2013 ANSYS, Inc. March 14, 2013 1

ANSYS Mechanical Advanced Connections

Lecture 2 Interface Treatments

2013 ANSYS, Inc. March 14, 2013 2

Chapter Overview

between contact and target surfaces to simulate more advanced relationships

The following will be covered in this Lecture:

A. Rigid Body Motion

B. Contact Surface Offset

C. Workshop 2A - Interference Fit

D. Contact Stabilization Damping

E. Workshop 2B - Contact Stabilization

F. MPC Interface Options

G. Time Step Controls

H. Friction

I. Workshop 2C -Friction

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A. Rigid Body Motion

Rigid body motion can occur in the beginning of a static analysis due to the fact that an initial contact condition is not well established.

The finite element method cannot reconcile rigid-body motion in a static structural analysis.

- If an initial gap is present and a force based loading is applied, initial contact may not another part.

Fn

Target

Contact

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For linear contact (bonded or no separation)behavior, a large enough Pinball Radius may allow any gap between Contact and Target surfaces to be ignored.

For nonlinear contact (frictional or frictionless) behavior, an initial gap is not automatically ignored.

Fn

Target

Contact

... Rigid Body Motion

Pinball Radius

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B. Contact Surface Offset

To alleviate situations where a negligible gap exists between contact and target surfaces, there are two options available under Interface Treatment to internally offset the Contact surfaces by a specified amount.

On the left is the original model (mesh). The top red mesh is the body associated with the Contact surfaces

The Contact surface can be mathematically offset by a certain amount, as shown on the right in light green. This adjustment will allow for initial contact to be established.

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Important notes on Contact Offset Feature:

This is a mathematical adjustment only. Nodes and elements are not altered.

amount.

It has the effect of a change in geometry at the contact interface since a

surface.

It is intended for applications where this adjustment is small enough to have a negligible effect on overall results.

It has proven to be a useful tool to establish initial contact in static analyses without having to modify the CAD geometry.

... Contact Surface Offset

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- Mechanical determines what contact offset amount is needed to close the gap and establish initial contact. Note that the contact status must be near field open (size of the Pinball Region must envelop the gap) for this to work. If all contact elements are far-field open, no adjustment will be made.

positive or negative distance to offset the contact surface. A positive value will tend to close a gap while a negative value will tend to open a gap.

This can also be used to model initial interference fits without modifying the geometry. Model the geometry in just-touching position and change the positive distance value to the interference value.

... Contact Surface Offset

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- Applies the interference gradually over several substeps within a load step.

This option is useful to enhance convergence for challenging interference problems.

- Applies the interference all at once in first substep.

... Contact Surface Offset

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C. Workshop

Please Refer to your Workshop Supplement

W2A: Interference Fit

../Workshop_instructions_Staff/Mech_AC_145_WS2a-offset.ppt../Workshop_instructions_Staff/Mech_AC_145_WS2a-offset.ppt

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D. Contact Stabilization Damping

be ignored).

- For such situations, Contact Stabilization Damping can be used to introduce a

viscous damping traction proportional to but opposite to the relative pseudo

velocities between the two surfaces along contact normal and/or tangential

directions.

Where: = damping coefficient in normal direction

= damping coefficient in tangential direction

= pseudo velocity

Fn

Target

Contact

Ft

22

11

udP

udP

udP

td

td

nndn

Pdn

Pd1,d2

u

d

d

t

n

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The damping coefficients are calculated as a function of the opening gap and pinball as follows:

Where: FDMN = scaling factor in normal direction (default =1.0)

FDMT = scaling factor in tangential direction (default = 0.001)

PINB = Pinball radius

0

/)(**

0

/)(*

PINBUPINBFDMTFDMNd

PINBUPINBFDMNd

n

t

n

n

pinbnn

pinbn

UUU

UU

gt or 0 If

If

pinbnn

pinbn

UUU

UU

gt or 0 If

If

... Contact Stabilization Damping

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Contact stabilization is inactive by default.

As an exception, it will be activated automatically if ALL of the following conditions are met:

Gauss point or surface projection based detection.

AND the entire contact pair is in near field open status.

AND a geometric penetration is detected at any contact nodal point, despite the initially open status.

... Contact Stabilization Damping

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Stabilization Damping Factor is applied in the contact normal direction and it is valid only for nonlinear contact (frictionless, rough and frictional contacts).

If this factor is 0 (default), the damping is activated under the conditions mentioned previously and only in the first load step

If its value is greater than 0, the damping is activated for all load steps.

Additional controls are available via KEYOPT(15) in a command object.

Tangential damping factor is not directly exposed in Mechanical, but can be manually defined via RMODIF in a command object.

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Note that the Energy introduced into the model by Contact Stabilization Damping is artificial.

