Cyclic stress factor f for ASME displacement checks
This article contains FAQ related to cyclic stress factor f.
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FAQ
How does CSiPlant determine the cyclic stress factor for distilled load cases of ASME displacement design checks?
Answer: The cyclic stress factor f can be either user-specified or program determined. This is controlled by the choice made on the "Design Preferences" tab for a given design request as shown in Figures 1 and 2.
Figure 1: User-specified cyclic stress factor
Figure 2: Program determined cyclic stress factor
When the cyclic stress factor is user-specified, its value is applied to all the displacement load cases of the design request, and all stations.
When the cyclic stress factor is program-determined, it is calculated in accordance with the specified design code. ASME B31.1 calculates the cyclic stress factor from the following two code equations (ASME B31.1 2018, Section 10.3.2, equations 1c and 2; page 18), the overall approach is very similar for the other ASME codes:
f=6/N^(0.2) ≤ 1
N=NE+Σ((qi^5)(Ni)) for i = 1, 2, ..., n
The calculation of N (the total number of equivalent reference displacement stress range cycles) is based on "defined" displacement loads case for the design request. Distilled load cased are not considered in the calculation of N, because the number of cycles for distilled displacement load cases is expected to be smaller than the number of cycles of defined load cases and thus not govern the calculation of N. If this assumption does not apply to a particular design request, then the user should use user-specified f factor. Please refer to the CSiPlant ASME Design Manual for explanation of "defined" and "distilled" load cases for displacement design checks.
The number of cycles for each defined displacement load case is provided on the Design tab of the load case definition as shown in Figure 3 below.
Figure 3: Specification of number of cycles for load case