9/9/2015 ADVANCED: Chapter 6: Element Birth and Death (UP19980818)

http://mostreal.sk/html/guide_55/g-adv/GADV6.htm 1/9

Chapter 6: Element Birth and Death

Go to the Next ChapterGo to the Previous Chapter

Go to the Table of Contents for This Manual

Go to the Guides Master Index

Chapter 1 * Chapter 2 * Chapter 3 * Chapter 4 * Chapter 5 * Chapter 6 * Chapter 7 * Chapter 8

6.1 What Is Element Birth and Death?

If material is added to (or removed from) a system, certain elements in your model may become "existent" (or

"nonexistent"). The element birth and death options can be used to deactivate or reactivate selected elements insuch cases. (Applicable element types are listed in Table 6-1.) This feature can be useful in analyzing excavation

(as in mining and tunneling), staged construction (as in shored bridge erection), sequential assembly (as in

fabrication of layered computer chips), and many other applications in which you can easily identify activated or

deactivated elements by their known locations. The birth and death feature is only available in the

ANSYS/Multiphysics, ANSYS/Mechanical, and ANSYS/Structural products.

Table 6-1 Elements with birth and death capability

LINK1 SURF19 SHELL41 SOLID64 PLANE83 SHELL143

PLANE2 PIPE20 PLANE42 SOLID65 SOLID87 SURF151

BEAM3 MASS21 SHELL43 PLANE67 SOLID90 SURF152

BEAM4 SURF22 BEAM44 LINK68 SOLID92 SURF153

SOLID5 BEAM23 SOLID45 SOLID69 SHELL93 SURF154

LINK8 BEAM24 PLANE53 SOLID70 SOLID95 SHELL157

LINK10 PLANE25 BEAM54 MASS71 SOLID96 TARGE169

LINK11 MATRIX27 PLANE55 SOLID72 SOLID97 TARGE170

9/9/2015 ADVANCED: Chapter 6: Element Birth and Death (UP19980818)

http://mostreal.sk/html/guide_55/g-adv/GADV6.htm 2/9

PLANE13 LINK31 SHELL57 SOLID73 SOLID98 CONTA171

COMBIN14 LINK32 PIPE59 PLANE75 SHELL99 CONTA172

PIPE16 LINK33 PIPE60 PLANE77 PLANE121 CONTA173

PIPE17 LINK34 SOLID62 PLANE78 SOLID122 CONTA174

PIPE18 PLANE35 SHELL63 PLANE82 SOLID123

User elements may also be given the birth and death capability. See the Guide to ANSYS User Programmable

Features for more information on user elements

In some circumstances, an element's birth and death status may be dependent on an ANSYS-calculatedquantity, such as temperature, stress, strain, etc. You can use commands such as ETABLE (Main

Menu>General Postproc>Element Table>Define Table) and ESEL (Utility Menu>Select>Entities) todetermine the value of such quantities in selected elements, and to change the status (melted, solidified, ruptured,

etc.) of those elements accordingly. This procedure can be useful for modeling effects due to phase changes (asin welding processes, when structurally inactive molten material solidifies and becomes structurally active),failure-surface propagation, and other analysis-dependent element changes.

6.1.1 How Does the Birth and Death Feature Work?

To achieve the "element death" effect, the ANSYS program does not actually remove "killed" elements. Instead,it deactivates them by multiplying their stiffness (or conductivity, or other analogous quantity) by a severe

reduction factor [ESTIF]. This factor is set to 1.0E-6 by default, but can be given other values (see Section6.2.2 for more information). Element loads associated with deactivated elements are zeroed out of the load

vector, however, they still appear in element-load lists. Similarly, mass, damping, specific heat, and other sucheffects are set to zero for deactivated elements. The mass and energy of deactivated elements are not included in

the summations over the model. An element's strain is also set to zero as soon as that element is killed.

In like manner, when elements are "born," they are not actually added to the model; they are simply reactivated.You must create all elements, including those to be born in later stages of your analysis, while in PREP7. You

cannot create new elements in SOLUTION. To "add" an element, you first deactivate it, then reactivate it at theproper load step.

