

sure that you have the proper element attributes active

(TYPE, REAL and MAT), and then issue the ESURF

command.

Last but not least, the Contact Wizard can be

used to generate node-to-surface contact elements

and is usually the easiest and quickest way of making

them.



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35Surface-to-Surface Elements

The latest evolution of contact element technology has

been in the area of surface-to-surface contact. This

allows contact to take place between one or more

edges in 2-D, or one or more surfaces in 3-D. There

are several important characteristics that make

surface-to-surface contact elements very different

from their less sophisticated ancestors.

Surface-to-surface contact is not defined by a

single element, but by two types of elements

called targets and contacts.

Any number of target and contact elements

can be identified as being a set or group.

Contact can take place between any contact

elements and any target elements that are in

this group.

ANSYS uses the real constant number to

identify the target and contact elements that

are in a group. All target and contact elements

in this group have the same real constant

number.

Two-dimensional contact problems can be

simulated using either CONTA171 or CONTA172 with

TARGE169, while three-dimensional problems would

use either CONTA173 or CONTA174 with TARGE170.

CONTA171 and CONTA173 are appropriate for edgesand surfaces made from linear (no midside nodes)

elements while CONTA172 and CONTA174 can be

used with edges and surfaces made from quadratic

(having midside nodes) elements. Both CONTA172

and CONTA174 can be used in a degenerate form on

surfaces made from linear elements.

The introduction of surface-to-surface contact

elements has brought about big improvements in

solution efficiency and has also broadened the types

of contact problems that can be modeled. They offer

many new and improved features, such as the ability

to contact and then bond two surfaces together,

automatic opening or closing of gaps to a uniform

value, and a variety of contact algorithms, to name just

a few.

You can generate surface-to-surface contact

elements by using series NSEL, ESEL and ESURF

commands. The Contact Wizard automates these

steps and makes the generation of surface-to-surface

contact elements quick and easy in both 2-D and 3-D.

Now that we have been introduced to the contact

elements that are at our disposal, well follow up next

time with some helpful hints on how to use them. s

Part two of this article, to appear in the next issue of ANSYSSolutions, will discuss various aspects of using contact

elements, including modeling tips and setting appropriate

stiffness.

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