Fatigue Analysis Methods |
 
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Fatigue Analysis Methods |
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Three methods are used when you run a LifeTime Mode 1 analysis to calculate fatigue damage and fatigue life, termed, Statistics, Spectrum and Rainflow, respectively.
•Statistics. Calculating the standard deviation (σ) and mean zero up-crossing period (Tz) of combined axial and bending stress directly from Flexcom time histories, and then calculating fatigue damage from these based on an assumption that stress peaks are distributed according to the Rayleigh probability density function (pdf).
•Spectrum. Calculating the spectrum of combined stress from the Flexcom time histories, and then evaluating the moments of this spectrum. These are then used to compute fatigue based on an assumption that stress peaks are distributed according to either the Rayleigh or Dirlik’s probability density function.
•Rainflow. Calculating damage directly from the Flexcom stress histories using the rainflow cycle counting technique (ASTM Standard E1049-85, 1985 (2005)).
Because combined stress varies throughout the cross-section, LifeTime actually applies the above three methods at 8 points, at 45° intervals, around the hot spot outer surface. So for each hot spot you nominate, a fatigue life estimate is actually calculated for 8 points around the circumference.
Note that of the three methods mentioned above, only the Rainflow method is relevant to VIV Induced Fatigue of Pipe-in-Pipe Systems as fatigue computations are based on regular/periodic time histories for this specialised case.
•*TD OPTIONS is used to specify a number of miscellaneous parameters, many of which relate to the calculation of stress spectra.