frequency domain analysis in ls-dyna_yun huang_jan-2013

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LS-DYNA

Frequency Domain Analysis

in LS-DYNA®

Yun Huang, Zhe Cui

Livermore Software Technology Corporation

16 January, 2013

Oasys LS-DYNA UK Users’ Meeting, Solihull



LS-DYNA

2

Outline

1. Introduction

2. Frequency response functions

3. Steady state dynamics

4. Random vibration and fatigue5. Response spectrum analysis

6. Acoustic analysis by BEM and FEM

7. Conclusion and future developments



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INTRODUCTION

1.

3



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4

*FREQUENCY_DOMAIN_FRF

*FREQUENCY_DOMAIN_SSD

*FREQUENCY_DOMAIN_RANDOM_VIBRATION_ {OPTION}

*FREQUENCY_DOMAIN_ACOUSTIC_BEM_ {OPTION}

*FREQUENCY_DOMAIN_ACOUSTIC_FEM*FREQUENCY_DOMAIN_RESPONSE_SPECTRUM

Frequency Domain Features in LS-DYNA

New keywo rds for frequency domain analysis



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Frequency domain v s. t ime domain

A time-domain graph shows how a signal changes over time

A frequency-domain graph shows the distribution of the energy

(magnitude, etc.) of a signal over a range of frequencies

Time domain analysis

Transient analysis (penetration)

Impact (crash simulation) Large deformation (fracture)

Non-linearity (contact)

Frequency domain analysis

Harmonic (steady state vibration)

Resonance Linear dynamics

Long history (fatigue testing)

Non-deterministic load (random analysis)



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Time domain excitation Frequency domain excitation

Fourier Transform

A given function or signal can be converted between the time and frequency

domains with a pair of mathematical operators called a transform.

dt et h H t i )()(

d e H t h t i)(2

1)(

Inverse Fourier

Transform



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Vehicle NVH• Interior noise

• Exterior radiated noise

• Vibration

Vehicle Durability

• Cumulative damage ratio• Expected life (mileage)

Aircraft / rocket / spacecraft vibro-acoustics

Durability analysis of machines and electronic devices

Acoustic design of athletic products

Civil Engineering• Architectural acoustics (auditorium, concert hall)

• Earthquake resistance

Off-shore platforms, wind turbine, etc.• Random vibration

• Random fatigue

App l icat ions o f the frequency domain features



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App l icat ion of LS-DYNA in automot ive industry

In automotive, one model for crash, durability, NVHshared and maintained across analysis groups

Manufacturing simulation results from LS-DYNA

used in crash, durability, and NVH modeling.

Crashworthiness

Occupant Safety

NVH

Durability



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New Databases in Frequency Domain

Keyword *DATABASE_FREQUENCY_BINARY _{OPTION}

Database lspcode used for

D3SSD 21 Steady state dynamics

D3SPCM 22 Response spectrum analysis

D3PSD 23 Random vibration

D3RMS 24 Random vibration

D3FTG 25 Random fatigue

D3ACS 26 FEM acoustics

ASCII databases

FRF: frf_amplitude, frf_angle, frf_real, frf_imag

BEM acoustics: Press_Pa, Press_dB, bepres, fringe_*,

panel_contribution_NID,

SSD: elout_ssd, nodout_ssd, …

BINARY databases



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FREQUENCY RESPONSE FUNCTIONS

2

10



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o The concept of Frequency Response Function is the foundation of modern

experimental system analysis and experimental modal analysis

o Keyword *FREQUENCY_DOMAIN_FRF

o Express structural response due to unit load as a function of frequency

o Shows the property of the structure system

o Is a complex function, with real and imaginary components. They mayalso be represented in terms of magnitude and phase angle

o Result files: frf_amplitude, frf_angle

o Support efficient restart

11

Transfer functionH(ω)

Input force

F(ω)

Displacement response

X(ω)

Introduction: FRF

ωFωHωX

ωF

ωXωH



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Accelerance, Inertance Acceleration

