how to carryout pressure vesses fatigue FEA 1

[brightinstinct]

Hi Everyone
I need to carryout a FEA using ANSYS on a designed PV to know the number of cycles that the vessel body and Locking ring can be subjected to under Fatigue.I am looking for some references for similar kind of analysis done.

VESSEL SPECIFIACTION
Design Pressure: 11,000 psi
Max Working Pressure: 10,000 psi
Volume: 7.5 Litres
Temperature: -20 to 150 Degrees Celsius
Internal dims (approx): 126mm bore x 600mm long

DESIGN CODE
PD5500 2003 Cat 1, Asme VIII Div 2 (For Fatigue)

I have never done any FEA for fatigue, so i needed some clue very urgently to start the work.Also please let me know where i can find code PD500.
could you please shed some light on this.
Regards
sanjay

 

[cbrn]

Hi,
here is the approach when we perform PV analysis for fatigue, here in my company (other methods may exist, probably faster than this):
1- Decompose the fatigue cycle in a series of partial constant-amplitude cycle-types (i.e. get the cycle spectrum or the cycle histogram). Determine all the loadcases from-to each cycle-type ranges
2- Run in Ansys all the needed loadcases (if you apply them as loadsteps in the same analysis, you have the advantage of having the opportunity to do loadsteps calculations/combinations directly in Ansys, if you need to; however, this is not strictly necessary)
3- Save the Principal Stresses results for all nodes of interest, for every loadcase (or loadstep, depending how you have managed them in Ansys)
4- Import these results in Excel (if you have more than 64000 nodes, split the results files)
5- For each loadcase, calculate the Stress Differences (ASME VIII-2 appendix 5, A02 5.110.3) at every node
6- For each cycle type (note that a cycle is defined by two loadcases), calculate the Stress Difference variation, at every node
7- For each cycle type, find the Maximum of the variation of the Stress Differences; the Alternating Stress Intensity (Salt) for the cycle is one half of this value. No calculation of the Average Stress is needed, since the ASME design fatigue curves (dfc) are already compensated for that.
8- For each cycle type, compare the Salt with the appropriate ASME dfc and get the number of allowed cycles Ni.
9- Apply Cumulative Damage Miner's rule and Gassner approach to calculate the fatigue curve corresponding to your assigned cumulative cycle.

Note that, if you want to VERIFY the fatigue resistance of your structure under your cumulative cycle, i.e. you know the design number of repetitions of the cumulative cycle, step 9 is different and easyer: you simply calculate, for each cycle-type, the Usage Factor Ui=ni/Ni, then you calculate the Cumulative Usage Factor U=SUM(Ui) and you verify that U<1.

Regards

 

[JacobL]

As a side note, I hope you are using a coupled solution for the fatigue, since thermal stresses are coupled with stress from the pressure.
Depending on the materials in question, the thermal factor might lead to lower life time.