contact beam

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Introduction

This tutorial was completed using ANSYS 7.0 The purpose of the tutorial is to describe how

to utilize contact elements to simulate how two beams react when they come into contact with

each other.

The beams, as shown below, are 00mm long, 0mm ! 0mm in cross"section, ha#e a

Young$s modulus of %00 &'a, and are rigidly constrained at the outer ends. A 0(N load is

applied to the center of the upper, causing it to bend and contact the lower.

Preprocessing: Defining the Problem

. Give example a Title

)tility *enu + ile + -hange Title .../title, Contact Elements

%. Open preprocessor menu

ANSYS *ain *enu + 'reprocessor/PREP7

. Define Areas

'reprocessor + *odeling + -reate + Area + /ectangle + y % -ornersBLC4,WP X, WP Y, Width, Height

1e are going to define % rectangles as described in the following table2

Rectangle Variables !P "#!P $#!idth#%eight&

30, 4, 00, 05

% 340, 0, 00, 05

'( Define the T)pe of *lement

o 'reprocessor + 6lement Type + Add6dit8elete...

or this problem we will use the '9AN6:% 3Solid, ;uad :node :%5 element.

This element has % degrees of freedom at each node 3translation along the t initiates a process during the solution calculations where the

time step or load step, depending on what the user has specified in the solution

controls, incremements slowly when contact is immenent. This way, one

surface won$t penetrate too far into the other and cause the solution to fail.

>t is important to note, -@NTA-: elements are created in the space between two

surfaces prescribed by the user. This will be co#ered below. As the surfaces approach

each other, the contact element is slowly BcrushedB until it$s upper node3s5 lie along

the same line as the lower node3s5. Thus, ANSYS can calculate when the two

prescribed surfaces ha#e made contact. @ther contact elements, such as -@NTA74,

reCuire a target element, such as TA/&6DE, to function. 1hen using contact

elements in your own analyses, be sure to understand how the elements wor=. The

ANSYS help file has plenty of useful information regarding contact elements and isworth reading.

2. Define Real .onstants for the .ontact *lements

'reprocessor + /eal -onstants... + Add...

>n the $/eal -onstants for -@NTA-:$ window, enter the following

properties2


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. Normal contact stiffness (N2 %00000

-@NTA-: elements basically use a penalty approach to model contact. 1hen one surface

comes into BcontactB with the other, ANSYS numerically puts a spring of stiffness (N

between the two. ANSYS recommends a #alue between 0.0 and 00 times Young$s modulus

for the material. Since this BspringB is so stiff, the beha#iour of the model is li=e the twosurfaces ha#e made contact. This (N #alue can greatly affect your solution, so be sure to read

the help file on contact so you can recognize when your solution is not con#erging and why.

A good rule of thumb is to start with a low #alue of (N and see how the solution con#erges

3start watching the ANSYS @utput 1indow5. >f there is too much penetration, you should

increase (N. >f it ta=es a lot of iterations to con#erge for a single substep, you should

decrease (N.

%. Target length tolerance T@9S2 0

/eal constant T@9S is used to add a small tolerance that will internally increase the length of

the target. This is useful for problems when node to node contact is li=ely to occur, rather thannode to element edge. >n this situation, the contact node may repeatedly BslipB off one of the

target nodes, resulting in con#ergence difficulties. A small #alue of T@9S, gi#en in F, is

usually enough to pre#ent such difficulties.

The other real constants can be used to model sliding friction, tolerances, etc. >nformation

about these other constants can be found in the help file.

. Define 4odes for .reating .ontact *lements

)nli=e the normal meshing seCuence used for most elements, contact elements must be

defined in a slightly different manner. Sets of nodes that are li=ely to come into contact must

be defined and used to generate the necessary elements. ANSYS has many recommendations

about which nodes to select and whether they should act as target nodes or source nodes. >n

this simple case, source nodes are those that will mo#e into contact with the other surface,

where as target nodes are those that are contacted. These terms are important when using the

automatic contact element mesher to ensure the elements will correctly model contact

between the surfaces. A strong understanding of how the elements wor= is important when

using contact elements for your own analysis.

irst, the source nodes will be selected.

)tility *enu + Select + 6ntities...

Select Areasand 5) 4um2Pic6from the pull down menus, select 7rom 7ullfrom the radio

buttons and clic= @(. Select the top beam and clic= @(. This will ensure any nodes that are

selected in the ne!t few steps will be from the upper beam. >n this case, it is not too hard to

ensure you select the correct nodes. ?owe#er, when the geometry is comple!, you may

inad#ertantly select a node from the wrong surface and it could cause problems during

element generation.


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Select 4odesand 5) 8ocationfrom the pull down menus, $ coordinatesand Reselectfrom

the radio buttons and enter a #alue of 4 and clic= @(. This will select all nodes along the

bottom of the upper beam.


Select 4odesand 5) 8ocationfrom the pull down menus, " coordinatesand Reselectfromthe radio buttons and enter #alues of 40,00. This will select the nodes abo#e the lower beam.


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Now if you list the selected nodes, 9tilit) /enu 8ist 4odes(((you should only ha#e the

following nodes remaining.


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>t is important to try and limit the number of nodes you use to create contact elements. >f you

ha#e a lot of contact elements, it ta=es a great deal of computational time to reach a solution.

>n this case, the only nodes that could ma=e contact with the lower beam are those directly

abo#e it, thus those are the only nodes we will use to create the contact elements.

