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Chapter 14: Explicit Dynamics Analysis

Go to the Previous ChapterGo 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 * Chapter 9

* Chapter 10 * Chapter 11 * Chapter 12 * Chapter 13 * Chapter 14

14.1 Overview of ANSYS/LS-DYNA ExplicitDynamics

ANSYS provides an interface to the LS-DYNA explicit dynamics finite element program. The explicit method of

solution used by LS-DYNA provides fast solutions for large deformation dynamics and complex contactproblems. Using this interface, you can model your structure in ANSYS, obtain the explicit dynamics solution via

LS-DYNA, and review results using the standard ANSYS postprocessing tools.

You can combine the capabilities of the ANSYS implicit program and the LS-DYNA explicit program. For

example, you can perform an explicit solution using ANSYS/LS-DYNA, and transfer the results into an implicit

solution in ANSYS (for example, to solve a springback problem). You can also run an implicit solution in

ANSYS, followed by an explicit solution in ANSYS/LS-DYNA (for example, to solve a dynamic problem inLS-DYNA of a preloaded structure in ANSYS). The procedures for both of these sequential solutions are

discussed later in this chapter.

14.2 Commands Used in an Explicit DynamicsAnalysis

You use the same set of commands to build a model and perform an explicit dynamics analysis that you use to

do any other type of finite element analysis. Likewise, you choose similar options from the ANSYS program'sgraphical user interface (GUI) to build and solve models no matter what type of analysis you are doing.

In addition, several commands are available specifically for an explicit dynamics analysis. These commands are

listed below.

EDBOUND Defines a boundary plane for sliding or cyclic symmetry.

EDBVIS Specifies bulk viscosity coefficients.

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EDCDELE Deletes contact surface specifications.

EDCGEN Specifies contact parameters.

EDCLIST Lists contact surface specifications.

EDCONTACT Specifies contact surface controls.

EDCPU Specifies CPU time limit.

EDCRB Merges two rigid bodies.

EDCSC Specifies whether subcycling will be used.

EDCTS Specifies mass scaling.

EDCURVE Specifies data curves.

EDDAMP Defines system damping.

EDDRELAX Activates dynamic relaxation or stress initialization.

EDENERGY Specifies energy dissipation controls.

EDFPLOT Specifies plotting of load symbols.

EDHGLS Specifies the hourglass coefficient.

EDHIST Specifies time-history output.

EDHTIME Specifies the time-history output interval.

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EDINT Specifies number of integration points for output.

EDIVELO Specifies initial nodal velocities.

EDLCS Defines a local coordinate system.

EDLDPLOT Plots explicit dynamics load curve.

EDLOAD Specifies loads.

EDMP Defines hourglass, rigid, orthotropic, and cable material properties.

EDNDTSD Smooths noisy data and provides a graphical representation of the data.

EDNROT Applies a rotated coordinate nodal constraint.

EDOPT Specifies the type of output (ANSYS or LS-DYNA).

EDOUT Specifies LS-DYNA output files.

EDREAD Reads output into variables in POST26.

EDRST Specifies time increment for output to the .RST file.

EDSHELL Specifies shell computation controls.

EDSOLV Specifies "explicit dynamics solution" as the subsequent status topic.

EDSTART Specifies status (new or restart) of the analysis.

EDWELD Defines a massless spotweld.

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EDWRITE Writes explicit dynamics input to an LS-DYNA input file or creates, updates, or lists parts.

REXPORT Exports displacements from an implicit analysis into ANSYS/LS-DYNA.

In addition to the above commands, the TB command has several options for materials that are unique to explicit

dynamics analysis.

For detailed, alphabetized descriptions of the ANSYS commands, see the ANSYS Commands Reference.

14.3 Overview of Steps in an Explicit DynamicsAnalysis

The procedure for an explicit dynamics analysis consists of three main steps:

1. Build the model.

2. Apply loads and obtain the solution.

3. Review the results.

14.3.1 Build the model

To build the model, you specify the jobname and analysis title, and then use PREP7 to define the element types,element real constants, material properties, and the model geometry. These tasks are common to most analyses.

