fatigue analysis in Pipe stress

Reply from another forum:

1) Fatigue in piping is high strain and low cycle .In other cases , fatigue is high cycle , involving no or little plastic action.

2) Example of high cycle are :- Ball bearings.Here in high cycle Fatigue , analysis is done to determine the endurance limit, which is stress level that can be applied infinite number of times without failure.

3)In low cycle fatigue , as in case of pipes , applied stress exceeds yield strength of material causing plastic instability.In piping stress anaysis, a alternating stress which is half of peak stress is calculated.
The idea is to ensure number of load cycles (N) , associated with specific alternating stress is less than the number allowed by S-N curve.

4) If you read the code , the formula for calculation of allowable displacement stress range :- THere is a factor "f" , which is called stress range factor , which takes into consideration , cyclic loading.( ie :- No of full displacement cycles during expected service life ).Cummulative usage factor is calculated ,which should be less than one.

5) Caesar , uses the above formula in calculating displacement stresses , so fatigue stresses are accounted for as required by code.

By Jatin Prabhu