When an element is reactivated, its stiffness, mass, element loads, etc. return to their full original values. Elements

are reactivated having no record of strain history (or heat storage, etc.). However, initial strain defined as a realconstant (for elements such as LINK1) will not be affected by birth and death operations. Also, unless large-

deformation effects are turned on [NLGEOM,ON], some element types will be reactivated in their originally

9/9/2015 ADVANCED: Chapter 6: Element Birth and Death (UP19980818)

http://mostreal.sk/html/guide_55/g-adv/GADV6.htm 3/9

specified geometric configuration (large-deformation effects should be included to obtain meaningful results).

Thermal strains are computed for newly-activated elements based on the current load step temperature and thereference temperature. Thus, newborn elements with thermal loads may not be stress-free as intended. The

material property REFT can be used instead of the global TREF to specify material-dependent referencetemperatures. This allows you to specify the activation temperature as a stress-free temperature.

6.2 How to Use the Birth and Death Feature

You can include element birth and death behavior in most static and nonlinear transient analyses, using the samebasic procedures described in the various analysis guides. As in any other analysis, the procedure consists ofthree main steps:

Build the modelApply loads and obtain the solution

Review the results

Modify your basic analysis procedure as follows to incorporate the element birth and death feature:

6.2.1 Build the Model

While in PREP7, create all elements-even those that won't be activated until later load steps. You cannot create

new elements outside of PREP7.

6.2.2 Apply Loads and Obtain the Solution

Perform the following actions within SOLUTION:

6.2.2.1 Define the First Load Step

In the first load step, you must choose the analysis type and all appropriate analysis options:

Command(s):

ANTYPE

GUI:

Main Menu>Solution>-Analysis Type-New Analysis

For a structural analysis, large-deflection effects should be on:

Command(s):

9/9/2015 ADVANCED: Chapter 6: Element Birth and Death (UP19980818)

http://mostreal.sk/html/guide_55/g-adv/GADV6.htm 4/9

NLGEOM,ON

GUI:

Main Menu>Solution>Analysis Options

For all birth and death applications, you must be sure to set the Newton-Raphson option explicitly in the firstload step, as the program cannot predict the presence of an EKILL command in a subsequent load step:

Command(s):

NROPT

GUI:

Main Menu>Solution>Analysis Options

Deactivate [EKILL] all the initially inactive elements that are to be added (reactivated) in later load steps:

Command(s):

EKILL

GUI:

Main Menu>Solution>-Load Step Opts-Other>Kill Elements

Elements are deactivated (or activated) in the first substep of the load step, and maintain that status through therest of the load step. Note that the default reduction factor used as a stiffness multiplier might not suffice for some

problems. Sometimes, you might have to consider using a more severe reduction factor. To provide a new value

for the reduction factor:

Command(s):

ESTIF

GUI:

Main Menu>Solution>Other>StiffnessMult

Nodes that are not connected to any active elements may "float," or pick up stray degree of freedom (DOF)

responses. In some cases, you might want to constrain inactive DOFs [D, CP, etc.] to reduce the number of

equations to be solved, and to avoid ill-conditioning. However, constraining inactive DOFs can become moreimportant for cases in which you want to reactivate elements with a specific shape (or temperature, etc.). Of

course, you must be sure to remove such artificial constraints when you reactivate elements. You must also

remove nodal loads from inactive DOFs (i.e., at nodes that are not connected to any active elements). Similarly,

you must specifically add nodal loads (if any) to reactivated DOFs.

Part of your input listing could look like this for your first load step:

9/9/2015 ADVANCED: Chapter 6: Element Birth and Death (UP19980818)

http://mostreal.sk/html/guide_55/g-adv/GADV6.htm 5/9

! First load stepTIME,... ! Sets TIME value (optional for static analyses)NLGEOM,ON ! Turns large-deflection effects onNROPT,FULL ! You must explicitly set the Newton-Raphson optionESTIF,... ! Sets non-default reduction factor (optional)ESEL,... ! Selects elements to be deactivated in this load stepEKILL,... ! Deactivates selected elementsESEL,S,LIVE ! Selects all active elementsNSLE,S ! Selects all active nodesNSEL,INVE ! Selects all inactive nodes (those not attached to any ! active elements)D,ALL,ALL,0 ! Constrains all inactive DOFs (optional)NSEL,ALL ! Selects ALL nodesESEL,ALL ! Selects ALL elementsD,... ! Adds constraints as appropriateF,... ! Adds nodal loads to active DOFs as appropriateSF,... ! Adds element loads as appropriateBF,... ! Adds body loads as appropriateSAVESOLVE