Force

Effective MassForce

Acceleration

Mobility VelocityForce

ImpedanceForce

Velocity

Dynamic Compliance,

Admittance, Receptance

Displacement

Force

Dynamic StiffnessForce

Displacement

FRF formulat ions



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Point A

Point B

Mode Damping ratio (0.01)

1 0.450

2 0.713

3 0.386

4 0.328

5 0.3406 0.624

7 0.072

8 0.083



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Example: nodal force/resul tant force

Left end of the beam is fixed and subjected

to z-directional unit acceleration

Nodal force and resultant force FRF at the

left end can be obtained

Resultant force FRF



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Nodal force applied and

displacement measuredExample: FRF for a tr immed BIW



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STEADY STATE DYNAMICS

3

17



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Harmonic excitation is often encountered in engineering systems. It is

commonly produced by the unbalance in rotating machinery.

The load may also come from periodical load, e.g. in fatigue test.

The excitation may also come from uneven base, e.g. the force on tires

running on a zig-zag road.

May be called as o Harmonic vibration

o Steady state vibration

o Steady state dynamics

Keyword *FREQUENCY_DOMAIN_SSD

Binary plot file d3ssd

)(sin)( 0 t F t F

Background

Introduction: SSD



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Example: a rectangular plate under pressure

X-Stress



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Enforced Motion

Relat ive disp lacement method

o base acceleration loading is applied through

inertial force.

o total displacement (velocity, acceleration) is

obtained by adding the relative displacement

(velocity, acceleration) to the base

displacement (velocity, acceleration).

baserelative

baserelative

baserelative

uuu

uuu

uuu

Large mass method

o a very large mass mL, which is usually 105-107

times of the mass of the entire structure, is

attached to the nodes under excitation

o a very large nodal force pL is applied to the

excitation dof to produce the desired enforced

motion. um p

umi p

um p

L L

L L

L L

2



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o the rectangular plate is excited by z -

direction acceleration at one end

o

steady state response at the other endis desired

o the excitation points are fixed inloading dof

o the resulted response is added

with base motion to get total

response

o a large mass with 106 times the originalmass is added to excitation points using

keyword *element_mass_node_set

o to produce desired acceleration, nodal

force is applied to the nodes in loading dof

o

the excitation points are free in loading dof

Relat ive disp . method Large mass method



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Example: auto model subjected to base acl.

Modal information

Number of nodes: 290k

Number of shells: 532k

Number of solids: 3k

Base acceleration spectrum is applied to

shaker table. 500 modes up to 211 Hz are

used in the simulation. The response is

computed up to 100 Hz.

Point A Point B



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Freq = 15 Hz Freq =25 Hz

Freq = 35 Hz



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o The loading on a structure is not known in a definite sense

oMany vibration environments are not related to a specific driving frequency

(may have input from multiple sources)

oProvide input data for random fatigue and durability analysis

oExamples:

Fatigue

Wind-turbine

Air flow over a wing or past a car body

Acoustic input from jet engine exhaust

Earthquake ground motion Wheels running over a rough road

Ocean wave loads on offshore platforms

Freq (Hz)

A c c e l e r a t i o n P S D

( g ^ 2 / H z )

Freq (Hz)

S P L ( d B )

Loading: PSD or SPL

Why we need random vibration analysis?

2,

2

df f

df f

Introduction: Random Vibration

Based on Boeing’s N -FEARA package



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Card 5 1 2 3 4 5 6 7 8

Variable SID STYPE DOF LDPSD LDVEL LDFLW LDSPN CID

Type I I I I I I I I

Default

Keyword

*FREQUENCY_DOMAIN_RANDOM_VIBRATION

The multiple excitations on structure can be

• uncorrelated

• partially correlated

• fully correlated

(cross PSD function needed)

When SID and STYPE are both < 0, they give the IDs of correlated

excitations

Correlat ion of m ult ip le exci tat ions



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The model is a simple pipe with 83700 elements and 105480 nodes. It is subjected to base

acceleration PSD 1) in x, y and z-directions uncorrelated; 2) in x, y and z-directions correlated.

28

RMS of Von Mises stress(the pipe is subject to x, y, z excitations, uncorrelated)

Example: pipe under base acceleration

Model provided by Parker Hannifin Corp.

