IGE/TD/12 does present specific requirements for true fatigue evaluation of systems subject to a cyclic loading threshold. Furthermore, ASME Section III, Subsection NB and ASME Section VIII Division 2 provide guidelines by which fatigue evaluation rules can be applied to piping and other pressure retaining equipment. These procedures have been adapted, where possible, to the methodology used by CAESAR II.
Perform fatigue analysis
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From the Allowable auxiliary dialog box, enter fatigue data or import it in from a text file. You can also define your own fatigue curves as discussed later in this section. By doing this, you assign the fatigue curve data to the piping material.
To help with your fatigue analysis, CAESAR II provides a number of commonly used curves.
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From either the Static or Dynamic Load Case Builders you must define, for every fatigue load case, the number of anticipated cycles.
A FAT stress type is also available
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Unless explicitly defined in the applicable code, CAESAR II calculates the fatigue stress the same way it calculates the stress intensity.
IGE/TD/12 is the only piping code supported by CAESAR II that has explicit instructions for calculating fatigue stresses. For more information
on IGE/TD/12, refer to IGE/TD/12.
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Allowable fatigue stresses are interpolated logarithmically from the fatigue curve based upon the number of cycles designated for the load case. For static load cases, the calculated stress is assumed to be a peak-to-peak cyclic value (for example, thermal expansion, settlement, pressure, and so forth), so the allowable stress is extracted directly from the fatigue curve. For harmonic and dynamic load cases, the calculated stress is assumed to be a zero-to-peak cyclic value (for example, vibration, earthquake, and so forth), so the extracted allowable is divided by two prior to use in the comparison.
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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. You can do this by logarithmically interpolating the "Cycles" axis of the fatigue curve based upon the calculated stress value. Because static stresses are assumed to be peak-to-peak cyclic values, the allowable number of cycles is interpolated directly from the fatigue curve. Because 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.
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CAESAR II provides two reports for viewing the results of load cases for the FAT stress type. The first of these is the standard Stress report that shows the calculated fatigue stress and fatigue allowable at each node.
You can generate individual stress reports for each load case to show whether any
of the individual load cases in isolation fail the system.
However, in those instances where there is more than one cyclic load case potentially contributing to a fatigue failure, the Cumulative Usage report is appropriate. To generate this report, select all the FAT load cases that contribute to the overall system degradation. The Cumulative Usage report lists for each node point the usage ratio actual cycle divided by allowable cycles, and then sums these to obtain the total cumulative usage. A total greater than 1.0 indicates a potential fatigue failure.