In either case, the calculation requires a power spectral density function (PSD) for response, obtained as follows.įor a frequency domain analysis, the PSD is obtained from a frequency domain simulation. Spectral analysisįor either of the spectral analysis methods, damage is calculated in the frequency domain by statistical methods. If one of the mean stress effects methods is used, then the equivalent stress range is used for each $S_i$. The S-N curve defines the number of cycles to failure, $N(S)$, for stress range $S$, and also defines an endurance limit, $F_\mathrm$ arises because the rainflow algorithm counts half-cycles rather than full-cycles. Finally, these total exposure damage values are summed over all load cases, to obtain the overall total damage value at that specific fatigue point.The load case damage values are scaled up according to the exposure associated with that load case, to give a total exposure damage value for that load case and fatigue pont.This value represents the damage at that fatigue point due to one occurrence of that load case. The value of load case damage corresponding to this response is calculated – see below for details.For mooring fatigue the response variable is effective tension. For stress factor fatigue then the stress is calculated based on the stress factors. For homogeneous pipes the ZZ stress variable is used.
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