Case Study for Fatigue Analysis in Caesar II

I have taken up this topic to explain the fatigue analysis (Click here to read the basic article on Fatigue Analysis) 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 the process/operation department. Let’s assume we received the following data from the process for a typical piping system.  

  • Operating cycle from ambient (40°C) to 425°C (400,000 cycles anticipated)
  • Shutdown external temperature variation from ambient (40°C) to -20°C (300,000 cycles anticipated)
  • Pressurization to 5.5 Bars (400,000 cycles anticipated)
  • Pressure fluctuations of plus/minus 1.5 Bars from the 5.5 Bars (1,000,000 cycles anticipated)

Now, in order to do a proper fatigue 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 (Refer Attached Figure, Fig.1 for proper understanding):  

Estimation of Number of Cycles for Fatigue Analysis
Fig.1: Estimation of Number of Cycles for Fatigue Analysis

The above figure (Fig. 1) explains calculation of the worst case cycle combination for fatigue analysis

  • From -20°C, 0 Bars to 425°C, 7 Bars.  300,000 Cycles
  • From 40°C, 0 Bars to 425°C, 7 Bars.:  100,000 Cycles
  • From 425°C, 4 Bars to 425°C, 7 Bars: 600,000 Cycles
  • From 425°C, 4 Bars to 425°C, 5.5 Bars: 400,000 Cycles

So in Caesar II we can define the above data as follows (Refer Fig. 2): T1= 425°C; T2= -20°CP1= 5.5 Bar; P2= 4 Bar  and P3= 7 Bar

Fatigue Input in Caesar II for Analysis
Fig. 2: Fatigue Input in Caesar II for Analysis

Fig.2 above shows the Caesar II spreadsheet explaining the input requirement   Now go to the load case editor and define load cases as shown in Fig.3 for fatigue analysis. Click on the load cycles button to input the number of cycles calculated above.  

Creating Load Cases for Fatigue Analysis
Fig. 3: Creating Load Cases for Fatigue Analysis

Fig.3 above shows the fatigue analysis Load cases that have to be created for Fatigue Analysis   Don’t forget that all load cases with stress type FAT (for fatigue) must have their expected number of Load Cycles specified.   After the load cases are prepared, run the analysis to find out the results from the output processor. Part of the output results are provided in the below-attached figures for your reference (Fig. 4 and Fig. 5)   The fatigue stress range (Maximum Stress Intensity as calculated in Expansion stress case) may be checked against the fatigue curve allowable for each fatigue load case as shown in Fig 4.    

Output Screen showing stress range
Fig 4: Output Screen showing stress range

However, this is not a true evaluation of the situation, because it is not a case of “either-or.” The piping system is subjected to all of these load cases throughout its expected design life, not just one of them. Therefore, we must review the Cumulative Usage 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. Refer to Fig 5 for example.  

Output Screen showing Cumulative usage factor
Fig. 5: Output Screen showing Cumulative 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 Usage Ratio (actual cycles divided by allowable cycles). The Usage Ratios are then summed for all selected load cases; if this sum exceeds 1.0, the system has exceeded its fatigue capabilities.  

Print Friendly, PDF & Email

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

6 thoughts on “Case Study for Fatigue Analysis in Caesar II

  1. Dear sir 
    as per your example pressure variation you have taken From 425°C, 4 Bars to 425°C, 7 Bars: 600,000 Cycles
    i think you it should be 1000,000 and other pressure case need not to be take care.

  2. Could anyone please explain the following conditions. I am confused with cycles for each condition. 

    From -20°C, 0 Bars to 425°C, 7 Bars.  300,000 Cycles
    From 40°C, 0 Bars to 425°C, 7 Bars.:  100,000 Cycles
    From 425°C, 4 Bars to 425°C, 7 Bars: 600,000 Cycles
    From 425°C, 4 Bars to 425°C, 5.5 Bars: 400,000 Cycles

  3. kindly explain it.

    From -20°C, 0 Bars to 425°C, 7 Bars. 300,000 Cycles
    From 40°C, 0 Bars to 425°C, 7 Bars.: 100,000 Cycles
    From 425°C, 4 Bars to 425°C, 7 Bars: 600,000 Cycles
    From 425°C, 4 Bars to 425°C, 5.5 Bars: 400,000 Cycles

    how we are getting those cycle values???

Leave a Reply

Your email address will not be published. Required fields are marked *

Recent Content

Enable Notifications    Ok No thanks