The ANSYS Modeling and Meshing Guide explains them in detail.

14.3.1.1 Points to Remember

Preferences

When using the GUI, you must first set the Preferences option (Main Menu>

Preferences) to "LS-DYNA Explicit" so that the menus are properly filtered to show explicit dynamics input

options. However, setting this option does not activate LS-DYNA capabilities; to do so, you must specify an

LS-DYNA element type such as SHELL163.

Element Types

For an explicit dynamics analysis, you must choose from the following element types:

LINK160BEAM161

SHELL163

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SOLID164

COMBI165

MASS166LINK167

Material Models

For an explicit dynamics analysis, you can use several existing ANSYS material models, as well as severalmaterial models specific to explicit dynamics analysis. Material models available for an explicit dynamics analysis

are listed below. See Chapter 7 of the ANSYS/LS-DYNA User's Guide for complete descriptions of all material

models available.

Elastic

Orthotropic Elastic

Anisotropic Elastic

Blatz-Ko RubberMooney-Rivlin Rubber

Viscoelastic

Isotropic Elastic PlasticPlastic Kinematic

Power Law Plasticity

Strain Rate Dependent Plasticity

Rate Sensitive Powerlaw Plasticity3-Parameter Barlat Plasticity

Barlat Anisotropic Plasticity

Piecewise Linear Plasticity

Transversely Anisotropic Elastic PlasticClosed Cell Foam

Low Density Foam

Viscous FoamCrushable Foam

Honeycomb

Composite Damage

RigidCable

Johnson-Cook Plasticity

Null

Note-Some material models require MP, TB, and TBDATA input and may also require additional data input

with the EDCURVE command. Use the EDMP command to specify hourglass, rigid, orthotropic, and cable

material properties (which are unique to an explicit dynamics analysis).

Contact

For an explicit dynamics analysis, you will probably want to include contact between surfaces. To include

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contact, you may need to define components (see Chapter 7 of the ANSYS Basic Analysis Procedures Guide

for more information on components). Specify the type of contact, the contact surfaces, and other contact

parameters.

Command(s):

EDCGEN

GUI:

Main Menu >Preprocessor>LS-DYNA Options>Contact Optns>Contact Param

ANSYS recommends using the following contact types:

Node-to-Surface (NTS): Contact is established when a contacting node penetrates a target surface. This

type is commonly used for general contact between two surfaces and is most efficient when a smaller

surface comes into contact with a larger surface, such as a thin rod impacting a flat plate.

Surface-to-Surface: (STS) Contact is established when a surface of one body penetrates the surface ofanother body. This type is commonly used for arbitrary bodies that have large contact areas and is very

efficient for bodies that experience large amounts of relative sliding with friction, such as a block sliding on

a plane.Automatic Single Surface (ASSC): Contact is established when a surface of one body contacts itself or

the surface of another body. This type is easy to use because no contact or target surface definitions are

required and is efficient for self-contacting problems or large deformation problems where general areas of

contact are not known beforehand.

For more complex analyses, the following contact options are also available:

Rigid Body Contact

Tiebreak Contact

Tied ContactEroding Contact

Single Edge Contact

14.3.2 Apply Loads and Obtain the Solution

1. Enter the ANSYS solution processor.

Command(s):

/SOLU

GUI:

Main Menu>Solution

2. Apply loads, initial velocities, constraints, and DOF coupling to the model.

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14.3.2.1 Loads

In an explicit dynamic analysis, all loads must be specified over time using component logic or part IDs,

array parameters, and the EDLOAD command. Basic input for this command is a component name orpart number and two array parameter names. The component specified must contain the nodes or

elements on which the load is being applied. The array parameters specified must contain time varying

load data (one parameter for time values and one for the corresponding load values, which must be the

same length). Valid loads are given in Table 14-1.

