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16.0 Release

Lecture 5:

Accessing Advanced Contact Features via

MAPDL

ANSYS Mechanical

Advanced Connections

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Chapter Overview Mechanical already exposes a rich library of options to meet the many challenges associated with simulating contact behavior.

However, there are still some contact applications requiring more advanced features not directly exposed in the Mechanical GUI. For example, consider the following cases:

Orthotropic or dynamic friction coefficient that varies as a function of relative velocity over a large displacement.

Friction where the underlying material shear strength is the weak link that dictates status change from sticking to sliding.

Surface cohesion (limited frictional resistance with zero normal force).

Fluid Pressure Penetration (Surface pressure based on contact status)

Archard surface wear model

and more

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Chapter Overview This lecture presents how to access these advanced contact features not directly available in the Mechanical GUI.

This is not intended to be a comprehensive presentation on all the advanced feature details, but rather a presentation on the general set-up procedure for using command objects successfully within contact regions in Mechanical.

The following topics will be covered:

A. Background on Contact Pair

B. Contact Documentation

C. MAPDL Command Syntax

D. General Procedure

E. Example Cases

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A. Background on contact pair

Recall that Face-to-Face, and Edge-to-Face contact use the concept of a contact pair, which is composed of target elements and contact elements

The contact elements overlie the underlying finite element model like a skin.

Face-to-Face populate the contact surface with Gauss points

Others use the node(s) on contact side directly (no gauss pts)

Separate element types define the target and contact surfaces.

The contact pair is identified through a shared real constant set.

Contact elements (REAL = N )

on the contact surface

Target elements (REAL = N )

on the target surface

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... Background on contact pair All the parameters that control the contact pair behavior are identified as either Key Option settings or as properties associated with the Real Constant Set.

A Key Option is analogous to a switch that controls a particular element behavior

Contact formulation, Type (bonded, frictionless, frictional), Interface Treatment, Normal stiffness updating schemes,etc, are examples of Key Option settings

A Real Constant Set is a list of properties of a particular contact pair that influences behavior and/or convergence

Normal contact stiffness value, contact surface offset value, pinball radius value,etc, are examples of Real Constant properties

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B. Contact Documentation

The Element Reference Manual (available on-line) contains a comprehensive list of all Key Options and Real Constants available for each contact element type.

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... Contact Documentation

Each Key Option and Real Constant listed in the Elements Manual also has a link to different sections of the Contact Technology Guide containing a comprehensive description of its use.

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C. MAPDL Command Syntax

The commands most commonly used to implement advanced contact technology features are:

KEYOPT for setting key options

RMODIF to modify a real constant value

MP, MPDATA for defining friction coefficients

TB and TBDATA for cohesive zone modeling parameters

Refer to Command Manual Documentation for details

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... MAPDL Command Syntax The syntax for the KEYOPT command is as follows:

KEYOPT, ITYPE, KNUM, VALUE

Where ITYPE is the Element type number KNUM is the number of the KEYOPT VALUE is the value of this KEYOPT

For example, setting CONTA174 element KEYOPT(12)=3, will enable bonded contact behavior, in which the target and contact surfaces only become bonded once contact is established, and they remain bonded for the remainder of the analysis.

This behavior can be activated for a contact element (with type number 5 as an example) using the following command:

KEYOPT,5,12,3

Element Type Number

Key Option Number (for controlling contact behaviors)

3 for bonded behavior

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... MAPDL Command Syntax The syntax for the RMODIF command is as follows:

RMODIF, NSET, STLOC, V1,,,,

Where NSET is the Real Constant Set number STLOC is Starting location in table for modifying data. VALUE is the new value assigned to constant in location STLOC.

For example, if real constant set #5 represents a frictional contact pair, a maximum limit on allowable shear stress at the contact interface can be defined with

RMODIF,5,9,TAUMAX

Real Set Number Location for max shear stress

Value for allowable shear stress

From CONTA174 docu:

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D. Procedure To introduce advanced contact features not directly available in Mechanical GUI, start by inserting a command object beneath a contact region

The expectation is that these commands apply to that specific contact region only.

