Fatigue Analysis in Caesar II

Fatigue Analysis Analysis in Caesar II this topic to explain the fatigue analysis ( Click here to read the basic article on Fatigue Analy An alysi siss) methodology using caesar II with an example problem of a typical piping system. To perform fatigue analysis we need to calculate the thermal and pressure fluctuations the  piping system will undergo in its design life. We have to calculate the worst possible cycles from preliminary data provided by process/operation department. Lets assume we received the following data from process for a typical piping system. perating cycle from ambient (!"#$) to !%&#$ (!""'""" cycles anticipated) hutdown external temperature variation from ambient (!"#$) to %"#$ (*""'""" cycles anticipated) • •

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+ressuri,ation to &.& -ars (!""'""" cycles anticipated) +ressure fluctuations of plus/minus .& -ars from the &.& -ars ('"""'""" cycles anticipated)

 ow' in order to do a proper fatigue analysis' analysis' these should be grouped in sets of load pairs which represent the worstcase combination of stress ranges between extreme states which we can do in the following way (0efer 1ttached 2igure' 2ig. for proper understanding)3

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Fig. 1: Explanation of worst case cycle combination for fatigue analysis 2rom %"#$' " -ars to !%&#$' 4 -ars. *""'""" $ycles $ycles 2rom !"#$' " -ars to !%&#$' 4 -ars.3 ""'""" $ycles $ycles

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2rom !%&#$' ! -ars to !%&#$' 4 -ars3 5""'""" $ycles

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2rom !%&#$' ! -ars to !%&#$' &.& -ars3 !""'""" $ycles

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o in $aesar II we can define the above data as follows (0efer 2ig. %)3 1! "#$%C& #! '#(%C )1! $.$ *ar& )#! )#! " *ar and )+! , *ar

Fig.#: Caesar II spreadsheet explaining the input re-uirement

 ow go to the load case editor and define load cases as shown in 2ig.* for fatigue analysis. $lic6 on load cycles button to input the number of cycles calculated above.

Fig.+: oad cases for Fatigue Analysis

7on8t forget that all load cases with stress type 21T (for fatigue) must have their expected number of Load $ycles specified.

1fter load cases are prepared run the analysis and find out the results from output processor. +art of the output results are provided in the below attached figures for your reference (2ig. ! and 2ig. &) The fatigue stress range (9aximum tress Intensity as calculated in :xpansion stress case) may be chec6ed against the fatigue curve allowable for each fatigue load case as shown in 2ig !.

Fig ": /utput 0creen showing stress range

;owever' this is not a true evaluation of the situation' because it is not a case of ected to all of these load cases throughout its expected design life' not >ust one of them. Therefore' we must review the $umulative ?sage report' which shows the total effect of all fatigue load cases (or any combination selected by the user) on the design life of the system. 0efer 2ig & for example.

Fig. $: /utput 0creen showing Cumulatie usage factor

This report lists for each load case the expected number of cycles' the allowable number of cycles (based upon the calculated stress)' and the ?sage 0atio (actual cycles divided by allowable cycles). The ?sage 0atios are then summed for all selected load cases@ if this sum exceeds ."' the system has exceeded its fatigue capabilities.