Command(s):

EDLOAD

GUI:

Main Menu>Solution>Loading Options>Specify Loads

Table 14-1 Loads applicable in an explicit dynamics analysis

Load Type Label

Displacements UX, UY, UZ

Rotations ROTX, ROTY, ROTZ

Forces FX, FY, FZ

Moments MX, MY, MZ

Velocities VX, VY, VZ

Accelerations (on nodes) AX, AY, AZ

Base Accelerations ACLX, ACLY, ACLZ

Angular Velocities OMGX, OMGY, OMGZ

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Pressures (applied to elements) PRESS

Displacements on Rigid Bodies RBUX, RBUY, RBUZ

Rotations on Rigid Bodies RBRX, RBRY, RBRZ

Velocities on Rigid Bodies RBVX, RBVY, RBVZ

Forces on Rigid Bodies RBFX, RBFY, RBFZ

Moments on Rigid Bodies RBMX, RBMY, RBMZ

The load symbol will appear automatically on the active window. To turn the display of this symbol on or off,

issue:

Command(s):

EDFPLOT

GUI:

Main Menu>Preprocessor>Loads>Show Forces

Main Menu>Solution>Loading Options>Show Forces

Utility Menu>PlotCtrls>Symbols

The load symbol is erased automatically when you replot.

We recommend that you specify velocity time histories instead of displacement time histories. Also, you should

not prescribe non-zero initial displacements. A piecewise linear displacement time history may lead to

discontinuous velocities and infinite accelerations. See Figure 14-1.

Figure 14-1 Effects of specifying displacement time history vs. velocity time history

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To visualize the applied load curve, use the ANSYS/LS-DYNA load curve display capability:

Command(s):

EDLDPLOT

GUI:

Main Menu>Solution>Loading Options>Plot Load Curve

14.3.2.2 Initial Velocities

You can also specify translational and rotational initial velocities for bodies in an explicit dynamic analysis

using component logic and the EDIVELO command. Valid initial velocity labels are given in Table 14-2.

Command(s):

EDIVELO

GUI:

Main Menu>Solution>-Init. Condition-Node Velocity

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Table 14-2 Initial velocity labels

Load Type Label

Initial Velocity VX, VY, VZ

Angular Velocity about Rotational Axis OMEGA

Coordinates on Rotational Axis XC, YC, ZC

Angle Relative o Global Axis ANGX, ANGY, ANGZ

14.3.2.3 Constraints

In addition to loads and initial velocities, constraints can also be applied to the model. Constraints can be

applied only to the displacement (UX, UY, UZ) and rotation (ROTX, ROTY, ROTZ) degrees of

freedom, and the constraint value must be zero. The F, SFE, and BF families of commands are not

applicable for an explicit dynamics analysis.

Command(s):

D

GUI:

Main Menu>Solution>-Constraints-Apply

14.3.2.4 DOF Coupling

DOF coupling and constraint equations are also allowed in an explicit dynamic analysis. Coupling isallowed only for the UX, UY, and UZ degrees of freedom. Constraint equations are allowed only for the

UX, UY, UZ, and ROTX, ROTY, ROTZ degrees of freedom.

Command(s):

CP, CE

GUI:

Main Menu>Preprocessor>Coupling/Ceqn>Couple DOFs

Main Menu>Preprocessor>Coupling/Ceqn>Constraint Eqn

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14.3.2.5 Data Smoothing

If you're working with noisy data (such as an earthquake excitation), you may want to "smooth" that data

to a set of data that provides an accurate approximation of the data points.

To smooth data, you must first create four vectors:

Vector 1 contains the noisy data from the independent variable.

Vector 2 contains the noisy data from the dependent variable and must be the same length as Vector 1.

Vector 3 contains the smoothed data from the independent variable.

Vector 4 contains the smoothed data from the dependent variable and must be the same length as Vector3.

You must always create the first two vectors (*DIM) and fill these vectors with the noisy data before

smoothing the data. If you are working in interactive mode, ANSYS automatically creates Vector 3 and

Vector 4, but if you are working in batch mode, you must create Vector 3 and Vector 4 (*DIM) before

smoothing the data. Vector 3 and Vector 4 are then filled automatically by ANSYS.

