Normally the fatigue analysis is performed for existing plants to evaluate actual cause for any failure. For new plants the analysis can be performed only if the project specification permits to do so. Refer project guidelines on the application requirement for fatigue analysis. Before starting the analysis be ready with following data which will be required during analysis:
- Fatigue Curve of the piping material
- Enough process data for finding the total number of cycles throughout the design life of the piping system.
Steps for Fatigue Analysis using Caesar II:
- Assigning the fatigue curve data to the Piping Material in use: This is done on the Allowable auxiliary screen. Fatigue data may be entered directly, or can be read from a text file by clicking the Fatigue Curves Button. Seven commonly used curves are available in \Caesar\System\*.Fat. (For Caesar version 2012, 2013 &2014 you may not find it in few computers, But these are available in earlier versions) Fatigue curves provide series of S-N data which define the allowable stress with given anticipated cycle and vise versa.
- Defining the fatigue load cases: For this purposes, a new stress type, FAT, has been already defined in Caesar II database. For every fatigue case, the number of cycles anticipated must also be entered in appropriate space.
- Calculation of the fatigue stresses: Caesar II automatically does this calculation for us. The fatigue stresses, unless explicitly defined by the applicable code are same as Caesar II calculated stress intensity (Max Stress Intensity), in order to conform to the requirement of ASME section VIII, Division 2 Appendix 5.
- Determination of the Fatigue stress allowable: The allowable stresses for fatigue analysis are required to be interpolated logarithmically from the fatigue curve based upon the number of cycles (throughout its life) designated in the fatigue load cases. The calculated stress is assumed to be a peak-to-peak cycle value (i.e., thermal expansion, settlement, pressure, etc) for static load cases, so the allowable stress can be extracted directly from fatigue curve. On the other hand for harmonic and dynamic load cases, the calculated stress is assumed to be a zero–to-peak cycle value (i.e., vibration, earthquake, etc), so the extracted allowable need to be divided by 2 prior to use in the comparison.
- Determination of the allowable number of cycles: The flip side of calculating the allowable fatigue stress for the designated number of cycles is the calculation of the allowable number of cycles for the calculated stress level. This is done be logarithmically interpolating the “Cycles” axis of the fatigue curve based upon the calculated stress value. Since static stresses are assumed to be peak-to-peak cycle values, the allowable number of cycles is interpolated directly from the fatigue curve. Since harmonic and dynamic stresses are assumed to be zero-to-peak cyclic values, the allowable number of cycles is interpolated using twice the calculated stress value.
- Reporting the analysis results: Caesar II provides two reports for viewing the results of load cases of stress type FAT; standard stress report and cumulative usage report. The first of these is the standard stress report for displaying the calculated fatigue stress and the fatigue allowable at each node. Stress reports could be generated individually for each load case and show whether any of the individual load cases in isolation would fail the system or not.
However, in situations where there is more than one cyclic load case potentially contributing to fatigue failure, the cumulative usage report is more appropriate. In order to generate this report, the user should select all of the FAT load cases which contributes to the overall system degradation (possible failure). The cumulative usage report lists for each node point the usage ratio (actual cycles divided by allowable cycles), and then sums (combines) these up for total cumulative Usage. A total value greater than 1.0 indicates a potential fatigue failure.