IMPLICIT ANALYSIS USING LS-DYNA - Oasys · PDF fileUse IMFLAG=1 on *CONTROL_IMPLICIT_GENERAL...

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT

In case of any queries: [email protected]

Copyright © 2017 by Ove Arup & Partners1

IMPLICIT ANALYSIS USING LS-DYNA

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT



Why Implicit?

Explicit: Short time dynamics (i.e., high frequency response, wave

propagation) like impact, crash, etc.

Implicit: Structural dynamics (i.e., low frequency response, vibration,

oscillation) like static loading, earth quake, metal forming, roof crush, door

sag, bolt preload, seat belt anchorage strength test, etc.

IMPLICIT ANALYSIS

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Why Implicit?...

Long Duration problems > 500,000 explicit time integration cycles → ?

Statically loading a structure (instead of performing the so called quasi-static

loading using an explicit time integration solver).

Different time scales in an event.

Static loading followed by transient loading or vice-versa, e.g., metal forming.

IMPLICIT ANALYSIS

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LS-DYNA ENVIRONMENT



Types of Implicit analysis using LS-DYNA

Linear Analysis

Static or Dynamic

Single-Step or Multi-Step

Eigen Value Analysis

Frequencies and Mode shapes

Linear buckling loads and modes

Modal analysis – Extraction and Superposition

Dynamic analysis by modal superposition

Non-Linear Analysis

Newton, Quasi-Newton, Arc-length solution

Static or dynamic

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How to invoke Implicit Solver in LS-DYNA?

IMFLAG – Implicit/ Explicit analysis type flag

= 0 → Explicit analysis (default)

= 1 → Implicit analysis

*CONTROL_IMPLICIT_GENERAL

Card-1 IMFLAG DT0 IMFORM NSBS IGS CNSTN FORM ZERO_V

This keyword is required for all implicit analysis in LS-DYNA.

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IMPLICIT ANALYSIS

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Eigen Value Analysis

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT



Eigen Value Analysis… Few Animations

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT




IMPLICIT ANALYSIS

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LS-DYNA ENVIRONMENT



How to carry out Eigen value analysis?

*CONTROL_IMPLICIT_EIGENVALUE

Card-1 NEIG CENTER LFLAG LFTEND RFLAG RHTEND EIGMTH SHFSCL

NEIG – number of Eigen values to be extracted.

The lowest NEIG Eigen values will be found by default.

If NEIG > 0, Eigen values will be computed at time = 0 and LS-DYNA will terminate.

Use IMFLAG=1 on *CONTROL_IMPLICIT_GENERAL (activates implicit method).

d3eigv – Eigen vectors (viewed in a post processor).

eigout – a summary table of eigen value, along with modal participation factors

and modal effective mass tables.

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d3eigv

similar to d3plot binary database.

Each state shows one mode shape

‘State times’ give info of circular frequency

eigout

An ASCII text file

Summary of frequencies found

*CONTROL_IMPLICIT_EIGENVALUE... Outputs

𝝀 𝝎 = 𝝀 𝒇 = 𝝎 𝟐𝝅 𝑻 = 𝟏 𝒇

IMPLICIT ANALYSIS

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LS-DYNA ENVIRONMENT



Non-Linear Static Implicit

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT



Non-linear Implicit analysis – Few Animations

IMPLICIT ANALYSIS

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IMPLICIT ANALYSIS

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IMFLAG – Implicit/ Explicit analysis type flag

= 1 → Implicit analysis

DT0 – Initial time step size for implicit

(default – none)

Non-linear Implicit analysis

*CONTROL_IMPLICIT_GENERAL

Card-1 IMFLAG DT0 IMFORM NSBS IGS CNSTN FORM ZERO_V

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EXPLICIT IMPLICIT

Timestep size and Stability Conditional

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C*Unconditional

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Non-linear Implicit analysis…

*CONTROL_IMPLICIT_SOLUTION

Card-1 NSOLVR ILIMIT MAXREF DCTOL ECTOL RCTOL LSTOL ABSTOL

NSOLVR – Solution method for implicit analysis

= 2 → Nonlinear with BFGS updates (obsolete)

= 12 → (new default from 9.0.1) Nonlinear with BFGS updates

+ optional arc length

+ different line search and integration schemes compared to solver 2.

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Arc length method can handle situations of load reversals, snap-through and snap-back,

which makes it ideal for dealing with non-linear limit load and post-buckling analyses.