After these vectors are created, you then smooth the data:

Command(s):

EDNDTSD

GUI:

Main Menu>Solution>Loading Options>Smooth Data

3. Specify explicit dynamics controls. Table 14-3 shows the LS-DYNA output control options that you should

specify for an explicit dynamics analysis.

Table 14-3 LS-Dyna output control options

Option Command GUI Path

Terminate (Time) TIME Main Menu>Solution>LS-DYNA Controls>Control Options

SHELL/BEAM Outpt EDINT Main Menu>Solution>LS-DYNA Controls>Control Options

Substep Controls EDRST Main Menu>Solution>LS-DYNA Controls>Control Options

Output Interval EDHTIME Main Menu>Solution>LS-DYNA Controls>Control Options

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Energy Options EDENERGY Main Menu>Solution>LS-DYNA Controls>Control Options

Note-Most of the default settings for the LS-DYNA control options (default controls, file controls,

damping options, etc.) are sufficient for most explicit dynamics analysis and need not be modified.

A brief description of those options that are recommended follows:

Terminate [TIME]

This option specifies time at the end of the analysis.

Substep Controls [EDRST]

This option specifies the number of results written to the Jobname.RST file. Because explicitdynamics analyses are only solved over very small time increments (i.e., 1e-7 seconds), only a

relatively small number of solutions should be written to the Jobname.RST file.

Output Interval [EDHTIME]

This option specifies the number of output steps for the history file (Jobname.HIS). The history fileresults are typically saved for a small subset of nodes or elements [EDHIST], but at a much higherfrequency than the results file (Jobname.RST) results.

4. Save a back-up copy of the database to a named file.

Command(s):

SAVE

GUI:

Utility Menu>File>Save as

5. Start solution calculations.

Command(s):

SOLVE

GUI:

Main Menu>Solution>-Solve-Current LS

6. Leave SOLUTION.

Command(s):

FINISH

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GUI:

Close the Solution menu.

14.3.3 Review the Results

14.3.3.1 Postprocessors

You can review these results using POST1, the general postprocessor, and POST26, the time-historyprocessor.

POST1 is used to review results over the entire model at specific time-points. Some typical POST1operations are explained below.POST26 is used to track specific nodal and element result items over a more detailed load history.

For a complete description of all postprocessing functions, see Chapter 4 of the ANSYS Basic AnalysisProcedures Guide.

14.3.3.2 Points to Remember

The database must contain the same model for which the solution was calculated.The results file (Jobname.RST) must be available for POST1.

The history file (Jobname.HIS) must be available for POST26.All stresses and strains output from LS-DYNA are in the global Cartesian coordinate system. Therefore,

use only RSYS,0 for stresses and strains. However, if you are using composite materials, stresses can bein a local (element) coordinate system.

14.3.3.3 Reviewing Results Using POST26

POST26 works with tables of result item versus time, known as variables. Each variable is assigned areference number, with variable number 1 reserved for time. To generate results for the Jobname.HIS file, you

first need to specify the output files [EDOUT], the time-history output [EDHIST], and the time-history outputinterval [EDHTIME].

1. Set file to Jobname.HIS (otherwise, results are read from the Jobname.RST file).

2. Define the variables.

Command(s):

NSOL (primary data, that is, nodal displacements)ESOL (derived data, that is, element solution data, such as stresses)

EDREAD (specific explicit dynamics data)

GUI:

Main Menu>TimeHist Postpro>Define Variables

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Main Menu>TimeHist Postpro>Read LSDYNA outp

3. Graph or list the variables. By reviewing the time-history results at strategic points throughout the model, youcan identify the critical time-points for further POST1 postprocessing.

Command(s):

PLVAR (graph variables)PRVAR, EXTREM (list variables)

GUI:

Main Menu>TimeHist Postpro>Graph VariablesMain Menu>TimeHist Postpro>List Variables

Main Menu>TimeHist Postpro>List Extremes

4. If you have noisy data, such as the resultant force of a metal stamping operation, you may want to smooth the

data while keeping the envelope of the curve. For more information on how to smooth data, see the discussion isSection 14.3.2.5.

Command(s):

EDNDTSD

GUI:

Main Menu>Solution>Loading Options>Smooth Data

14.3.3.4 Other Capabilities

Many other postprocessing functions, such as performing math operations among variables, moving variables into

array parameters, and moving array parameters into variables, are available in POST26. See the ANSYS BasicAnalysis Procedures Guide for details.

14.3.3.5 Reviewing Results Using POST1

1. Read in the database from the database file.

Command(s):

RESUME

GUI:

Utility Menu>File>Resume from

2. Read in the desired set of results. Identify the data set by step numbers or by time.

Command(s):

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SET

GUI:

Main Menu>General Postproc>-Read Results-By Load Step

The substeps correspond to the time intervals for output specified with the substep output control option(EDRST). For example, if the analysis end time is 10 (TIME,10) and EDRST,10 is issued, then output

will be saved to the Jobname.RST file every second ( i.e., at times t=0,1,2,...10 seconds).

3. Perform the necessary POST1 operations. Typical POST1 operations for an explicit dynamics analysis areexplained below.

For faster plotting in an explicit dynamics analysis, select the following graphics options: (UtilityMenu>PlotCtrls>Style>Hidden-Line Options):

Z-buffered plot type (/TYPE,1,6)PowerGraphics (/GRAPHICS, POWER)

Using the above graphics options provides faster plotting for any of the POST1 options describes below.

14.3.3.6 Option: Display Deformed Shape

Command(s):

PLDISP

GUI:

Main Menu>General Postproc>Plot Results>Deformed Shape

The KUND field on PLDISP gives you the option of overlaying the undeformed shape on the display.

14.3.3.7 Option: Display Animated Shapes

Command(s):

ANIM

GUI:

Utility Menu>PlotCtrls>Animate>Dynamic Results

14.3.3.8 Option: Contour Displays

Command(s):

PLNSOL or PLESOL

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GUI:

Main Menu>General Postproc>Plot Results>-Contour Plot-Nodal Solu or Element Solu

Use these options to contour almost any result item, such as stresses (SX, SY, SZ...), strains (EPELX,EPELY, EPELZ...), and displacements (UX, UY, UZ...).

The KUND field on PLNSOL and PLESOL gives you the option of overlaying the undeformed shape onthe display.

You can also contour element table data and line element data:

Command(s):

PLETAB, PLLS

GUI:

Main Menu>General Postproc>Element Table>Plot Element Table Main Menu>General Postproc>Plot Results>-Contour Plot-Line Elem Res

14.3.3.9 Option: Vector Displays

Command(s):

PLVECT (vector displays), PRVECT (vector listings)

GUI:

Main Menu>General Postproc>Plot Results>-Vector Plot-Predefined

Main Menu>General Postproc>List Results>Vector Data

Vector displays (not to be confused with vector mode) are an effective way of viewing vector quantities,

such as displacement (DISP), rotation (ROT), and principal stresses (S1, S2, S3).

14.3.3.10 Option: Tabular Listings

Command(s):

PRNSOL (nodal results)PRESOL (element-by-element results)

PRRSOL (reaction data), etc.NSORT, ESORT

GUI:

Main Menu>General Postproc>List Results>solution option Main Menu>General Postproc>List Results>-Sorted Listing-Sort Nodes or Sort Elems

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Use the NSORT and ESORT commands to sort the data before listing them.

14.3.3.11 Other Capabilities

Many other postprocessing functions are available in POST1. See Chapter 4 of the ANSYS Basic Analysis

Procedures Guide for details.

14.4 Where to Find Explicit Dynamics ExampleProblems

The ANSYS Tutorials describes a sample explicit dynamics analysis problem.

14.5 Sequential Solutions

The simulation of some engineering processes require the capabilities of both implicit (ANSYS) and explicit(ANSYS/LS-DYNA) analyses. To solve these problems, you need to use both solution methods, e.g., an

explicit solution followed by an implicit solution or vice-versa. The LS-DYNA program is a dynamics programintended to solve dynamic problems. If an engineering process contains phases that are essentially static or quasi-static (such as a preload before a dynamic phase or a springback after a metalforming phase), these phases are

easier to simulate using the ANSYS implicit code. Procedures combining the ANSYS implicit solver with theANSYS/LS-DYNA explicit solver to solve engineering processes are described below.

14.5.1 Explicit-to-Implicit Sequential Solution

In this type of sequential solution, useful for springback calculations after a metalforming analysis, you first run anexplicit analysis to simulate a metalforming process. You then read the stresses and thicknesses into ANSYS and

obtain a geometrically non-linear but materially linear equilibrium solution to simulate elastic springback of thework piece.

The procedure follows.

1. Run the explicit analysis as described earlier in this chapter, using Jobname1. You must use SHELL163 tomodel the working piece in order to analyze the springback effect in a subsequent ANSYS implicit analysis. In

addition, you must use one of the following element formulations for the SHELL163 elements: KEYOPT(1) = 2,8, or 10. Solve and finish the analysis.

You should always check your explicit analysis solution from ANSYS/LS-DYNA carefully beforeproceeding with the ANSYS implicit analysis. Specifically, check whether there is any undesirabledynamic effect left in the structure at the end of the explicit run (using POST26).

2. Save the explicit analysis database to file Jobname1.DB.

Command(s):

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SAVE

GUI:

Utility Menu>File>Save as

Note-If you do not save your Jobname1.DB file at this point, then the database for this explicit run will not be

saved. Only the database file for the subsequent implicit run will be saved.

3. Change to Jobname2 to prevent the explicit results files from being overwritten.

Command(s):

/FILNAME,Jobname2

GUI:

Utility Menu>File>Change Jobname

4. Re-enter the preprocessor.

Command(s):

/PREP7

GUI:

Main Menu>Preprocessor

5. Convert explicit element types to corresponding companion implicit element types. The companion explicit-implicit element type pairs are:

Explicit Element Type Implicit Element Type

LINK160 LINK8

BEAM161 BEAM4

SHELL163 SHELL181

SOLID164 SOLID45

COMBI165 COMBIN14

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MASS166 MASS21

LINK167 LINK10

Although all explicit element types are converted, only SHELL163 data (stresses and thicknesses) istransferred to SHELL181 (via the RIMPORT command; see step 12).

Command(s):

ETCHG,ETI

GUI:

Main Menu>Preprocessor>Switch Elem Type

6. Redefine the key options, real constants, material properties, boundary conditions, and loading values on anyimplicit elements that are converted from explicit element types. (For SHELL163 elements that were converted

to SHELL181, you do not need to redefine the real constants, but you do need to redefine the other values.)The TYPE, REAL, MAT, and ESYS numbers from the explicit elements are retained, but the actual key option

and real constant values are reset to zero or the default settings.

Command(s):

KEYOPT

RMP

etc.

GUI:

Main Menu>Preprocessor>Element Type/Real Constants/Material Properties/Loads

Note-Only linear elastic material properties (as specified with the MP command) can remain active in theANSYS implicit phase. Delete any inelastic material properties (as specified with the TB command) from the

ANSYS/LS-DYNA run.

7. Turn off shape checking because elements may have undergone considerable deformation during the explicitanalysis.

Command(s):

SHPP,OFF

GUI:

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Main Menu>Preprocessor>Checking Ctrls>Shape Checking

8. Redefine the implicit elements to the deformed configuration.

Command(s):

UPGEOM

GUI:

Main Menu>Preprocessor>Create>Nodes>From Results

9. Unselect or delete any unnecessary elements (mainly those making up any rigid bodies from the explicit

analysis), or convert them to null elements. Any explicit elements that are not either unselected, deleted,converted to null elements, or converted to implicit will remain active in ANSYS, which will produce an error

and terminate the analysis. Also, if the rigid bodies in the explicit analysis were made up of SHELL163 elements,these elements must be unselected, deleted, or converted to NULL elements before importing stresses and

thicknesses (from SHELL163 to SHELL181) by the RIMPORT command (See Step 12); otherwise, theimplicit analysis will be terminated.

