Fatigue damage estimation

This calculation generates fatigue damage estimates from a stress history or a previously generated rainflow cycle count, by taking account of the fatigue properties of the material.

A suitable stress history can be generated from the relevant loads using the Channel combination calculation.

Click the Select... button to select channels representing the stress signal to be processed, or a previously generated rainflow cycle count (see 8.10) (not available for Multiple Channels). If you have selected multiple channels you will be able to specify a number of load cases and variables to be processed in a single calculation, and the results for each load case will then be stored as additional outputs of that load case, and/or accumulated over the turbine lifetime.

Then enter the material properties as follows:

Fatigue Model

• S-N curve: Select either the log-log or the lookup table option. The fatigue properties are then entered as follows: If Log-log relationship is selected: enter the inverse slope (m) and intercept (c) of the S-N curve, such that the stress range (S) giving N cycles to failure is given by:

\[ \begin{equation} \log(S) = \log(c) - \frac{1}{m} \log(N). \end{equation} \]

• If Lookup table is selected, click the define fatigue data lookup table button for a pop-up window. Use Add or Insert to add points to the lookup table, and enter the stress values and the corresponding number of cycles to failure by double-clicking on the appropriate table entries. The stress entries must be monotonically decreasing, with the corresponding cycles to failure monotonically increasing.

• Goodman correction: Use the check box to enable the Goodman correction if required. If required, enter the ultimate strength of the material, as a stress.

If a stress history rather than a previously generated rainflow cycle count is to be processed, enter the following data which is required for the cycle counting:

• Minimum value: the start of the stress distribution (should be less than the minimum stress).

• Maximum value: the end of the stress distribution (should be greater than the maximum stress).

• Number of bins: suggest 20. A higher number (maximum 128) will give finer resolution, provided there is sufficient data.

• Minimum range: the smallest signal range to be counted as a cycle. A non-zero value may be useful to remove the effect of any spurious noise on the signal.

Last updated 10-09-2024