Dear Mr. Ay,

Thank you for your response.

However, could you please be more specific, and clarify how is established that "smallest allowed cycles", probably on the highest loaded/stressed node/element?

For one cyclic load and one stress range accounted, the matter is simple indeed, and no explanations are needed.

But, for two or more cyclic loads accounted, that "smallest allowed cycles" number is not so clear anymore, at least from my point of view.

For Mr. Waheed case, there are 3 cyclic stress ranges accounted: S1,S2,S3. If, from the applicable fatigue curve, the corresponding maximum Allowed No. of Cycles are Na1, Na2 and Na3 respectively, and the effective applied No. of ccycles are N1, N2 and N3 respectively, then the "Usage Ratio" = Accumulated Damage Ratio is:

D = N1/Na1 + N2/Na2 + N3/Na3 (that should be < 1.00).

Such assessment is performed for each Node/Element, and the highest "D" value is stated, corresponding to the highest "fatigue loaded/damaged" section of the piping system.

However, is still unclear how that "smallest allowed cycles" number is established in such case.

In general, N1, N2, N3 applied cycles are different, the S1, S2, S3 stresses vary from one node/element to another, which means that Na1, Na2 and Na3 also vary from one node to another, and, it may happen the highest "stressed" Node/Element (e.g. where the MAX value from S1, S2, S3 is found) NOT TO COINCIDE with the Node/Element where the highest "D" usage ratio is found.
In such situation, which is the "smallest allowed cycles" concept significance, and what approach is employed to evaluate it?

In my opinion, an "Allowed No of Cycles" should be related to a Stress Range Value and to an applicable Fatigue Curve. In the situation described above, there are 3 Cyclic Stress Range values, with 3 different applied numbers of load-unload cycles.
Therefore, the "smallest allowed cycles" concept is not anymore so clear.

Thank you for your time,

Kind regards,