Command(s):

ESELEDELE

GUI:

Utility Menu>Select>Entities or Main Menu>Preprocessor>Delete>Elements

10. Re-enter the solution processor.

Command(s):

/SOLU

GUI:

Main Menu>Solution

11. Set any necessary constraints on the model by modifying or adding to the boundary conditions defined duringthe explicit analysis (for example, in a metalforming analysis, you need to constrain the blank).

Command(s):

D, etc.

GUI:

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Main Menu>Solution>Apply>-Structural-Displacement>On Nodesetc.

12. Import stresses and changed thicknesses (from SHELL163 to SHELL181 only). The deformed integration

point thicknesses are averaged before being transferred to the implicit analysis.

Command(s):

RIMPORT

GUI:

Main Menu>Solution>-Loads-Apply>-Structural-Other>Import Stress

Note-The stresses and thicknesses that are imported from LS-DYNA are some of the ETABLE items forSHELL163. These include moments (SMISC items 1, 2, and 3), in-plane forces (SMISC items 6, 7, and 8),

and changed thickness (NMISC item n+1). See the ANSYS Elements Reference for more information onSHELL163 output data.

13. Turn large deformation effects on.

Command(s):

NLGEOM,ON

GUI:

Main Menu>Solution>Analysis Options

14. Solve and finish the analysis.

Command(s):

SOLVE

FINISH

GUI:

Main Menu>Solution>Current LS

Main Menu>Finish

Once you have solved the analysis, you can use any of the standard ANSYS post-processing functions to reviewyour results.

14.5.2 Implicit-to-Explicit Sequential Solution

In this type of sequential solution, useful for analyzing a birdstrike on rotating engine blades, or a droptest

simulation of preloaded consumer goods, you first run an ANSYS implicit analysis to apply a preload (for

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example, spin of a jet engine). You then read the nodal displacements, rotations, and temperatures from theANSYS results file and write them to an LS-DYNA file ("drelax") for subsequent stress initialization. Finally, youobtain the dynamic solution (for example, of the bird impact or the drop of a phone set).

The procedure follows.

1. Run the implicit analysis as described in other chapters of this guide, using Jobname1. Keep in mind that thisanalysis must be small strain with linear material behavior. The only element types that can be used for an

implicit-to-explicit sequential solution are:

LINK8BEAM4

SHELL181SOLID45

COMBIN14MASS21

LINK10

2. Define any additional nodes and elements that are necessary to complete the explicit solution (for example, thebird in a birdstrike simulation, or a rigid surface that a phone would impact in a droptest). These additional nodes

and elements may not be part of the implicit analysis, but they need to be defined here nonetheless. Theseadditional nodes must be constrained (using D,ALL,ALL,0).

Command(s):

NE

GUI:

Main Menu>Preprocessor>Create>Nodes/Elements

3. Solve and finish the analysis.

Command(s):

SOLVE

FINISH

GUI:

Main Menu>Solution>Current LSMain Menu>Finish

4. Save the implicit analysis database to file Jobname1.DB.

Command(s):

SAVE

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GUI:

Utility Menu>File>Save as

Note-If you do not save your Jobname1.DB file at this point, then the database for this implicit run will not be

saved. Only the database file for the subsequent explicit run will be saved.