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Card-1 NSOLVR ILIMIT MAXREF DCTOL ECTOL RCTOL LSTOL ABSTOL

DCTOL – Displacement relative convergence tolerance (default = 0.001)

ECTOL – Energy relative convergence tolerance (default = 0.01)

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*CONTROL_IMPLICIT_AUTO

Card-1 IAUTO ITEOPT ITEWIN DTMIN DTMAX DTEXP KFAIL KCYCLE

IAUTO – Automatic time step control

= 0 → constant time step size (default)

= 1 → automatically adjusted timestep size

Automatic timestep controller controls the timestep

based on convergence criteria, NOT based on accuracy.

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*CONTROL_IMPLICIT_AUTO

Card-1 IAUTO ITEOPT ITEWIN DTMIN DTMAX DTEXP KFAIL KCYCLE

DTMIN – Minimum allowable timestep size (default = DT0/1000)

Simulation stops with error termination, if time step falls below DTMIN

DTMAX – Maximum allowable timestep (default = DT0*10)

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Convergence info in d3hsp file

Not converged Converged Not Active

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Convergence info in d3hsp & d3iter files


Optional-1 DNORM DIVERG ISTIF NLPRINT NLNORM D3ITCTL CPCHK

NLPRINT – Nonlinear solver print flag

= 3 → print iteration, norm and line search info.

D3ITCTL – Controls D3ITER database (default = 0)

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Residual force plots in d3plot & d3iter

*DATABASE_EXTENT_BINARY

Card-4 DTDT RESPLT NEIPB

RESPLT – Output of translational and rotational residual forces to d3plot & d3iter.

= 1 → Output residualC

IMPLICIT ANALYSIS

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*CONTROL_ACCURACY

Card-1 OSU INN PIDOSU IACC

OSU – 2nd order objective stress update

= 0 → Off (default)

= 1 → On

INN – Invariant node numbering

= 2 → On for shell and thick shell elements (default for implicit)

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*CONTROL_ACCURACY

Card-1 OSU INN PIDOSU IACC

IACC – Implicit accuracy flag, turns on some specific accuracy considerations in

implicit analysis at an extra CPU cost.

= 0 → Off (default)

= 1 → OnC Strong objective treatment of some tied contact interfaces.

Fully iterative treatment of MAT_024 & MAT_123, including

smooth decay of stresses down to zero when including failure.

Strong objective treatment of some elements in the context of

large rotations, applies to shell element types –16, 16 & 4, beam

element types 1, 2 & 9 and solid element types –2, –1, 1, 2 & 16.

IMPLICIT ANALYSIS

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LS-DYNA ENVIRONMENT



Non-Linear Implicit – Recommended ELFORMs

Element ELFORM Comments

Beam

1 Generic use

9 For bolts with pre-tension or for thick and short beams

6 For springs and dampers

Shell

–16 1st order. NIP ≥ 5 recommended.

23/ 24 2nd order

Solid

–2 1st order Hexa

23 2nd order Hexa

16 2nd order Tetra

IMPLICIT ANALYSIS

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LS-DYNA ENVIRONMENT



*CONTACT – Penalty based (_MORTAR)

*CONTACT_AUTOMATIC_SINGLE_SURFACE_MORTAR

*CONTACT_AUTOMATIC_SURFACE_TO_SURFACE_MORTAR

*CONTACT_FORMING_SURFACE_TO_SURFACE_MORTAR

The MORTAR contacts are segment based, using a penalty formulation and

specially developed for implicit analyses, which can handle edge-to-edge

contacts for shells and solids, as well as beam-to-beam and beam-to-edge

situations and these are active by default with the automatic contacts.

The Mortar contacts features smoothness and continuity that is highly

appreciated in implicit analysis.

IMPLICIT ANALYSIS

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LS-DYNA ENVIRONMENT



*CONTACT – Penalty based (_MORTAR)…

𝝈𝒄 = 𝜶𝜺𝑲𝒔𝒇𝒅

𝜺𝒅𝒄

Stiffness scaling factor (SFS*SLSFAC) 0.03 Stiffness modulus of slave segment

Penetration distance

Characteristic length of slave segment

Contact pressure in Mortar contact

as a function of penetration

𝒇 𝒙 = 𝟎𝟎𝟎𝑪𝒖𝒃𝒊𝒄 𝒇𝒖𝒏𝒄𝒕𝒊𝒐𝒏 𝒕𝒉𝒂𝒕 𝒅𝒆𝒑𝒆𝒏𝒅𝒔 𝒐𝒏 𝑰𝑮𝑨𝑷

𝟏

𝟒𝒙𝟐 𝒙 < 𝒅𝒎𝒂𝒙 𝟐𝜺𝒅𝒄

𝒙 ≥ 𝒅𝒎𝒂𝒙 𝟐𝜺𝒅𝒄

Co

nta

ct

Str

ess

Penetration

IGAP = 1

IGAP = 2

IGAP = 5

IGAP = 10

For shells = Shell thickness

For solids = median (50%ile)

of the edges in the slave side

95% of the average characteristic lengths on master and slave sides

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In _MORTAR, the VDC input is ignored.

