Fatigue life prediction of structures whose natural frequencies lie near the frequencies of the input loading requires the stress results from dynamic analyses. These analyses can be performed in either the time or the frequency domains depending on the nature or the loading. This presentation will outline the approaches available for fatigue life prediction using Altair OptiStruct and APA product, nCode DesignLife for both time and frequency based dynamic analyses. The strengths, weaknesses and assumptions of each approach will be presented, along with typical applications enabling the user to select the proper technique for their particular use case.

1. 2015 Americas Altair Technology Conference
Fatigue Life from Sine-on-Random Excitation
2015 Americas ATC
May 7th, 2015
Jeffrey Mentley
HBM-nCode
©2015HBM

2. Agenda
1. Problem description
2. Engine speed profile
3. Extraction of sine on random loading
4. DesignLife vibration fatigue
1. Creation of FRF
2. Sine on random loading
3. Fatigue results
5. Summary
6. Questions
©2015HBM

3. • Components on or near rotating machinery can be subjected to
dynamic loads
• These loads can be random, sinusoidal, or a combination of both,
and are important in understanding the fatigue life of the
component
Problem description
©2015HBM
Example test spec from MIL STD 810
Acceleration
Log frequency Hzf3 f4
A3 A4
W0
W1
f5
A5
Random PSD (g2/Hz)
Sinusoidal tones (g)
f2
A2
f1
A1

4. • Components on or near rotating machinery can be subjected to
dynamic loads
• These loads can be random, sinusoidal, or a combination of both,
and are important in understanding the fatigue life of the
component
Problem description
©2015HBM
Engine order plot of acceleration

5. • The actual engine loading environment is swept sine on random
• The analysis approach being used in DesignLife is sine on random
• The RPM history is separated into blocks, each at a constant RPM
• Visually
• Time at level
• Each RPM block is represented as a sine dwell
Engine test schedule
©2015HBM
Time
RPM

6. • For each constant RPM block
• Calculate time spent at this RPM ( engine usage )
• Extract acceleration time history at this RPM ( measured data )
• Determine order or orders for sine tones
Engine test schedule
©2015HBM
Engine Speed Profile

7. • For each RPMs acceleration time history
• Calculate frequency for sine tone based on order and RPM
• Remove sine tone frequency from acceleration history using band stop
frequency filter, and create a PSD for random loading
• Pass only the sine tone frequency from the acceleration history using a
band pass frequency filter
• Process statistics of sine tone history to obtain RMS of signal, and
calculate appropriate sine tone amplitude
Extracting sine on random loading
©2015HBM
Random
Sine Dwells

8. Vibration fatigue in DesignLife Version 11.0
©2015HBM

9. Analysis path for sine on random loading
©2015HBM
OptiStruct
nCode DSP
nCode DesignLife

10. • Accessory bracket mounted on engine
• Modal analysis ( 1st 8 modes )
• Unit vertical acceleration sweep across frequency range ( 1 - 400 Hz )
OptiStruct analysis to obtain FRF
©2015HBM

11. Frequency Response Functions from OptiStruct
©2015HBM
Modal
Coordinates
PCH
Modal
Stresses
H3D,OP2
Modal Analysis
Frequency Response
Function
Harmonic
Analysis
Harmonic
Analysis
Complex FRF
OP2
Frequency Response
Function
Direct FRF Modal FRF

12. • Duty cycle used to sequence sine on random analyses
• Engine speed profile used to set each RPM duration
• Each sine on random analysis with its own PSD and sine dwell
Sine on random loading
©2015HBM
Engine Speed Profile

13. • OptiStruct input
• Sine on random duty cycle
• SN fatigue properties
Fatigue analysis process
©2015HBM

14. • Fatigue results for entire component
• Damage
• Life
• RMS stress
• Expected max stress
• Spectral Moments, Irregularity factor
• Results for overall engine test, and each RPM event
• Damage percentages across events
Fatigue results
©2015HBM

15. • Approach for fatigue life using both random and sinusoidal loading
• Ability to use actual measured engine data to define loading
• Vibration duty cycle created from engine speed profile, and sine on
random input for each RPM condition
• OptiStruct used to define the frequency response function of the bracket
• DesignLife combines the FRF and vibration duty cycle, and computes the
fatigue damage
Summary
©2015HBM

16. Questions
©2015HBM

17. measure and predict with confidence
www.ncode.com
Connect with us on:
linkedin.com/company/hbm-ncode | youtube.com/hbmncode | @hbmncode | +nCode
© 2015 HBM
Jeff Mentley
HBM-nCode
jeff.mentley@hbmncode.com