Please see the TIME, NLGEOM, NROPT, ESTIF, ESEL, EKILL, NSLE, NSEL, D, F, SF, and BF

command descriptions for more information.

6.2.2.2 Subsequent Load Steps

In your remaining load steps, you can deactivate and reactivate elements as desired. As before, be sure to applyand delete constraints and nodal loads as appropriate.

To deactivate elements, use one of these methods:

Command(s):

EKILL

GUI:

Main Menu>Solution>-Load Step Opts-Other>Kill Elements

To reactivate elements, use one of these methods:

Command(s):

EALIVE

GUI:

Main Menu>Solution>-Load Step Opts-Other>Activate Elem

! Second (or subsequent) load step:TIME,...ESEL,...

9/9/2015 ADVANCED: Chapter 6: Element Birth and Death (UP19980818)

http://mostreal.sk/html/guide_55/g-adv/GADV6.htm 6/9

EKILL,... ! Deactivates selected elements

ESEL,...EALIVE,... ! Reactivates selected elements... FDELE,... ! Deletes nodal loads at inactive DOFsD,... ! Constrains inactive DOFs...F,... ! Adds nodal loads as appropriate to active DOFsDDELE,... ! Deletes constraints from reactivated DOFsSAVESOLVE

Please see the TIME, ESEL, EKILL, EALIVE, FDELE, D, F, and DDELE command descriptions for

more information.

6.2.3 Review the Results

For the most part, you will follow standard procedures when postprocessing an analysis that contains

deactivated or reactivated elements. However, you should realize that "killed" elements are still present in yourmodel, even though they make an insignificant contribution to the stiffness (conductivity, etc.) matrix. Therefore,

they will be included in element displays, output listings, etc. For example, deactivated elements will be included

in nodal results averaging (PLNSOL command or menu path Main Menu>General Postproc>PlotResults>Nodal Solu) and will "smear" the results. The entire element printout for deactivated elements should

be ignored, since many items computed make little physical sense. A suggestion is to use selecting (ESEL

command or menu path Utility Menu>Select>Entities) to remove deactivated elements for element displays

and for other postprocessing operations.

6.2.4 Using ANSYS Results to Control Birth and Death

At times, you will not explicitly know the location of elements that you need to deactivate or reactivate. For

instance, if you want to "kill" melted elements in a thermal analysis (i.e., to model the removal of melted material),

you will not know the location of those elements beforehand; you will have to identify them on the basis of their

ANSYS-calculated temperatures. When the decision to deactivate or reactivate an element depends on the

value of an ANSYS result item (such as temperature, stress, strain, etc.), you can use commands to identify and

select the critical elements.

To identify the critical elements:

Command(s):

ETABLE

GUI:

Main Menu>General Postproc>Element Table>Define Table

To select the critical elements:

9/9/2015 ADVANCED: Chapter 6: Element Birth and Death (UP19980818)

http://mostreal.sk/html/guide_55/g-adv/GADV6.htm 7/9

Command(s):

ESEL

GUI:

Utility Menu>Select>Entities

You could then deactivate or reactivate the selected elements. (You could also use the ANSYS ParametricDesign Language to write a macro to perform such an operation. See the APDL Programmer's Guide for more

information.)