In _MORTAR, the SOFT input is ignored.

BSORT = 1 automatically applied, i.e., bucket sorting is done every cycle.

SNLOG = 1 automatically applied (Shooting node logic in thickness offset

contact is skipped).

IMPLICIT ANALYSIS

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LS-DYNA ENVIRONMENT



*CONTACT – Kinematic Constraint based…

Constraint based formulation with moment coupling, i.e.,

*CONTACT_TIED_SHELL_EDGE_TO_SURFACE_CONSTRAINED_OFFSET &

penalty based formulation with moment coupling, i.e.,

*CONTACT_TIED_SHELL_EDGE_TO_SURFACE_BEAM_OFFSET often suffice.

Ensure all the nodes intended are tied properly in case of Tied contacts.

By setting IACC = 1 on *CONTROL_ACCURACY a strongly objective

formulation of these contacts is activated.

IMPLICIT ANALYSIS

USING LS_DYNA

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Implicit Dynamics

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT



Implicit Dynamics

*CONTROL_IMPLICIT_DYNAMICS

Card-1 IMASS GAMMA BETA TDYBIR TDYDTH TDYBUR IRATE ALPHA

IMASS – Implicit analysis type

= 0 → static analysis (default)

= 1 → dynamic analysis using Newmark time integration

𝜸 = 𝟎. 𝟓 & 𝜷 = 𝟎. 𝟐𝟓 will ensure energy is conversed, but no numerical damping.

𝜸 = 𝟎. 𝟔 & 𝜷 = 𝟎. 𝟑𝟖 if the dynamic results are of no interest but just a way to

proceed to the static solution (say for quasi-static simulations), then it is

recommended to use numerical damping to prevent unnecessary oscillations.

IMPLICIT ANALYSIS

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LS-DYNA ENVIRONMENT





Card-1 IMASS GAMMA BETA TDYBIR TDYDTH TDYBUR IRATE ALPHA

TDYBIR – Birth time for application of dynamic terms

TDYDTH – Death time

TDYBUR – Burial time

Solution Time →

Dyn

am

ic e

ffe

cts

(ma

ss

& d

am

pin

g)

100%

0%TDYBIR TDYDTH TDYBUR

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT



Non-linear Implicit Dynamics – Few Animations

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT



General Remarks

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT



Examples & More info on – IMPLICIT LS-DYNA

You can download some Implicit LS-DYNA examples from:

http://www.dynasupport.com/howtos/implicit/some-guidelines-for-

implicit-analyses-using-ls-dyna/ImplicitPackage.zip

You can get more info regarding implicit from:

http://www.dynasupport.com/howtos/implicit

Also refer Appendix ‘P’: Implicit Solver in Volume-I manual of LS-DYNA.

http://www.dynasupport.com/howtos/implicit/some-guidelines-for-implicit-analyses-using-ls-dyna/ImplicitPackage.zip

http://www.dynasupport.com/howtos/implicit

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT



Golden Rules

Preferably always use the latest release version

of Double precision LS-DYNA (preferably MPP-

DYNA) for implicit simulations as the implicit

solver tends to get better by the day.

GOLDEN RULE

IMPLICIT ANALYSIS

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LS-DYNA ENVIRONMENT



Golden Rules

For best implicit performance, it is important

to provide enough memory to allow the

stiffness matrix factorization to run in-core.

GOLDEN RULE

IMPLICIT ANALYSIS

USING LS_DYNA

LS-DYNA ENVIRONMENT



Golden Rules

Preferably add all the implicit related control and

database cards as an include file. Make different

include files (i.e., with the relevant control cards)

for different kinds of Implicit simulations.

GOLDEN RULE

IMPLICIT ANALYSIS

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IMPLICIT ANALYSIS USING LS-DYNA - Oasys · PDF fileUse IMFLAG=1 on *CONTROL_IMPLICIT_GENERAL...

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