Static Analysis Fatigue Example - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II Version
13

Consider a sample job that potentially has several different cyclic load variations:

  • Operating cycle from ambient 70°F to 500°F, 12,000 cycles anticipated

  • Shut down external temperature variation from ambient 70°F to -20°F, 200 cycles anticipated

  • Pressurization to 1800 psig, 12,000 cycles anticipated

  • Pressure fluctuations of +/- 30 psi from the 1800 psig, 200,000 cycles anticipated

To do a proper fatigue analysis, you must group the load pairs that represent the worst-case combination of stress ranges between extreme states. These load variations can be laid out in graphical form. The figure below shows a sketch of the various operating ranges this system experiences. Each horizontal line represents an operating range. At the end of each horizontal line, the temperatures and pressures defining the range are noted. At the center of each horizontal line, the number of cycles for each range is defined.

Using this sketch of the operating ranges, the four fatigue load cases can be determined. Case 1: Cover the absolute extreme, from -20°F and 0 psi to 500°F and 1830 psi. This occurs 200 times. As a result of this case, the cycles for the ranges defined must be reduced by 200. The first range (-20, 0 to 70, 0) is reduced to zero, and has no contribution to additional load cases. The second range (70, 0 to 500, 1800) is reduced to 11,800 cycles. The third and fourth ranges are similarly reduced to 199,800 cycles.

These same steps can be used to arrive at cases 2 through 4, reducing the number of considered cycles at each step. This procedure is summarized in the table below.

Segment

-20, 0 to 70, 0

70, 0 to 500, 1800

500, 1700 to 500, 1800

500, 1800 to 500, 1830

Case

Initial

200

12,000

200,000

200,000

After 1

0

11,800

200,000

199,800

After 2

0

0

200,000

188,000

After 3

0

0

12,000

0

After 4

0

0

0

0

This table is then used to set the load cases as cycles between the following load values:

  • Between -20°F, 0 psig and 500°F, 1830 psig (200 cycles)

  • Between 70°F, 0 psig and 500°F, 1830 psig (11,800 cycles)

  • Between 500°F, 1770 psig and 500°F, 1830 psig (188,000 cycles)

  • Between 500°F, 1770 psig and 500°F, 1800 psig (12,000 cycles)

These temperatures and pressures are entered as operating conditions accordingly:

Next enter the fatigue curve data for the material. This is done by clicking Fatigue Curves to activate the Material Fatigue Curve dialog box. This dialog box can be used to enter the fatigue curve for the materials.

For IGE/ TD/12, you only need to enter five sets of fatigue curves for fatigue classes D, E, F, G, and W.

  1. Enter up to eight Cycle versus Stress data points to define the curve. Interpolations are made logarithmically.

  2. Enter Cycle/Stress pairs in ascending cycle order.

  3. Enter stress values as the allowable stress range, rather than the allowable Stress Amplitude.

SHARED Tip You can enter fatigue curve data from a text file, by clicking Read from file. This displays a list of all \CAESAR\SYSTEM\*.FAT files.

The following fatigue curve files are delivered with CAESAR II. You can also construct additional fatigue curve files. For more information on fatigue curve files, see Appendix A below:

  • 5-110-1A.FAT ASME Section VIII Division 2 Figure 5-110.1, UTS < 80 ksi

  • 5-110-1B.FAT ASME Section VIII Division 2 Figure 5-110.1, UTS = 115-130 ksi

  • 5-110-2A.FAT ASME Section VIII Division 2 Figure 5-110.2, Curve A

  • 5-110-2B.FAT ASME Section VIII Division 2 Figure 5-110.2, Curve B

  • 5-110-2C.FAT ASME Section VIII Division 2 Figure 5-110.2, Curve C

In this case for A106B low carbon steel operating at 500°F, 5-110-1A.FAT is the appropriate selection. This populates the fatigue curve data boxes in the dialog box:

Error check the job, and set up your load cases.

The static load case builder offers a new stress type, FAT (fatigue). Selecting this stress type does the following:

  1. Enables you to define the number of cycles for the load case. Dragging the FAT stress type into the load case or clicking the Load Cycles button opens the Load Cycles field.

  2. Calculates the stress range as per the Fatigue Stress method of the applicable code. This is the stress intensity for all codes except IGE/TD/12.

  3. Compares the calculated stress range to the full value extracted from the fatigue curve. Indicates that the load case may be included in the Cumulative Usage report.

The last four load cases represent the load set pairs defined earlier.

After you run the job the presence of a FAT stress type adds the Cumulative Usage report to the list of available reports.

You can check the fatigue stress range against the fatigue curve allowable for each load case by selecting it along with the Stresses report. A review of each load case confirms that all stress levels passed.

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 also review the Cumulative Usage Report report, which shows the total effect of all fatigue load cases, or any user-selected combination, on the design life of the system. 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. In this case, it is apparent that with the maximum cumulative usage ratio of 0.87 at node 115, this system is not predicted to fail due to fatigue: