international conference on engineering vibration … of fatigue cycle identification methods...
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International Conference onEngineering Vibration
Comparison of Fatigue Cycle Identification Methods
Ljubljana, Slovenia
7-10 September 2015
Curtis E. Larsen, Ph.D., P.E.
NASA Technical Fellow for Loads and Dynamics
Tom Irvine
Dynamic Concepts, Inc.,
Industry Member, NESC Loads and Dynamics Technical Discipline Team
https://ntrs.nasa.gov/search.jsp?R=20160006936 2018-05-10T16:59:14+00:00Z
Comparison of Fatigue Cycle Identification Methods • Simple range count vs. range pair count vs. rainflow count
• ASTM E1049 − 85 (2011) “Standard Practices for Cycle Counting in Fatigue Analysis
• “The rain flow cycle counting method allows satisfactory predictions of the effects of different block sizes, different sequences of applying the same strain peaks, and superimposed loadings.”
• “The range pair counting method is nearly identical to the rain flow method, but the use of any of the other well known cycle counting methods, such as peak counting, level crossing counting, or range counting, can result in large differences between predicted and actual fatigue lives.”
• “The use of any method of cycle counting other than the range pair or rain flow methods can result in inconsistencies and gross differences between predicted and actual fatigue lives.”• Dowling, N. E., “Fatigue Failure Predictions for Complicated Stress-Strain Histories”, Department of Theoretical
and Applied Mechanics, University of Illinois, Urbana, Illinois, T. & A. M. REPORT NO. 337, January 1971. (Sponsored by Naval Air Development Center, Warminster, Pennsylvania 18974, Contract No. N00156-70-C-1256)
Comparison of Fatigue Cycle Identification Methods • Practical range of fatigue exponent: m = 3.324 and m = 7.3
• Power Spectral Densities Studied• Single Block
• Unimodal
• Two-Block
• Bimodal
Damage Rate ComparisonSingle Block PSDs
1.00
10.00
1.00 10.00
Damage Rate vs. RainflowSingle Block PSDs
m = 3.324
Simple Range Count
Range Pair Count
Rainflow Count
30
300
30.00 300.00
Damage Rate vs RainflowSingle Block PSDs
m = 7.3
Simple Range Count
Range Pair Count
Rainflow Count
Damage Rate ComparisonUnimodal PSDs
3.00
3.00
Damage Rate vs. RainflowUnimodal PSDs
m = 3.324
Simple Range Count
Range Pair Count
Rainflow Count
100
100.00
Damage Rate vs. RainflowUnimodal PSDs
m = 7.3
Simple Range Count
Range Pair Count
Rainflow Count
Damage Rate ComparisonUnimodal PSDs
3
3.00
Damage Rate vs. RainflowUnimodal PSDs
m = 7.3
Simple Range Count
Range Pair Count
Rainflow Count
0.10
0.10
Damage Rate vs. RainflowUnimodal PSDs
m = 3.324
Simple Range Count
Range Pair Count
Rainflow count
Damage Rate ComparisonTwo-Block PSDs
0.1
1
10
100
0.10 1.00 10.00 100.00
Damage Rate vs. RainflowTwo-block PSDs
m = 3.324
Simple Range Count
Range Pair Count
Rainflow Count
1
10
100
1000
1.00 10.00 100.00 1000.00
Damage Rate vs. RainflowTwo-Block PSDs
m = 7.3
Simple Range Count
Range Pair Count
Rainflow Count
Damage Rate ComparisonBimodal PSDs
0.5
5
0.50 5.00
Damage Rate vs. RainflowBimodal PSDs
m = 3.324
Simple Range Count
Range Pair Count
Rainflow Count
10
100
10.00 100.00
Damage Rate vs. RainflowBimodal PSDs
m = 7.3
Simpe Range Count
Range Pair Count
Rainflow Count
Conclusion
• Reiterate conclusions of Dowling and others• Rainflow cycle identification method preferred (conservative)
• Range-pair method nearly equivalent
• Simple range counting can be very unconservative• Small cycles can do “negative damage” as superimposed on large cycles that are
consequently not counted
• If a specific usage of simple range counting must be validated by rainflow, why not use rainflow in the first place?
Appendix
PSD and Time History Example PlotsSingle Block
PSD and Time History Example PlotsUnimodal
PSD and Time History Example PlotsUnimodal
PSD and Time History Example PlotsTwo-block
PSD and Time History Example PlotsBimodal
Comparison of Spectral Fatigue Methods
• Various published spectral methods compared vs. rainflow cycle identification• Narrow band or Rayleigh approximation (NB)
• Dirlik (DK)
• Alpha 0.75 (AL)
• Ortiz and Chen (OC)
• Zhao and Baker (ZB)
• Single Moment (SM)
• Wirsching and Light (WL)
• Benasciutti and Tovo (BT)
Damage Rate ComparisonSingle Block PSDs
1
10
1 10
Spectral Methods vs. Rainflow Damage RateSingle Block PSDs
m = 3.324
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
1
10
100
1000
1 10 100 1000
Spectral Methods vs Rainflow Damage RateSingle Block PSDs
m = 7.3
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
Damage Rate ComparisonUnimodal PSDs
3
3
Spectral Methods vs. Rainflow Damage RateUnimodal PSDs
m = 3.324
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
90
90
Spectral Methods vs Rainflow Damage RateUnimodal PSDs
m = 7.3
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
Damage Rate ComparisonUnimodal PSDs
0.1
0.1
Spectral Methods vs. Rainflow Damage RateUnimodal PSDs
m = 3.324
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
4
4
Spectral Methods vs Rainflow Damage RateUnimodal PSDs
m = 7.3
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
Damage Rate ComparisonTwo-Block PSDs
0.1
1
10
100
0.1 1 10 100
Spectral Methods vs. Rainflow Damage RateTwo-Block PSDs
m = 3.324
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
10
100
1000
10 100 1000
Spectral Methods vs Rainflow Damage RateTwo-Block PSDs
m = 7.3
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
Damage Rate ComparisonBimodal PSDs
0.5
5
0.5 5
Spectral Methods vs. Rainflow Damage RateBimodal PSDs
m = 3.324
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
20
200
20 200
Spectral Methods vs Rainflow Damage RateBimodal PSDs
m = 7.3
rainflow
DNB
DDK
DAL
DOC
DZB
DSM
DWL
DBT
Questions and Discussion