ANSYS LS-DYNA

Freq

(Hz)

Acceleration

psd (g^2/Hz)

5 0.01

40 0.01

100 0.04

500 0.041000 0.0065

2000 0.001

Max Von Mises RMS stress (MPa)

ANSYS LS-DYNA

99.4 100.7



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RMS of Von Mises stress(the pipe is subject to x, y, z excitations, correlated)

ANSYS LS-DYNA

Max Von Mises RMS stress (MPa)

ANSYS LS-DYNA

140.3 142.4



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The tube was fixed to the shaker tables using

aluminum blocks which surrounded the tube

and were tightened using screws.

Shaker table A Shaker table B

CH 6

CH 3 CH 7 CH 5CH 1

x

y

z

CH 1

Tube thickness = 3.3 mm

Example: shaker table

Cou rtesy of Rafael, Israel.

Base accelerat ion PSD load



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GRMS=1.63 g^2/Hz

Hz g^2/Hz

10 0.001

20 0.003

40 0.003

80 0.02

120 0.02

200 0.0015

500 0.0015

Input PSD

A cluster server is analyzed

by LS-DYNA to understandthe location of vibration

damage under standard

random vibration condition.

Example: cluster server



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1σ 3σ

U (mm) 0.78 2.34

Sv-m (MPa) 41.2 123.6

D i s p l a c e m e n t

S t r e s s

It is found that the 3

Von-Mises stress is less

than the yield stress of

the material (176 Mpa).

Maximum values



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Nodes: 800k

Elements: 660k

Modes: 1000

Model courtesy of Predict ive Engineering

MPP Random Vibration Analysis



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• Keyword *FREQUENCY_DOMAIN_RANDOM_VIBRATION_FATIGUE

• Calculate fatigue life of structures under random vibration• Based on S-N fatigue curve

• Based on probability distribution & Miner’s Rule of Cumulative DamageRatio

• Schemes:

Steinberg’s Three-band technique considering the number of stress cyclesat the 1 , 2 , and 3 levels.

Dirlik method based on the 4 Moments of PSD.

Narrow band method

Wirsching method

…

Introduction: Random Fatigue

i i

i N

n R

PDF( probability density function)

Typical SN (or Wöhler) curve



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S-N fatigue curve definition

• By *define_curve

• By equation

aS N m

)log()log( N baS

SNLIMT Fatigue life for stress lower than the lowest stress on S-N curve.

EQ.0: use the life at the last point on S-N curve

EQ.1: extrapolation from the last two points on S-N curve

EQ.2: infinity.

• Fatigue life of stress below fatigue threshold

Source of information: http://www.efunda.com

N: number of cycles for fatigue failure

S: stress



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Steinberg’s Three Band Technique

Standard Deviation Percentage of Occurrence Number of cycles for failure

1σ stress 68.3% N1

2σ stress 27.1% N2

3σ stress 4.33% N3

3

3

2

2

1

1][ N n

N n

N n D E

0433.0)0(

271.0)0(

683.0)0(

3

2

1

T E n

T E n

T E n E(0): Zero-crossing frequency with

positive slope

Reference

Steinberg, D.S., Vibration Analysis for Electronic Equipment (2nd edition), John Wiley & Sons, New

York, 1988.



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Time of exposure: 4 hours

Frequency (Hz)

P S D (

g 2 / H z

)

Z-acceleration PSD

PSD = 2 g2/Hz

100.0 2000.0

Example: Aluminum bracket

0

5

10

15

20

25

30

35

40

45

50

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09

No. of cycles

S t r e s s ( K s i )

S-N fatigue curve

Nodes constrained to

shaker table

Aluminum 2014 T6

= 2800 Kg/m3

E = 72,400 MPa

= 0.33



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39

Accumulative damage ratio(by Steinberg’s method)

RMS of Von-Mises stress (unit: GPa)

(given in d3ftg) (given in d3rms)



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40Model cou rtesy of CIMES France

Aluminum alloy 5754

= 2700 Kg/m3

E = 70,000 MPa

= 0.33

Base acceleration is applied at the edge of the hole

Acceleration PSD(exposure time: 1800 seconds)

