Question:

Explain Fatigue Analysis performed in AutoPIPE as per ASME Section III, Subsection NB

Answer: 

This article explains the fatigue analysis procedure in AutoPIPE in accordance with ASME Section III, Subsection NB. It describes load set generation, fatigue pairing logic, and cumulative usage factor calculation as performed by AutoPIPE.

• Fatigue Evaluation Tables:
  1. Each stress combination in the summary is assigned a Load Set number.
  2. Some thermal cases appear multiple times because they are part of different transients (different equation indices).
  3. Each load set is assigned a number of cycles based on the transient definition.

• Load Set Pairing Logic:
  AutoPIPE automatically generates a Load Set Pairing Table using the following logic:
  1. Pair each load set with all other load sets.
  2. Identify the pair producing the maximum alternating stress (SALT).
  3. Add the critical pair to the Usage Summary.
  4. Remove load sets whose cycles are exhausted.
  5. Carry forward remaining cycles to the next iteration.
  6. If multiple load sets belong to the same transient and two are exhausted, all remaining sets from that transient are removed.

• Iterative Fatigue Evaluation:
  1. Revised fatigue tables are created after each iteration.
  2. The worst load set pair (highest SALT) is selected.
  3. Usage factor for that pair is added to the Cumulative Usage Factor.
  4. Cycles are deducted, and exhausted load sets are removed.
  5. The process repeats until no valid load pairs remain.

• Usage Factor Summary:
  
  The Usage Factor Summary Table lists:
  1. All critical load set pairs
  2. Alternating stress values
  3. Number of cycles used
  4. Individual and cumulative usage factors

This table represents the final fatigue damage calculation. 

What is the exact stopping criterion for the ASME NB fatigue iteration process?

The fatigue iteration process in AutoPIPE terminates when the calculated stress or cycle values exceed the valid range of the selected fatigue curve.

For Example, in AutoPIPE model the fatigue curve used is “ASME III – ADLPIPE (Default)”. Since this internally generated curve is limited to 1E6 cycles for carbon steel, the corresponding alternating stress limit is 12,250 psi.

Accordingly, the fatigue iteration stops once the calculated alternating stress falls below 12,250 psi, which indicates that the allowable number of cycles exceeds 1,000,000, thereby going beyond the scope of the curve. In this case, only the first few load set pairs are included in the fatigue usage calculation, while the remaining load set pairs are ignored because their alternating stress values are below the curve limit.

Similarly, for ASME III I‑9.1 and ASME III I‑9.2, the fatigue curves are valid only within the cycle range of 10 to 10¹¹. If the calculated values fall outside this range, AutoPIPE will automatically terminate the fatigue iterations.

What is the workaround to avoid automatic termination of fatigue iterations?

Workaround 1: Since the “ASME III – ADLPIPE (Default)” fatigue curve is limited to 10⁶ cycles, user may consider using alternative ASME III fatigue curves that support a wider cycle range.

Workaround 2: User may create user‑defined fatigue curves (*.FAT) using a text editor with an extended cycle range, provided they follow the required fatigue curve format. These files can be saved either in the model directory or in the AutoPIPE program directory. AutoPIPE assumes log‑log interpolation between the defined data points.