To deactivate the selected elements:

Command(s):

EKILL, ALL

GUI:

Main Menu>Solution>-Load Step Opts-Other>Kill Elements

To reactivate the selected elements, use one of these methods:

Command(s):

EALIVE,ALL

GUI:

Main Menu>Solution>-Load Step Opts-Other>Activate Elem

The following simplified input listing demonstrates how you might deactivate elements that rupture when their totalstrain has exceeded some critical value:

/SOLU ! Enter SOLUTION... ! Typical solution procedureSOLVEFINISH!/POST1 ! Enter POST1SET,...ETABLE,STRAIN,EPTO,EQV ! Store total equivalent strain in ETABLEESEL,S,ETAB,STRAIN,0.20 ! Select all elements with total equivalent strain ! greater than or equal to 0.20FINISH!/SOLU ! Re-enter SOLUTIONANTYPE,,RESTEKILL,ALL ! Deactivate selected (overstrained) elementsESEL,ALL ! Restore full element set... ! Continue with solution

9/9/2015 ADVANCED: Chapter 6: Element Birth and Death (UP19980818)

http://mostreal.sk/html/guide_55/g-adv/GADV6.htm 8/9

Please see the ETABLE, ESEL, ANTYPE, and EKILL command descriptions for more information.

6.3 Further Guidelines

Constraint equations [CE, CEINTF, etc.] cannot be applied to inactive DOFs. (Inactive DOFs occur

when a node has no active ("alive") elements attached to it.)You can model stress-relieving operations (such as annealing) by deactivating and then reactivating

elements.

In nonlinear analyses, be careful not to deactivate or reactivate elements in such a way as to create

singularities (such as sharp re-entrant corners in a structural analysis) or sudden large changes in stiffness.

Such situations are likely to cause convergence difficulties.

If the model is entirely linear (has no contact or other nonlinear element present, and the material is linear)

except for birth and death elements, ANSYS will treat the analysis as linear, and will not setSOLCONTROL,ON.

The FULL Newton-Raphson option with adaptive descent turned on often yields good results in analyses

that incorporate element birth and death. To use this option with adaptive descent turned on:

Command(s):

NROPT,FULL,,ON

GUI:

Main Menu>Solution>Analysis Options

You can retrieve a parameter whose value will indicate the status (active or inactive) of an element

[*GET,Par,ELEM, n, ATTR, LIVE] (Utility Menu> Parameters>Get Scalar Data). This parameter

could be used in APDL logical branching ([*IF], etc.) or in other applications in which you need tomonitor the birth-death status of an element.

You might be tempted to try to deactivate or reactivate elements by changing their material properties

[MPCHG] (Main Menu>Preprocessor> Material Props>Change Mat Num).

However, you must proceed cautiously if you attempt such a procedure. The safeguards and restrictions

that affect "killed" elements will not apply to elements that have their material properties changed in

SOLUTION. (Element forces will not be automatically zeroed out; nor will strains, mass, specific heat,

etc.) Many problems could result from careless use of MPCHG. For instance, if you reduce an element'sstiffness to almost zero, but retain its mass, it could result in a singularity if subjected to acceleration or

inertial effects.

One application of MPCHG would be in modeling construction sequences in which the strain history of a

"born" element is maintained. Using MPCHG in such cases will enable you to capture the initial strain

experienced by elements as they are fitted into the displaced nodal configuration.

The load step file method [LSWRITE] for solving multiple load steps cannot be used with the birth-death

9/9/2015 ADVANCED: Chapter 6: Element Birth and Death (UP19980818)

http://mostreal.sk/html/guide_55/g-adv/GADV6.htm 9/9

option, because it will not write the status of deactivated or reactivated elements to the load step file. Birthand death analyses having multiple load steps must therefore be performed using a series of explicit

SOLVE commands (Main Menu>Solution>Current LS).

6.4 Where to Find Examples

The ANSYS Verification Manual consists of test case analyses demonstrating the analysis capabilities of theANSYS program. While these test cases demonstrate solutions to realistic analysis problems, the ANSYS

Verification Manual does not present them as step-by-step examples with lengthy data input instructions and

printouts. However, most ANSYS users who have at least limited finite element experience should be able to fill

in the missing details by reviewing each test case's finite element model and input data with accompanying

comments.

The following is the element birth and death test case that the ANSYS Verification Manual includes:

VM194 Element Birth/Death in a Fixed Bar with Thermal Loading

Go to the beginning of this chapter