Example: beam with predefined notch



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RMS Sx = 35.0 MPa at critical point

Analysis method Expected life Damage ratio

Experiment 7mn 25s -

Steinberg 4mn 10s 7.19

Dirlik 5mn 25s 5.54

Narrow Band 2mn 05s 14.41Wirsching 5mn 45s 5.08

Chaudhury & Dover 6mn 03s 6.03

Tunna 4mn 06s 7.31

Hancock 22mn 18s 1.35

CODE RMS Sxx

ANSYS 33.5 MPa

RADIOSS® BULK 35.7 MPa

LS-DYNA 35.0 MPa



LS-DYNA

42Cumulative damage ratio by Dirlik method

Cumulative damage ratio by Steinberg’s method

Damage ratio = 5.540

Damage ratio = 7.188



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43

Experiment setup

Failure at the notched point in experiment



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Initial failure area

A model from railway application

The structure is subjected to base acceleration defined by the International Standard

IEC61373 to simulate the long-life test. This standard intends to highlight any weakness

which may result in problem as a consequence of operation under environment where

vibrations are known to occur in service on a railway vehicle.

Example: support antenna



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Area 1 - damage ratio = 25.5

Area 2 - damage ratio = 3.45

Area 1

Area 2



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Initial Damage Ratio

Card1 1 2 3 4 5 6 7 8

Variable MDMIN MDMAX FNMIM FNMAX RESTRT MFTG RESTRM INFTG

Type I I F F I I I I

Default 1 0.0 0 0 0 0

*FREQUENCY_DOMAIN_RANDOM_VIBRATION_ FATIGUE

Card7 1 2 3 4 5 6 7 8

Variable FILENAME

Type C

Default d3ftg

Define Card 7 if option FATIGUE is used and INFTG=1.

INFTG Flag for including initial damage ratio.

EQ.0: no initial damage ratio,

EQ.1: read existing d3ftg file to get initial damage ratio.

FILENAME Path and name of existing binary database (by default, D3FTG) for

initial damage ratio.

VARIABLE DESCRIPTION



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Time of exposure: 4 hours

0

5

10

15

20

25

30

35

40

45

50

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09

No. of cycles

S t r e s s ( K s i )

S-N fatigue curve

Edge fixed toshaker table.

Acceleration

PSD = 1g2/Hz

for 1-2000 Hz

RMS - von mises for x-acl PSD RMS -von mises for z-acl PSD



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Damage ratio for x-load only

Damage ratio for z-load only

Damage ratio for x+z load

Dirlik method is used (MFTG = 2).

1.152e-01

6.019e-01

7.171e-01



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RESPONSE SPECTRUM ANALYSIS

5

49



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Introduction: Response Spectrum

*FREQUENCY_DOMAIN_RESPONSE_SPECTRUM Use various mode combination methods to evaluate peak

response of structure due to input spectrum.

The input spectrum is the peak response (acceleration, velocity

or displacement) of single degree freedom system with different

natural frequencies. The input spectrum is dependent on damping (using

*DEFINE_TABLE to define the series of excitation spectrum

corresponding to each damping ratio).

Output binary database: d3spcm (accessible by LS-PREPOST).

It is an approximate method. It has important application in earthquake engineering, nuclear

power plants design etc.



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Capabilities

o

SRSS methodoNRC Grouping method

oCQC method

oDouble Sum methods Rosenblueth-Elorduy coefficient

Gupta-Cordero coefficient

Modified Gupta-Cordero coefficient

oNRL SUM method

oRosenblueth method

Mode comb inat ion

oBase velocityoBase acceleration

oBase displacement

oNodal force

oPressure

Input sp ectrum

o

LogarithmicoSemi-logarithmic

o Linear

Frequenc y interpolat ion

oBINARY plot file: d3spcm

o ASCII files: nodout_spcm, elout_spcm

Results

oCivil and hydraulic buildings Hydraulic dams

Bridges

High buildings

oNuclear power plants

Appl icat ions



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Example: multi story tower

60 m /15 story towerLoading conditionsBase acceleration input spectrum

Mode combination method

SRSS

Damping = 1%

Frequency (Hz) Acceleration (m/s2)

