explicit dynamics

EXPLICIT DYNAMICS

with

ANSYS/LS-DYNA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual

March 15, 2001Inventory #001449

1-2

What is ANSYS/LS-DYNA?

• General purpose explicit dynamic finite element program

• Used to solve highly nonlinear transient dynamic problems

– Efficient for a wide range of contact types

– Advanced material modeling capabilities

– Robust for very large deformation analyses

• Seamless interface of the ANSYS and LS-DYNA programs

– Full integration of the LS-DYNA solver into ANSYS

– All pre and post-processing performed using standard ANSYS conventions

– GUI has look and feel of general ANSYS

– Supports capability of implicit - explicit sequential solutions

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-3

Benefits of ANSYS/LS-DYNA

• Excellent combination of explicit and implicit solution technology

• ANSYS Pre and Post Processing:

– All explicit dynamic specific commands begin with EDxx prefix

– Customized ANSYS GUI for efficient execution of explicit problems

– Supports all ANSYS Solid Modeling and Boolean Operations

– Allows direct geometry import from IGES, Pro/E, ACIS, Parasolid, etc.

– Supports all ANSYS automatic meshing features

– APDL and design optimization can be used

– Supports all general postprocessing features and animation macros

– Specialized time-history postprocessing

• LS-DYNA Solver

– Fastest explicit solver in marketplace

– More features than any other explicit code

– Full version of LS-DYNA (with airbags, seatbelts, explosives, etc.)

– Full versions of LS-TAURUS and LS-POST postprocessors

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-4

Applications of ANSYS/LS-DYNA

• Crashworthiness Analysis

– ANSYS/LS-DYNA:

• Full Car Crash

• Car Component Analyses

• Crash in ALL Vehicle Industries

– Car

– Truck

– Bus

– Train

– Ship

– Aircraft

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-5

(continued)


• Manufacturing Process Simulation

• Deep drawing

• Hydro forming

• Superplastic forming

• Rolling

• Extrusion

• Stamping

• Machining

• Drilling

• Almost all forming processes have been simulated with LS-DYNA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-6

• Pipe whip (ANSYS News 3/93):• Impact of a pipe with a rotational

velocity of 50 rad/sec• CPU time (SGI Octane R12000) < 20

seconds

(continued)


• Contact/Impact– Drop test – Pendulum impact test– Jet engine fan

containment

• A wide range of contact types are possible

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-7

PUNCH

DIE

BLANK

STATIC ‘QUASI’ STATIC DYNAMIC

Structural Problems Metal Forming Impact Problems

F = 0 F 0 F = ma

IMPLICIT METHODEXPLICIT METHOD

Comparison of Implicit and Explicit Methods

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-8


Implicit Time Integration:

• Inertia effects ([C] and [M]) are typically not included

• Average acceleration - displacements evaluated at time t+t:

Linear Problems:– Unconditionally stable when [K] is linear

– Large time steps can be taken

Nonlinear problems:– Solution obtained using a series of linear approximations (Newton-

Raphson)

– Requires inversion of nonlinear stiffness matrix [K]

– Small iterative time steps are required to achieve convergence

– Convergence is not guaranteed for highly nonlinear problems

att

1tt FKu

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-9

Comparison of Implicit and Explicit MethodsExplicit Time Integration

• Central difference method used - accelerations evaluated at time t:

where {Ftext} is the applied external and body force vector,

{Ftint} is the internal force vector which is given by:

• Fhg is the hourglass resistance force (see ELEMENTS Chapter) and Fcont is the contact force.

• The velocities and displacements are then evaluated:

intt

extt

1t FFMa

contacthgn

T FFdBF

int

tttttt tavv 2/2/

2/2/ ttttttt tvuu

where tt+t/2=.5(tt+ tt+ t) and tt- t/2=.5(tt- tt+ t)

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-10

Explicit Time Integration (continued):

• The geometry is updated by adding the displacement increments to the initial geometry {xo}:


• Nonlinear problems:

– Lumped mass matrix required for simple inversion

– Equations become uncoupled and can be solved for directly (explicitly)

– No inversion of stiffness matrix is required. All nonlinearities (including contact) are included in the internal force vector.

– Major computational expense is in calculating the internal forces.

