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Fatigue Life from Sine-on-Random Excitation

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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.

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Fatigue Life from Sine-on-Random Excitation

  1. 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. 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. 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. 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. 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. 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. 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. 8. Vibration fatigue in DesignLife Version 11.0 ©2015HBM
  9. 9. Analysis path for sine on random loading ©2015HBM OptiStruct nCode DSP nCode DesignLife
  10. 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. 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. 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. 13. • OptiStruct input • Sine on random duty cycle • SN fatigue properties Fatigue analysis process ©2015HBM
  14. 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. 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. 16. Questions ©2015HBM
  17. 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