Back on Fatigue Analysis subject

Just a few general queries about how Caesar approaches Fatigue Calcs.

1. Within the documentation it says that the Stress intensities are calculated and used in accordance with ASME VIII Div 2. However ASME VIII Div 2 utilises Stress Ranges not Stress Intensities. It used to use Stress Intensities but since the 2007 edition it uses Stress Ranges. does this mean Caesar is not complying with the designated code within the documentation?

2. Does Caesar actually calculate the stress intensity within a piping system correctly particularly at bends? At bends the Hoop pressure stress should be added to the calculated bending stress as the bending stress is actually in the circumferential direction. Does Caesar consider this?

Regards

DSB123

Any comments?

Additionally - How valid are the approaches in Caesar in light of the ASME VIII Div 2 re-write of the fatigue methodology and the new fatigue curve equations provided?

Anyone from Intergraph willing to respond on what the software does or doesn't do?

I am unsure of your terms here.
Stress intensity is one of many ways to look at stress. Here, stress intensity is the diameter of Mohr's Circle, which is twice maximum shear stress. This value can can be compared with the yield allowable limit in satisfying the Tresca theory of failure.
Stress range is not a stress equation. Stress range is a change in stress as opposed to a state of stress. This range is used for the simplified fatigue evaluation found in most piping codes. Stress state is normally used to evaluate collapse loads (sustained & occasional).
I don't know what C2 text you are referencing regarding ASME Section VIII Division 2 fatigue evaluation. The fatigue curves have been updated in VIII-2 and these have not been changed yet in C2. But you can enter your own.
C2, as a piping program, calculates stress in accordance with the piping codes. The stress we calculate is a nominal stress - using a calculated bending moment along the pipe centerline and a stress intensification factor(SIF). This code-defined SIF accounts for the shell distortion (ovalization) caused by elbow bending. The Code does not accommodate the change in hoop stress associated with this distortion. The Code is a simplified method of assuring piping safety. You can employ more rigorous evaluation if you wish (i.e. FEA), but not with C2 alone.

Hi Dave,
Thanks for the reply but I understand the differences between stress intensity and stress range. I have asked two specific questions which are clear and concise. could you please answer the specific questions on whether Caesar is performing the calculations correctly or not. I beleive Caesar is not complying with the approaches of ASME VIII Div 2 for fatigue assessment and not adding the calculated bending stress to the hoop stress before calculating the stress intensity even if the ASME VIII Div 2 still used stress intensities which it does not.

Please give direct answers to my two original questions to avoid ambiguity.

Regards

Hi Dave,
You say "I don't know what CAESAR II text you are referencing regarding ASME Section VIII Division 2 fatigue evaluation."

The answer is that in the CII Technical Reference Manual in chapter 6- Fatigue Analysis of Piping Systems - Performing Fatigue Analysis it says

"3.Calculation of the fatigue stresses: This is done automatically by CAESAR II – the fatigue stresses, unless explicitly
defined by the applicable code are calculated the same as CAESAR II calculates stress intensity, in order to conform to the
requirements of ASME Section VIII, Division 2 Appendix 5"

Which implies that the requirements of ASME VIII Div 2 are followed. However as stated above I question whether this is the case especially for the later editions of ASME VIII Div 2.

Comments please.

C2 does not use a specific piping code stress equation for VIII-2 fatigue evaluation, instead, a general 3D stress intensity is calculated at 4 points through the pipe cross section - the outside surfaces opposite each other where bending moment is maximum and the inside surfaces at these positions. Since C2 provides no inclusion of a stress concentration factor, this stress is not of the peak stress sort for rigorous fatigue evaluation. This is not the stress calculation found in VIII-2.
Nor does C2 offer a local hoop stress calculation to incorporate hoop stress caused by elbow bending.
C2 evaluates stress in accordance with the requirements of the pre-2007 edition of VIII-2. C2 does not calculate a stress that can be directly compared to those fatigue curves. Answer to question 1, then, is yes and no.
On question 2 - no.
I will see that the sentence you quote be changed.

Many thanks Dave for the direct answers - its just as I thought but did want Intergraph confirmation.