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*TD Options |
 
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*TD Options
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*TD Options |
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To specify a number of miscellaneous parameters, many of which relate to the calculation of stress spectra.
Refer to Fatigue Analysis (Mode 1) for further information on this feature.
One line with three values, one of which is optional:
Start Time, ENSEMBLES=No. of Ensembles [, SCALE=Scale Factor]
No. of Ensembles must be greater than 1. Scale Factor defaults to 1.
Input: |
Description |
Scale Factor for Stress: |
This is used to transform stresses in the Flexcom units to units consistent with the S-N curve data. For Imperial units, a value of 6.9444E-06 would be typical to transform lb/ft2 to ksi. For SI units, a value of 1.E-06 would be typical to transform N/m2 to MPa. In some cases LifeTime is in a position to decide the appropriate value to use, in which case it is not necessary to explicitly specify a scale factor. See Note (a). |
Start Time: |
This input is used to exclude initial transients from the fatigue calculations. Any data from solution times prior to this start time is ignored in the LifeTime calculations. |
Number of Ensembles: |
The number of ensembles to be used in calculating stress spectra. This number must be greater than 1, and in fact defaults to 4. See Note (b). |
(a)Flexcom outputs the value you specify for g, the gravitational constant, to every analysis database. If any of the Flexcom analyses you list in your Seastate File – Specify data generated a database file, then LifeTime retrieves this value and uses it to determine if you used Imperial or SI units in your data. Specifically, if 9 <= g <= 10, then LifeTime decides you employed SI units; if 32 <= g <= 33, then LifeTime decides you used Imperial units. In either case, LifeTime automatically determines the scale factor required to transform stresses to MPa or ksi as appropriate, so you do not need to explicitly specify a Scale Factor for Stress - just let LifeTime determine the appropriate scale. But if none of your Flexcom analyses generated a database, you should explicitly specify a scale factor – otherwise no factor will be applied (or in other words, a factor of 1.0 will be applied) – and this may not be consistent with your S-N curve specification.
(b)The procedure used by Flexcom in calculating spectra is as follows. Firstly, the output timetrace is divided equally into a number of smaller timetraces or ensembles. A spectrum for each ensemble is then calculated using the Fast Fourier Transform (FFT) algorithm. Finally, the actual spectrum to be output is found as an average of the spectra calculated for each ensemble. This standard procedure minimises the statistical error associated with the FFT process. You specify the number of ensembles to be used in this process using the Number of Ensembles entry above. This value should always be greater than 1.