5. Change to Jobname2 to prevent the implicit results files from being overwritten.

Command(s):

/FILNAME,Jobname2

GUI:

Utility Menu>File>Change Jobname

6. Re-enter the preprocessor.

Command(s):

/PREP7

GUI:

Main Menu>Preprocessor

7. Convert implicit element types to corresponding companion explicit element types. The correspondingcompanion implicit-explicit element type pairs are:

Implicit Element Type Explicit Element Type

LINK8 LINK160

BEAM4 BEAM161

SHELL181 SHELL163

SOLID45 SOLID164

COMBI14 COMBIN165

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MASS21 MASS166

LINK10 LINK167

Command(s):

ETCHG,ITE

GUI:

Main Menu>Preprocessor>Switch Elem Type

Implicit elements not listed above can also be used, as long as they are defined by the same number ofnodes, but they will not automatically be converted to explicit elements when ETCHG is issued. Theseelements must be converted manually using EMODIF. Higher-order implicit elements can also be used,but must also be converted manually using EMODIF with the corner nodes only. Do not delete or

unselect the midside nodes - these nodes must be written to the LS-DYNA input file. The "drelax" filecontains solutions for these nodes, but the ANSYS/LS-DYNA explicit elements do not use these nodes intheir definition.

Command(s):

EMODIF

GUI:

Main Menu>Preprocessor>Move / Modify>Modify Nodes

Note-Element types LINK8 and LINK10 lack a third node; however, their corresponding companion explicitelement types, LINK160 and LINK167, require a third (orientation) node. If you are using element typesLINK8 or LINK10, you must first convert the element type using ETCHG,ITE, and then manually define the

third node of LINK160 or LINK167 elements using N and EMODIF.

Note-Also, if you are converting BEAM4 to BEAM161, you may need to manually define the third node ofBEAM161 elements as well. However, BEAM4 allows you to define a third, optional node. If you have definedthis third node on BEAM4, then the conversion to BEAM161 will be completed automatically when you issue

ETCHG,ITE. If you did not define the third node on BEAM4, then you must manually define it on BEAM161using N and EMODIF.

8. Associate explicit dynamics material models with the material properties defined during the implicit run. Ifworking in the GUI, you will automatically be prompted to associate a material model when you convert theelements from implicit to explicit element types.

Command(s):

MPMOD

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GUI:

Main Menu>Preprocessor>Material Props>Define MAT Model

9. Redefine the key options, real constants, boundary conditions, and loading values on the explicit elements. TheTYPE, REAL, MAT, and ESYS numbers from the implicit elements are retained, but the actual key option and

real constant values are reset to zero or the default settings.

Command(s):

KEYOPTRMP

etc.

GUI:

Main Menu>Preprocessor>Element Type/Real Constants/Material Properties/Loads

10. Remove constraints from the additional nodes or elements defined in Step 2, above.

Command(s):

DDELE

GUI:

Main Menu>Preprocessor>Loads>Delete>Displacements

11. Re-enter the solution processor.

Command(s):

/SOLU

GUI:

Main Menu>Solution

12. Read nodal displacements, rotations, and temperatures from the implicit results file, and write this informationto an ASCII LS-DYNA file, "drelax."

Command(s):

REXPORT

GUI:

Main Menu>Solution>-Load Step Opts-Read Disp

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13. Initialize the structure to the prescribed geometry according to the displacements, rotations, and temperaturescontained in the "drelax" file. In this step, LS-DYNA applies the load information (displacements, rotations, andtemperatures) from the "drelax" file to the original geometry and calculates the deformed geometry, which it thenuses as a starting point for the explicit analysis.

Note-The temperature degree of freedom, although contained in the "drelax" file, is not directly supported by

ANSYS/LS-DYNA.

Command(s):

EDDRELAX

GUI:

Main Menu>Solution>LS-DYNA Controls>Default Ctrls>Dynamic Relaxation

14. Apply any necessary loading for the explicit run.

Command(s):

EDIVELOEDLOADEDCURVE

etc.

GUI:

Main Menu>Solution>Node VelocityMain Menu>Solution>Loading Options>Specify LoadsMain Menu>Preprocessor>Material Props>Curve ID

15. Solve and finish the explicit dynamics analysis. You can then return to the implicit solution, if necessary.

14.6 Additional Information

For detailed instructions on using LS-DYNA, see the ANSYS/LS-DYNA User's Guide. For additionalinformation on the ANSYS/LS-DYNA product, see the ANSYS Elements Reference, the ANSYS CommandsReference, and Livermore Software Technology Corporation's LS-DYNA Theoretical Manual.

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