mech ac 160 l02 interface treatments

1 2015 ANSYS, Inc. April 16, 2015 ANSYS Confidential

16.0 Release

Lecture 2:

Interface Treatments

ANSYS Mechanical

Advanced Connections


Chapter Overview

In this Lecture, we will discuss tools available for adjusting the Interface 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


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 be established, and one part may fly away relative to another part.

Fn

Target

Contact


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


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.


Important notes on Contact Offset Feature:

This is a mathematical adjustment only. Nodes and elements are not altered. The position of the contact surface is interpreted as being offset by specified amount.

It has the effect of a change in geometry at the contact interface since a rigid region will exist between the actual mesh and the offset contact 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


In the Details view, the user can select Adjust to Touch or Add Offset

Adjusted to Touch - 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.

Add Offset allows the user to specify a 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.



Add Offset, Ramped Effects - Applies the interference gradually over several substeps within a load step.

Useful to enhance convergence for challenging interference problems.

Add Offset, No Ramping - Applies the interference all at once in first substep.


Time

Offset Ramped

Substeps

Substeps

Time

Offset Stepped


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


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

Contact Stabilization introduces 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: FDMN = real constant scaling factor in normal direction (default =1.0) FDMT = real constant scaling factor in tangential direction (default = 0.001)

Vn, Vt = pseudo velocity

Contact Stabilization Damping

Fn

Target

Contact

Ft

Fdn Fdt


Click to edit Master text styles

Enhanced Contact Stabilization Scheme

Prior to this release, automatic contact damping was activated based on the contact status of the entire contact pair in the previous substep.

Now, automatic damping is activated based on the contact status of the current iteration, and damping is deactivated if any contact detection point has a closed status.

In addition, the default damping coefficient has been reduced, minimizing the risk of degraded accuracy while still providing effective stabilization.



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

* This seems like a contradiction,

but it can happen on curved

surfaces with tight clearances and

with a coarse mesh that does not

capture the curvature of contact

and target surfaces accurately.



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.


Contact Stabilization Damping 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



Example: Consider a fixed pin interfacing with a hole in plate with initial radial

clearance and under a force based load Stabilization captures localized stress distribution more accurately than Adjust to Touch

Conventional Adjust to Touch Contact Stabilization Damping


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

to-touch option. Note also, it introduces some artificial energy into the model.



Example: Consider a hyperelastic seal against a hard surface


Mathematical adjustment to close gap causes rigid region to exist

No Gap



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


Click to edit Master text styles

MPC is a true linear contact for small-deflection applications

When convergence is difficult switching to MPC is an attractive alternative to changing the contact stiffness.

MPC contact prevents artificial stiffness when gaps exist between curved surfaces. (Another alternative is to use joints)

Ideal for shell-solid, shell-shell, and beam-shell contacts

A caveat is that MPC is the most sensitive contact type to overconstraint, so avoid it when there are other contacts or boundary conditions with shared topology!

Gap between bonded parts

F. MPC Interface Options


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.

- Constraint Type options only appear in Details Window when there are Rotational DOFs in the MPC relationship (i.e. MPC contact involving shell and/or beam elements)

- Target Normal, Couple U to ROT (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

... MPC Interface Options


- Target Normal, Uncouple U to ROT 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.



For example, consider the case of two surface body edges connected together with MPCs

- The default, Target Normal, Couple U to ROT, 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

- Target Normal, Uncouple U to ROT produces expected results



Inside Pinball, Couple U to ROT 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

Inside Pinball, Couple U to ROT 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


Shell elements

Bea

m N

orm

al

Dir

ecti

on

Shell elements

Bea

m N

orm

al

Dir

ecti

on


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


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). Hence, the default is appropriate.

Time Step Controls

Step 1 Step 2

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


Time Step Controls 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 using a bisected time increment (reduced by half).

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


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 in a full transient dynamic analysis.

Recommended if dynamic impact is anticipated.

Time Step Controls

Substep 1

Substep 2

Substep 3

Contact missed due to coarse time step

Time step is auto decreased to capture the contact status change


Time Step Controls

Use Impact Constraints: - A Newmark method is used to achieve a more accurate 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.

Can be more efficient than the Predict for Impact option presented on previous slide.


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


Friction dissipates energy, and is therefore a path-dependent behavior.

The loading must be applied in the same manner as it occurs on the physical parts.

Time steps must be small for good accuracy.

Note that, unlike for plasticity, auto time stepping does not take into account the size of the frictional response increment.

Small time steps

True displacement path

Coarse time steps

A

B

Frictional Contact Options


Physical friction is a complex phenomenon that is a function of:

The contacting materials (including lubricants).

Surface roughness.

Temperature.

Relative velocity of the bodies.

The complex mechanisms involved in friction can only be mathematically approximated.

In fact, a single friction test, run at low speed with constant pressure, will frequently display a fairly erratic force-displacement behavior:

F

u




Friction is accounted for with Coulombs Law:

Where: m =coefficient of static friction

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

n o r m a lt a n g en t i a l FF m

Fn Ft

m Fn


Contact problems involving friction can produce an unsymmetric stiffness matrix.

For example, a 2D gap element will have the following unsymmetric element matrix when sliding:

However, using an unsymmetric equation solver is more computationally expensive than a symmetric solver.

For this reason, Mechanical uses a symmetrization algorithm by which most contact problems involving friction can be solved with symmetrized matrices



Newton-Raphson Option

If you experience convergence difficulties with a frictional contact involving large sliding, consider

activating the unsymmetric Newton-Raphson Option in

Details of Analysis Settings window.

Unsymmetric option can enhance convergence

Requires more computational time to obtain a

solution per iteration vs a symmetric solver.

Recommend starting with Program Controlled default and only force Unsymmetric option to improve

convergence as necessary.




For frictional contact, a friction coefficient must be input

A Friction Coefficient m of 0.0 results in the same behavior as frictionless

contact

The default contact formulation is Augmented Lagrange


Reviewing Friction Related Results

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

For Contact Status, Sticking vs. Sliding results differentiate which contacting areas are moving


I. Workshop Contact with Friction

Please refer to your Workshop Supplement for instructions on:

W2C-Contact with Friction
../Workshop_instructions_Staff/Mech_AC_145_WS2c-Friction.ppt../Workshop_instructions_Staff/Mech_AC_145_WS2c-Friction.ppt../Workshop_instructions_Staff/Mech_AC_145_WS2c-Friction.ppt

mech ac 160 l02 interface treatments

Documents

contact interface

contact surfaces

contact elements

contact surface offset

offset contact surface

contact status

contact offset feature

initial contact condition