FATIGUE COUNTS

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Michel Olagnon w w w . i f r e m e r . f r l f r e m e r FATIGUE COUNTS Rainflow, zero-crossing, bandwidth, etc.

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FATIGUE COUNTS. Rainflow, zero-crossing, bandwidth, etc. RAINFLOW COUNTS. The on-going study: 1. West-Africa spectra are different from those that we are used to. We need first to check our analysis methods in such cases. - PowerPoint PPT Presentation

Transcript of FATIGUE COUNTS

Michel Olagnon

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FATIGUE COUNTS

Rainflow, zero-crossing, bandwidth, etc.

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RAINFLOW COUNTS

The on-going study:1. West-Africa spectra are different from those that we are used to. We need first to check our analysis methods in such cases.2. It seems impossible to calculate through all combinations of the values of a dozen parameters describing sea states. We need ways to combine the damages created by the components of the sea states at the final stage only.3. How do we select the calculation cases from the climate description and from the combination formulas ?

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RAINFLOW COUNTS

• Time domain analysis in a composite signal context (Hélène Pineau - Actimar)• Spectral domain - Simple summation and alternatives (Michel Olagnon - Ifremer)• Preliminary results with combination formulas (Zakoua Guédé - Ifremer)

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With respect to counting all transitions, zero-crossing removes most of the small ranges, and rainflow turns medium-small ranges into smaller ones and, as we will see better on the next figure, into enlargement of the largest ones.

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Zero-crossing finds the same large ranges as alltransitions, whereas they are larger with rainflow: rainflow associates high peaks to low valleys and smaller peaks to less deep valleys.

-> Hélène

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Simple Summation: add independent damages...

…yet:

10 + 1 = 11 cycles range 2,m=4 -> damage ~ 176

9 cycles range 1.78 in average, 1 cycle range 4m=4 -> damage ~ 346

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Simple summation is not conservative, yet it would be nice to still be able to compute separately damage for each component and to have a (non-linear) way to make the combination.

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RAINFLOW COUNTS

Separate the turning points into two sets:* The maximum (and resp. minimum) over each interval where the low frequency signal is positive (resp. negative)* The other turning points

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RAINFLOW COUNTS

The Simple Summation formula D = DH + DL

The Combined Spectrum formula D = NH+L((DH/NH)2/m + (DL/NL)2/m)m/2

The DNB formula D = (NH-NL)/NH DH + NL((DH/NH)1/m + (DL/NL)1/m)m

The new formulae D = (NH-NL)/NH DH + Z(L/H+L ,NL/NH, m) DL

The CNB (Jiao-Moan) formula

D = F1(L/H+L ,NL/NH, m) DH + F2(L/H+L ,NL/NH, , m) DL

The “exact” formula D = WAFO (Combined Spectrum)

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RAINFLOW COUNTS

• ML distributed as the maximum of NH/Nl

maxima of the global signal : D = (1-)DH

+ Z1(m, ) ((1-2)/(1-2))m/2 DL

• ML distributed as the highest NH/Nl

maxima of the global signal :

D = (1-)DH + Z2(m, ) ((1-2)/(1-2))m/2 DL

• ML distributed as the highest NH/Nl

narrow-band maxima of the global signal:

D = (1-)DH + 1/ Q(m/2+1, -Ln()) / (1-

2)m/2 DL

•ML distributed as the sum of two

Rayleigh variables (Igor).

• DDNB = (1-) DH + (1+ /(1-2))m DL

The new formulae D = (NH-NL)/NH DH + F(, , m) DL

-> Zakoua

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RAINFLOW COUNTS

What does accuracy mean ?

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RAINFLOW COUNTS

Sensitivity to spectral bandwidth secondary wrt aleatory uncertainty.

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How much conservatism is needed ?How much is too much ?

92%

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West Africa Spectra -> Wide band wave actionMooring responses -> Composite signalsWhat does wide-band mean ?

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Is this a wideband spectrum ?If yes, is it because of the ears or of the tail ?

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Which is the widest ?

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= 0.776

= 0.604

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Effect on the rainflow ranges distribution when removing the spectrum tail: very similar to removing the smallest cycles.

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What kind of S-N curve slopes are we designing for ?

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Do we need double slopes ? Can we use thresholds ?

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Are the components independent ?