– No convergence checks are needed

– Very small time steps are required to maintain stability limit

ttott uxx

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-11

t t crit 2

max

Stability Limit

Implicit Time Integration

• For linear problems, the time step can be arbitrarily large (always stable)

• For nonlinear problems, time step size may become small due to convergence difficulties

Explicit Time Integration

• Only stable if time step size is smaller than critical time step size

• Where max = largest natural circular frequency

• Due to this very small time step size, explicit is useful only for very short transients

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-12

• Critical time step size of a rod

– Natural frequency:

• Critical time step:

– Courant-Friedrichs-Levy-criterion

– Δt is the time needed of the wave to propagate through the rod of

length l

Note: The critical time step size for explicit time integration depends on element length and material properties (sonic speed).

l

c=ω 2max with

ρ

Ec= (wave propagation velocity)

c

lt=Δ

Critical Time Step Size

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-13

• ANSYS/LS-DYNA checks all elements when calculating the required time step. For stability reasons a scale factor of 0.9 (default) is used to decrease the time step:

• The characteristic length l and the wave propagation velocity c are dependent on element type:

clt 9.0

ρ

Ec=elementtheoflength=l

)-1ρ(

E

)LL(LmaxA2

)LLL(LmaxA

2

3214321

νc=

,,l=shells: triangular for ,

,,,l=

L1

L4

L3

L2

A

Shell elements:

Beam elements:

ANSYS/LS-DYNA Time Step Size

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-14

File Organization

ANSYS /SOLU or LS-DYNAsolver taskreads jobname.K

ANSYS /PREP7 INTpreprocessingwrites jobname.K(standard LS-DYNA input)

ANSYS Resultsjobname.rstBinary Result FileEDRST,FREQ

ANSYS/POST26EDREAD, ...

LS-TAURUSLS-POST phs3

LS-TAURUS, LS-POST, ASCIIGLSTAT, MATSUM, …ASCII Result FilesEDOUT,File

ANSYS/POST1

ANSYS Resultsjobname.hisTime History DataEDHIST,Comp and EDHTIME,FREQ

LS-TAURUS & LS-POST phs1

LS-TAURUS & LS-POST binary d3plotBinary Result Filessimilar to jobname.rst

LS-TAURUS & LS-POST phs2

LS-TAURUS and LS-POST binary d3thdtBinary Result Filessimilar to jobname.his

ANSYS/POST26

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-15

(continued)

File OrganizationDescription of ANSYS Files Generated During an ANSYS/LS-DYNA run:

Jobname.k• LS-DYNA input stream that is automatically generated upon execution of the ANSYS

SOLVE command.

• Contains all geometry, load, and material data that exists in the ANSYS database

• The file is 100% compatible with LS-DYNA version 950e

• File can also be manually generated using the EDWRITE command: Solution: Write Jobname.k

Jobname.rst• Explicit dynamics results file that is nearly

identical to standard ANSYS .rst

• Primarily used to review results in the general ANSYS postprocessor (POST1)

• Contains results at a relatively small number of time steps (e.g., 10 - 1000)

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-16

(continued)

File OrganizationDescription of ANSYS Files Generated During an ANSYS/LS-DYNA run:

Jobname.his

• Explicit dynamics time history results files used in POST26

• Contains results for a subset of nodes and/or elements of the model

• Typically will contain results at significantly more time steps than Jobname.rst

Time history ASCII files

• Specialized files containing additional information about the explicit analysis

• User must specify which files are written before solution

• ASCII files include:

GLSTAT: Global energies

MATSUM: Material energies

SPCFORC: Nodal constraint reaction forces

RCFORC: Resultant contact interface forces

RBDOUT: Rigid body data

NODOUT: Nodal data

ELOUT: Element data

etc....

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

Exp

licit D

yn

am

ics w

ith A

NS

YS

/LS

-DY

NA

5

.75

.7

Training Manual


1-17

(continued)

File Organization

• Since the LS-TAURUS and LS-POST postprocessors automatically come with ANSYS/LS-DYNA, the following two DYNA results files are easily generated during an explicit dynamic analysis:

• D3PLOT– LS-TAURUS and LS-POST binary results file

– Similar to ANSYS Jobname.rst

• D3DHDT– LS-TAURUS and LS-POST time-history results file

– Similar to ANSYS Jobname.his

explicit dynamics

Education

Transcript of explicit dynamics