)tility *enu + Select + -ompAssembly + -reate -omponent

6nter the component name %o&ceas shown below, and clic= @(. Now we can use this

component, Source, as a list of nodes to be used in other functions. This can be #ery useful in

other applications as well.

Now select the target nodes.

)sing the same procedure as abo#e, select the nodes on the lower beam directly under the

upper beam. e sure to reselect all nodes before starting to select others. This is done by

opening the entity select menu, 9tilit) /enu 0elect *ntities(((, clic=ing the Also 0elect

radio button, and clic= the 0ele Allbutton.

These #alues will be the ones you$ll use.

o -lic= the lower area for the area select.

o The Y coordinate is 0

o The < coordinates #ary from 40 to 00.

1hen creating the component this time, enter the name Target.

I/PORTA4T2 e sure to reselect all the nodes before continuing. This is done by opening

the entity select menu, 9tilit) /enu 0elect *ntities(((, clic=ing the Also 0elect radio

button, and clic= the 0ele Allbutton.

;( Generate .ontact *lements

*ain *enu + 'reprocessor + *odeling + -reate + 6lements + 6lem Attributes

ill the window in as shown below. This ensures ANSYS =nows that you are dealing with the

contact elements and the associated real constants.


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*ain *enu + 'reprocessor + *odeling+ -reate + 6lements + Surf -ontact + Node to Surf

The following window will pop up. Select the node set S@)/-6 from the first drop down

menu 3-comp5 and TA/&6T from the second drop down menu 3Tcomp5. The rest of the

selections remain unchanged.

At this point, your model should loo= li=e the following.


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)nfortunately, the contact elements don$t get plotted on the screen so it is sometimes difficult

to tell they are there. >f you wish, you can plot the elements 3)tility *enu + 'lot + 6lements5

and turn on element numbering 3)tility *enu + 'lot-trls + Numbering + 6lemAttrib

numbering + 6lement Type Numbers5. >f you zoom in on the contact areas, you can see little

purple stars 3-ontact Nodes5 and thin purple lines 3Target 6lements5 numbered B%B which

correspond to the contact elements, shown below.

The preprocessor stage is now complete.

0olution Phase: Assigning 8oads and 0olving


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6nsure the following selections are made under the $asic$ tab 3as shown abo#e5

A. 6nsure Automatic time stepping is on. Automatic time stepping allows

ANSYS to determine appropriate sizes to brea= the load steps into.

8ecreasing the step size usually ensures better accuracy, howe#er, this

ta=es time. The Automatic Time Step feature will determine anappropriate balance. This feature also acti#ates the ANSYS bisection

feature which will allow reco#ery if con#ergence fails.

. 6nter 00 as the number of substeps. This will set the initial substep to

00 thof the total load.

-. 6nter a ma!imum number of substeps of 000. This stops the program

if the solution does not con#erge after 000 steps.

8. 6nter a minimum number of substeps of %0.

6. 6nsure all solution items are writen to a results file.

6nsure the following selection is made under the $Nonlinear$ tab 3as shown below5

. 6nsure *a!imum Number of >terations is set to 00

4OT*

There are se#eral options which ha#e not been changed from their default

#alues. or more information about these commands, type help followed by

the command into the command line.

. These solution control #alues are e!tremely important in determining if your analysis

will succeed or fail. >f you ha#e too few substeps, the contact nodes may be dri#en

through the target elements before ANSYS BrealizesB it has happened. >n this case the

solution will resemble that of an analysis that didn$t ha#e contact elements defined at

all. Therefore it is important to choose a relati#ely large number of substeps initially to

ensure the model is defined properly. @nce e#erything is wor=ing, you can reduce thenumber of substeps to optimize the computational time. Also, if the ma!imum number


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of substeps or iterations is left too low, ANSYS may stop the analysis before it has a

chance to con#erge to a solution. Again, lea#e these relati#ely high at first.

'( Appl) .onstraints

Solution + 8efine 9oads + Apply + Structural + 8isplacement + @n 9ines

i! the left end of the upper beam and the right end of the lower beam 3ie all

8@ constrained5

-( Appl) 8oads

Solution + 8efine 9oads + Apply + Structural + orce*oment + @n Nodes

Apply a load of "0000 in the Y direction to the center of the top surface of

the upper beam. Note, this is a point load on a %8 surface. This type of loading

should be a#oided since it will cause a singularity. ?owe#er, the displacement

or stress near the load is not of interest in this analyis, thus we will use a pointload for simplicity.

The applied loads and constraints should now appear as shown in the figure below.

>( 0olve the 0)stem

Solution + Sol#e + -urrent 9S%*L+E

Postprocessing: Vie?ing the Results

. Open postprocessor menu


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ANSYS *ain *enu + &eneral 'ostproc/P*%T

;( Ad@ust Graphical 0caling

)tility *enu + 'lot-trls + Style + 8isplacement Scaling

-lic= the lost many

hours trying to figure out why the contact elements weren$t wor=ing, when in fact it was Hust

due to the displacement scaling to which ANSYS defaulted. >f you lea#e the scaling as

default, many times it will loo= li=e your contact nodes ha#e gone through the target

elements.

. 0ho? the 0tress Distribution in the 5eams

&eneral 'ostproc + 'lot /esults + -ontour 'lot + Nodal Solu + Stress + #on *ises

:. Ad@ust .ontour 0cale

)tility *enu + 'lot-trls + Style + -ontours + Non")niform -ontours

ill in the window as follows2

This should produce the following stress distribution plot2


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. As seen in the figure, the load on the upper beam caused it to deflect and come in

contact with the lower beam, producing a stress distribution in both.

contact beam

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