1) The document describes a workflow for improving fatigue analysis quality through virtual iteration of multi-body system (MBS) models using load data from physical testing. 2) Sensors measure response data like accelerations, displacements, and strains during physical testing, which is analyzed to generate load data to drive virtual MBS models through an iterative process. 3) The virtual iteration technique aims to match simulated response data from the MBS model with the physical test response data, allowing internal loads and stresses from the MBS to be used for more accurate fatigue analysis.

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

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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

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General Workflow
Author: FEMFAT SUPPORT 3Date: 05.05.2015
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

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General Workflow
Author: FEMFAT SUPPORT 4Date: 05.05.2015
Workflow of fatigue testing and analysis
based on road load data
Application & calibration
Data acquisition on proving ground
Load data analysis
Fatigue testing
Fatigue analysis
Virtual iteration MBS model
M U L T I P H Y S I C
W O R K F L O W

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• 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
– MotionSolve
and additional 3rd party MBD products
General Workflow
Author: FEMFAT SUPPORT 5Date: 05.05.2015
black…measurement
red…...simulation

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General Workflow
Author: FEMFAT SUPPORT 6Date: 05.05.2015
Workflow of fatigue testing and analysis
based on road load data
Application & calibration
Data acquisition on proving ground
Load data analysis
Fatigue testing
Fatigue analysis
Virtual iteration MBS model
M U L T I P H Y S I C
W O R K F L O W

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General Workflow
Author: FEMFAT SUPPORT 7Date: 05.05.2015
• 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)

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General Workflow
Author: FEMFAT SUPPORT 8Date: 05.05.2015
Measurement signals (responses)
Damper force
Tie rod force
Ball joint forces Spring displacement
MBS: requests

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General Workflow
Author: FEMFAT SUPPORT 9Date: 05.05.2015
Workflow of fatigue testing and analysis
based on road load data
Application & calibration
Data acquisition on proving ground
Load data analysis
Fatigue testing
Fatigue analysis
Virtual iteration MBS model
M U L T I P H Y S I C
W O R K F L O W

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FEMFAT LAB vi Introduction
Author: FEMFAT SUPPORT 10Date: 05.05.2015
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 problem
find load for given response
MotionSolve
Input un Output yn
Load
(drive)
Desired
(response)

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FEMFAT LAB vi Introduction
Author: FEMFAT SUPPORT 11Date: 05.05.2015
MotionSolve
Input 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))

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FEMFAT LAB vi Introduction
Author: FEMFAT SUPPORT 12Date: 05.05.2015
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

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FEMFAT LAB vi Results
Author: FEMFAT SUPPORT 13Date: 05.05.2015
Results: 10. iteration, rough road

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FEMFAT LAB vi Results
Author: FEMFAT SUPPORT 14Date: 05.05.2015
Damper force – time domain 15th to 20th second Ball joint force lateral – time domain 15th to 20th second
Ball joint force longitudinal – time domain 15th to 20th second Tie rod force axial – time domain 15th to 20th second
black…measurement
red…...simulation
Results: 10. iteration, rough road

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FEMFAT LAB vi Results
Author: FEMFAT SUPPORT 15Date: 05.05.2015
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…measurement
red…...simulation

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FEMFAT LAB vi Results
Author: FEMFAT SUPPORT 16Date: 05.05.2015
Spring displacement
(model check)
Spring displacement – time domain
Spring displacement – time domain 15th to 20th second Spring displacement – frequency domain
Results: 10. iteration, rough road
black…measurement
red…...simulation

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FEMFAT LAB vi Results
Author: FEMFAT SUPPORT 17Date: 05.05.2015
Results: 10. Iteration – relative damage values (signal based)
Relative damage comparison
simulation to measurement
1,02
1,03
0,99
0,970,97
0,90
0,95
1,00
1,05
1,10
Damper force Bolt force lateral Bolt force
longitudinal
Tie rod force Spring deflection
relativedamagevalue

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General Workflow
Author: FEMFAT SUPPORT 18Date: 05.05.2015
Workflow of fatigue testing and analysis
based on road load data
Application & calibration
Data acquisition on proving ground
Load data analysis
Fatigue testing
Fatigue analysis
Virtual iteration MBS model
M U L T I P H Y S I C
W O R K F L O W

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Fatigue analysis
Author: FEMFAT SUPPORT 19Date: 05.05.2015
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

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Fatigue analysis
Author: FEMFAT SUPPORT 20Date: 05.05.2015
Stress Tensors
Material Properties
Stress Gradient
Mean Stress Influence
MultiAXial Load
Technological Influences
Size Influence
Temperature Influence
PLASTic Deformations
SPOT Joints s
Anisotropical Behaviour
of Arc WELDs
etc.
S/N1 modified
by FEMFAT
Load Cycles
StressAmplitude
S/N material
from specimen tests

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Fatigue analysis
Author: FEMFAT SUPPORT 21Date: 05.05.2015
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

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Fatigue analysis
Author: FEMFAT SUPPORT 22Date: 05.05.2015
• Transformation of all stress tensors
into several planes (2D,3D)
• Filtering of interesting planes
• Generation of the load histories
of the stress components
• Rainflow counting in all selected planes
• Damage analysis
(Influence Parameter Method)
• The cutting plane with maximum damage is
assumed 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

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Result comparison
Author: FEMFAT SUPPORT 23Date: 05.05.2015
Crack observed
after 97 h
Test bench
Crack predicted
after 28 h
FEMFAT with WFT
Crack predicted
after 60 h
FEMFAT with Virtual Iteration

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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 !
Author: FEMFAT SUPPORT 24Date: 05.05.2015

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The future is ours to make.
Author: FEMFAT SUPPORT 25Date: 05.05.2015