For convenience, Mechanical automatically parameterizes the attributes (element type number, real set number, material id number) for that particular contact region for use in commands.

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... Procedure

Hence, there is usually no need to look up the actual type and real set numbers to execute a command. Simply use the parameter names directly.

Below is an example command line added to a frictional contact region to define a limit on the max allowable shear stress (TAUMAX) at the contact interface

Optional notes can be added to the right of ! for further clarification.

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Element Type for

docu research

... Procedure If you are unfamiliar with the element type being used for a particular contact region and you need to research the documentation for proper key option or real constant table references, it is possible to extract this information from the Solver output (Solution Information).

For example:

From Contact tool:

From Solver Output:

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... Procedure

If this is a new run and there is no Solver Output, add a command object to the Environment branch to execute a partial solve with the following commands:

Refer to Command Manual Documentation for additional information on PSOLVE and /EOF

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... Procedure

There are also a number of contact results not directly available in a Mechanical Tool that can be extracted via a Command Object strategically placed in the Solution Branch.

Refer to Contact Element Documentation as well as commands manual documentation on ETABLE, ESOL, PRESOL, and PRNSOL,,, commands

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E. Example Case -TAUMAX

Consider the case of a large deflection extrusion process that involves gross deformation of the work piece as it is extruded thru the tooling.

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... Example Case - TAUMAX With conventional frictional contact, the model fails to converge because the contact status locks up in a closed and sticking mode as the frictional resistance load (m*N) becomes prohibitively large.

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... Example Case - TAUMAX

Adding a command object to the frictional contact region to define a limit on frictional shear at the interface resolves the convergence issue and more accurately represents the real world limit on the shear strength of the underlying material at this location.

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Example Case TAUMAX Full displacement results with TAUMAX limit included.

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Example Case Applying fluid pressure Consider the case of a piston-cylinder assembly in which you want to apply a fluid pressure to one side of the O-ring after the parts are all assembled and access how far the fluid will penetrate across the seal interface.

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It is possible to simulate the assembly of these parts with conventional contact and the load and BC options available in Mechanical.

This was done in previous Chapter (WS2C).

To apply the fluid pressure to open contact requires APDL in command objects

Example Case Applying fluid pressure

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The first command object simply preserves the contact element type number associated with the O-ring surface to a permanent parameter name cid_1 to be used later

Example Case Applying fluid pressure

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Two Named Selections are created to identify the total potential surface (name=pressure) onto which pressure load will be applied along with a reference starting point (name=start_pt)

Example Case Applying fluid pressure

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A command object is added to the Static Structural Environment:

To execute the necessary select logic on contact elements to receive the pressure load

To execute the SFE commands to apply a pressure load to the lower surface of the O-ring after the parts are assembled (at load step 3)

Example Case Applying fluid pressure

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A command object can also be added to the Solution branch:

To visually confirm the correct application of the fluid pressure load in load step 3.

Example Case Applying fluid pressure

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LS 2: Before pressure

Example Case Applying fluid pressure

LS 3: After Pressure

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Verification of fluid pressure loads with MAPDL plots

Example Case Applying fluid pressure

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Example Case - Contact Surface Wear

where:

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Defined by TB and TBDATA commands:

Applicable to nonlinear contact only (frictional and frictionless)

Asymmetric behavior recommended

Penalty based formulation recommended for convergence

Nodal detection necessary

Example Case - Contact Surface Wear

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No Wear Archard Wear

Sliding block with frictional contact

Example Case - Contact Surface Wear

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Brake Pad Model:

Example Case - Contact Surface Wear

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Please refer to your Workshop Supplement for instructions on:

W5A: Fluid Pressure Penetration

W5B: TAUMAX

W5C: Archard Wear Model

Workshops Accessing Advanced Contact

../Workshop_instructions_Staff/Mech_AC_145_WS3a-Fluid Pressure.ppt../Workshop_instructions_Staff/Mech_AC_145_WS3b-TAUMAX.ppt