0.1 19.4

5.0 22.23

10.0 24.10

15.0 25.5

20.0 26.6

6 0 m



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Z displacement results



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Von Mises stress results



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Example: arch dam

o 464.88 feet high

o Rigid foundation

o Subjected to x -directional ground acceleration

spectrum

The pseudo-acceleration spectrum of El

Centro earthquake ground motion (=5%)

The 1940 El Centro earthquake (or 1940 Imperial

Valley earthquake) occurred on May 18 in the

Imperial Valley in Southern California near the

border of the United States and Mexico. It had a

magnitude of 6.9.

X-directional acceleration



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ACOUSTIC ANALYSIS BY BEM/FEM

6

56



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57

*FREQUENCY_DOMAIN_ACOUSTIC_BEM_ {OPTION}

Available options include:

PANEL_CONTRIBUTION

HALF_SPACE

Introduction: BEM Acoustics

SSD

Structure loading

V (P) in time domain

V (P) in frequency domain

Acoustic pressure and SPL (dB) at field points

FFT

FEM transient analysis

BEM acoustic analysis

User data



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BEM (accurate)

Indirect variational boundary element method

Collocation boundary element method

They used to be time consuming

A fast solver based on domain decomposition MPP version

Approximate (simplified) methods

Rayleigh method

Kirchhoff method Assumptions and simplification in formulation

Very fast since no equation system to solve



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59

N

j

j

j

N

j

P p

d n

G pn

pG P p

j

1

1

)(

)(

real

i m a g i n a r y

p p

j

O

Projection in the direction

of the total pressure

Acoustic Panel Contribution



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61

Freq = 11Hz

Freq = 101 Hz

6 m

3 m

The plate is 0.2 m away from the vehicle

Sound Pressure Level distribution (dB)



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62

Half-space Problem

r eG

ikr

4

r

e R

r

eG

r ik

H

ikr

H

44

Free space Green’s function

Half space Green’s function

pr

r’

S

H H n ds

n

G pGvi P )()(

Helmholtz integral equation

R H = 1: rigid reflection plane, zero velocity

-1: soft reflection plane, zero sound pressure (water-air interface in

underwater acoustics)

The reflection plane is defined by *DEFINE_PLANE.



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63

Muffler Transmission Loss Analysis

W i

W t

t

i

W

W TL 10log10

TL (Transmission loss) is the difference in the sound power level

between the incident wave entering and the transmitted wave

exiting the muffler when the muffler termination is anechoic (no

reflection of sound).



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64

pi

pr

P 1 P 2 P 3

Muffler

Anechoic

Termination (Z=c)

po

Muffler P 2

v2

P 1

v1

Three-point method Four-pole method

Incoming wave is given as

1

21

12

2

)](sin[2

1 ikxikx

i e pe p x xk i

p

o

i

o

i

s

s

p

pTL 1010 log10log20

2

2

1

1

v

p

DC

B A

v

p

o

i

s

s

DcC c

B ATL

10

10

log10

1

2

1log20

Where, si and so are the inlet and outlet tube

areas, respectively

21

21

21

21

/

/

/

/

vv D

pvC

v p B

p p A

1,0|

1,0|

1,0|

1,0|

12

12

12

12

v p

vv

v p

vv

The four pole parameters A, B, C, D, can

be obtained from



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65

1. F. Fahy, Foundations of Engineering Acoustics. Elsevier Academic Press, 2001.

2. Z. Tao and A.F. Seybert, a Review of Current Techniques for Measuring Muffler Transmission Loss.

SAE International , 2003

0

10

20

30

40

0 1000 2000 3000

T r a n s

m i s s i o n

L o s s ( d B )

Frequency (Hz)

Plane Wave theory [1]

Experiment [2]

LS-DYNA (Three-Point Method)

LS-DYNA (Four-Pole Method)

Cutoff frequency for plane wave theory f =1119 Hz

Muffler transmission loss by different methods

References



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66

Double expansion chamber



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67

Z. Tao and A.F. Seybert, a Review of Current Techniques for Measuring Muffler Transmission

Loss. SAE International , 2003

References

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68

*BOUNDARY_ACOUSTIC_MAPPING

VARIABLE DESCRIPTION

SSID Set or part ID

STYP Set type:

EQ.0: part set ID, see *SET_PART,EQ.1: part ID, see *PART,

EQ.2: segment set ID, see *SET_SEGMENT.

