Improving the assessment quality of a fatigue analysis

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Improving the assessment quality of a fatigue analysisALTAIR Motion solve, FEMFAT LAB virtual iteration and FEMFAT


General Workflow

Author: FEMFAT SUPPORT 2Date: 05.05.2015

Application & calibration

Data acquisition on proving ground

Load data analysis

Fatigue testing

Fatigue analysis

Virtual iteration MBS model

Workflow of fatigue testing and analysis based on road load data

M U L T I P H Y S I C

W O R K F L O W


General Workflow


Acceleration

Typical responses• Accelerations

– 1-axial– 3-axial

• Displacements – Draw wire displacement sensor

• Frame torsion• Strains (directly/calibrated to

forces)– Axle– Ball joint– Link– Rod– Spring– Stabilizer

• Load cells– Mount

• Wheel force transducers

Displacement

Strain – ball jointStrain – twist beam

Load cell Wheel force transducer


General Workflow





Load data analysis

Fatigue testing

Fatigue analysis



W O R K F L O W


• FEMFAT LAB – Load data analysis software

• Module Virtual Iteration: – Load data generation for simulation models

based on measurement data (test track or test bench)

• General approach– Generate external load based on internal,

measured response

• Same approach as the iteration process in the laboratory (test bench)

• Excellent convergence between measurement and simulation

• Method is automated for – MotionSolveand additional 3 rd party MBD products

General Workflow


black…measurementred…...simulation


General Workflow





Load data analysis

Fatigue testing

Fatigue analysis



W O R K F L O W


General Workflow


• MotionSolve model of front axle of a passenger car (half axle)

• Computing internal forces for fatigue analysis of the knuckle with

• Measurement signals from test track• Desired (measurement)

• Damper force• Ball joint force longitudinal• Ball joint force lateral• Tie rod force axial • Spring displacement (used for model-check)

• Goal: load at wheel (4 channels)• Vertical displacement (wheel center) • Longitudinal force (wheel center) • Steering torque (wheel center) • Lateral force (tire patch)


General Workflow


Measurement signals (responses)

Damper force

Tie rod force

Ball joint forces Spring displacement

MBS: requests


General Workflow





Load data analysis

Fatigue testing

Fatigue analysis



W O R K F L O W


FEMFAT LAB vi Introduction


Response

Often simply and cheaply measureable• Accelerations• Displacements (relative)• Strains• Forces (internal)

Drive

High effort or not measureable• Forces (external)

• WFT• Load cell• Strain gauges

• Displacements (absolute)

Inverse non-linear problemfind load for given response

MotionSolveInput un Output yn

Load(drive )

Desired(response)




MotionSolveInput un Output yn

Load(drive )

Desired(response)

Calculation of the transfer function (MBS):

F(s) = y0(s) / u0(s) noise signal and its response

Calculation of first drive:

u1(s)= F-1(s) yDesired(s)

Calculation of further iterations:

un+1(s)=un(s)+ F-1(s) (yDesired(s) – yn(s))




Iteration process

un+1 = un + F -1 ( yDesired - yn )

1. Noise

2. Response of noise

6. Response = desired 5. Response

3. Transfer function 4. Drive signal


FEMFAT LAB vi Results


Results: 10. iteration, rough road




Damper force – time domain 15 th to 20 th second Ball joint force lateral – time domain 15 th to 20 th second

Ball joint force longitudinal – time domain 15 th to 20 th second Tie rod force axial – time domain 15 th to 20 th second






Damper force - frequency domain Ball joint force lateral - frequency domain

Ball joint force longitudinal - frequency domain Tie rod force axial - frequency domain

Results: 10. iteration, rough road black…measurementred…...simulation




Spring displacement(model check)

Spring displacement – time domain

Spring displacement – time domain 15 th to 20 th second Spring displacement – frequency domain






Results: 10. Iteration – relative damage values (signal based)

Relative damage comparisonsimulation to measurement

1,021,03

0,99

0,970,97

0,90

0,95

1,00

1,05

1,10

Damper force Bolt force lateral Bolt forcelongitudinal

Tie rod force Spring deflection

rela

tive

dam

age

valu

e


General Workflow





Load data analysis

Fatigue testing

Fatigue analysis



W O R K F L O W


Fatigue analysis


Internal forces of MOTION SOLVE

using VI:

Rough road

Specimen material data

FE structure & FE stresses for each load case

Multi axial fatigue assessment

RESULT:

Damage Values


Fatigue analysis


� Stress Tensors

� Material Properties

� Stress Gradient

� Mean Stress Influence

� MultiAXial Load

� Technological Influences

� Size Influence

� Temperature Influence

� PLASTic Deformations

� SPOT Joints s

� Anisotropical Behaviourof Arc WELDs

� etc.

S/N1 modifiedby FEMFAT

Load CyclesS

tres

s A

mpl

itude

S/N materialfrom specimen tests


Fatigue analysis


� Stress Tensors

� Stress Gradient

� Mean Stress Influence

� MultiAXial Load

� Temperature Influence

� PLASTic Deformations

� etc.

Influences in FEMFAT

Finally : Component S/N curve including all influences

FOR EACH NODE

Stress Amplitude

Load cycles

Mean Stress

Stress Amplitude

UTSUCS

Specimen Material Data

Mean Stress

Stress Amplitude

UTSUCS Load cycles


Fatigue analysis


• Transformation of all stress tensors into several planes (2D,3D)

• Filtering of interesting planes

• Generation of the load historiesof the stress components

• Rainflow counting in all selected planes

• Damage analysis(Influence Parameter Method)

• The cutting plane with maximum damage isassumed to be the critical plane for fatigue failure

)(

)(

)(

zzzzyyzxxz

yzzyyyyxxy

xzzxyyxxxx

nnne

nnne

nnne

σσσ

σσσ

σσσσ

++×+

++×+

++×=

γ

σa n

σm n

τ a

τ m

ϕ


Result comparison


Crack observed after 97 h

Test bench

Crack predicted after 28 h

FEMFAT with WFT

Crack predicted after 60 h

FEMFAT with Virtual Iteration


Conclusion

• Fully automated co-simulation FEMFAT LAB and MotionSolve• Easier and cheaper measurements can be used to get a better

correlation• Including FE-structures in a MotionSolve model can account also

for dynamic effects in the fatigue analysis• Including a MotionSolve simulation can also account for nonlinear

effects (nonlinear dampers, springs…)• No limitations for the used flex body co-simulation (attachment

parts to full vehicle)• Major increase of the accuracy in fatigue with FEMFAT with a

minor increase of the FEA simulation effort • FEMFAT is available in the partner program• FEMFAT LAB will follow soon !



The future is ours to make.


Improving the assessment quality of a fatigue analysis

Technology

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