It can alleviate convergence problems, but it can also affect solution accuracy if the applied stabilization energy generated by the damping forces are too large

In most cases, the program automatically activates and deactivates contact stabilization damping and estimates reasonable damping forces.

However, it is a good practice to check the stabilization energy and reaction forces.

The contact stabilization energy can be post processed via the ETABLE command using the AENE label in a command object. This should be compared to element potential energy via SENE label on ETABLE.

For example: ETABLE,AE,AENE !save artificial energies associated with stabilization

ETABLE,SE,SENE !save strain energies to element table

SSUM !sum all element energies for comparison

PRETAB,AE,SE !print element table values

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Example: Consider a fixed pin interfacing with a hole in plate with initial radial clearance and under a force based load

Contact Stabilization Damping

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Contact Stabilization can be more challenging to converge as compared to -to-

into the model.

Conventional Adjust to Touch Contact Stabilization Damping

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Mathematical adjustment to close

gap causes rigid region to exist No Gap

Conventional Adjust to Touch Contact Stabilization Damping

Example: Consider a hyperelastic seal against a hard surface

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E. Workshops

Please refer to your Workshop Supplement

W2B: Contact Stabilization

../Workshop_instructions_Staff/Mech_AC_145_WS2b-Stabilization.ppt../Workshop_instructions_Staff/Mech_AC_145_WS2b-Stabilization.ppt

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With MPC Formulation, there are different Constraint Types

available to address unique challenges that can arise when using

CEs (constraint equations) to connect entities together.

- (default in WB-Mechanical) is

sufficient for most applications

- Note, the typical MPC equation listed below establishes a relationship

between the rotational DOF of node 6212 on one surface body edge

with displacement DOFs on adjacent surface

F. MPC Interface Options

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- creates CEs that separate the

rotational and displacement DOFs into separate equations to improve

results for special applications to remove artificial stiffness at the

connection.

... MPC Interface Options

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For example, consider the case of two

surface body edges connected together

with MPCs

- The default, , creates too many

constraints, causing an artificial stiffness at the connection and

resulting in a discontinuity of stress and strain distribution that should

not be there

- produces expected results

... MPC Interface Options

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allows

contact detection regardless of element

normal direction

- This option is especially helpful for unique

applications of connecting a line body edge

with a shell or solid face or edge

option

properly creates CEs using nodes around

entire perimeter of shell edge, regardless of

beam element normal direction

MPCs created with Default based on beam

element normal direction

... MPC Interface Options

Shell elements

Be

am

No

rma

l D

irect

ion

Be

am

No

rma

l D

irect

ion

Shell elements

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Time Step Controls offers an additional layer of convergence enhancement that allows bisections and adjustments to time step size based on changes in contact behavior.

This choice is displayed only for nonlinear contact:

Frictionless

Rough

Frictional

G. Time Step Controls

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By default, changes in contact behavior do not influence automatic time stepping.

Appropriate for most analyses

Bisections triggered by contact status change might be an unnecessary detriment to

overall run time efficiency.

In the example below, there is no benefit to reducing the time step because of the contact status change (from closed to open). Yet, auto time stepping for other criteria often still is desirable.

Step 1 Step 2

Status = Closed Status = Closed Status = Open-near field

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Automatic Bisection - Contact behavior is reviewed at the end of each substep to determine whether excessive penetration or drastic changes in contact status have occurred. If so, the current substep is re-evaluated

In the example below, automatic bisection might enhance convergence when a localized contact status change results in a dramatic change in the stiffness response of the overall structure.

Use auto time

step for contact

status change in

this region only.

F

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Predict for Impact - Same as the Automatic Bisection option except that this option also predicts the minimal time increment needed to detect future changes in contact status.

Recommended if impact is anticipated.

Substep 1

Substep 2

Substep 3

Contact

missed due

to coarse

time step

Time step is

auto decreased

to capture the

contact status

change

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Use Impact Constraints: - Newmarkaccurate energy balance at the contact interface.

Helpful for high impact transient dynamic problems where inertia effects are important.

Applicable to situations where the energy created from a local contact penetration at time of impact exceeds the total potential energy of the original system at time zero.

slide.

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H. Frictional Contact Options

In general, the tangential or sliding behavior of two contacting bodies may be frictionless or involve friction.

Frictionless behavior allows the bodies to slide relative to one another without any resistance.

When friction is included, shear forces can develop between the two bodies.

Frictional contact may be used with small-deflection or large-deflection analyses

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Where: =coefficient of static friction

Once the tangential force Ftangential exceeds the above value, sliding will occur

normaltangential FF

Fn

Ft

Fn

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A Friction Coefficient of 0.0 results in

contact

The default contact formulation

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If frictional contact is present, additional contact output is available

Contact Frictional Stress and Contact Sliding Distance can be reviewed to get a better

understanding of frictional effects

moving