Card 1 2 3 4 5 6 7 8

Variable SSID STYP

Type I I

Default none 0

*BOUNDARY_ACOUSTIC_MAPPING

Purpose: Define a set of elements or segments on structure for mapping structural nodal velocity to boundary of acoustic volume.

LS DYNA



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69

Mesh A: 20 30 (600) Mesh B: 15 20 (300) Mesh C: 7 11 (77)

Mesh A 16 min 34 sec

Mesh B 10 min 10 sec

Mesh C 6 min 49 sec

CPU time (Intel Xeon 1.6 GHz)

Also mesh for structure surface

LS DYNA*FREQUENCY DOMAIN ACOUSTIC BEM ATV



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70

*FREQUENCY_DOMAIN_ACOUSTIC_BEM_ ATV

Acoustic Transfer Vector can be obtained by including the option ATV in the keyword. It calculates acoustic pressure (and sound pressure level) at field points due to unit

normal velocity of each surface node.

ATV is dependent on structure model, properties of acoustic fluid as well as location

of field points.

When ATV option is included, the structure does not need any external excitation,

and the curve IDs LC1 and LC2 are ignored.

ATV_DB_FIELD_PT_* (decibel due to unit vn at each node)

ATV_FIELD_PT_* (complex pressure due to unit vn at each node) ATV_DB_FIELD_PT_FRINGE_* (user fringe plot file for decibel due to unit vn at

each node)

Output files

LS DYNA



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71

ATV is useful if the same structure needs to be studied under multiple

load cases.

n

j

nmnm jmmm

ni jiii

n j

n j

m

i

V

V

V V

P

P

P P

2

1

,,2,1,

,,2,1,

,2,22,21,2

,1,12,11,1

2

1

Need to be computed only once

ATV at field points 1-m, due to unit normal velocity at node j

Change from case to case

1 2 m

LS DYNAA i FRF



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72

Acoustic FRF

Unit force excitation

Driver ear position

A simplified auto body model without any inside details

Analysis steps:

1.Modal analysis2.Steady state dynamics

3.Boundary element acoustics

All done in one run



LS DYNA



LS-DYNAPressu re distr ibu t ion

f =100 Hz

f =400 Hz

f =200 Hz

f =500 Hz

75

LS DYNAE l li d



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76

Introduction

Nodal force 0.01N is applied for frequency range of 10-20000 Hz.

Diameter 10mm, length 31.4mmThis edge is

fixed in x, y, z

translation dof.

To solve an interior acoustic problem by variational indirect BEM,collocation BEM and FEM. The cylinder duct is excited by harmonic nodal

force at one end.

*FREQUENCY_DOMAIN_SSD

*FREQUENCY_DOMAIN_ACOUSTIC_BEM

*FREQUENCY_DOMAIN_ACOUSTIC_FEM

or

Example: cylinder



LS DYNA



LS-DYNA

78f =20000 Hz

f =5000 Hz

f =15000 Hz

f =10000 Hz

Acoust ic pressure dist r ibut ion(by d3acs)

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CONCLUSION &FUTURE DEVELOPMENTS

7

79

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LS-DYNA

A set of frequency domain features have been imp lemented,

towards NVH, durabi l i ty analys is of vehicles and other vibrat ion

and acous t ic analysis

Frequency Response Function

Steady State Dynamics

Random Vibration and Fatigue

BEM & FEM Acoustics Response spectrum analysis

Future wo rk

SEA method for high frequency acoustics

Fast multi-pole BEM for acoustics

Infinite FEM

Fatigue analysis with strains

Feedbacks and suggestions from usersTh k !

frequency domain analysis in ls-dyna_yun huang_jan-2013

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