Engineering guide
This module performs the fatigue analysis per ASME BPVC Section VIII Division 2, Appendix 5, Paragraph 5-110 in the 2013 Edition — the classical fatigue-curve-based method of the "old" Division 2. This part of the code has been withdrawn: with the restructuring of Division 2 in 2007, the fatigue evaluation was transferred to Part 5 (today 5.5). For re-analyses, remaining-life assessments and modifications of existing equipment designed to the old Division 2, however, the method remains relevant.
From the stress intensity range (Max Sij), the module determines the alternating stress intensity Salt, corrects it by the ratio of the curve modulus of elasticity to the actual modulus, and reads the maximum allowable number of load cycles from the applicable design fatigue curve of the respective material group. Ultimate tensile strength and yield strength govern the curve selection; queries on the weld seam distance (less than 3 times the wall thickness) and on the condition (PL + Pb + Q) < 188 MPa control the additional corrections required for the mean stress treatment.
You need to calculate a fatigue analysis to ASME VIII-2 whenever equipment is loaded cyclically — start-up and shutdown, pressure and temperature cycles, batch operation — and the screening criteria for waiving the fatigue evaluation are not met.
Scope
- Fatigue analysis
Typical workflow
Determine the stress intensity range
From the stress analysis of the governing load cycles (superposition of load cases), the stress intensity range is determined for each cycle as the maximum of the principal stress differences Sij between the extreme states.
Form the alternating stress intensity
The alternating stress intensity S_alt is half the range. Stress concentrations at notches and welds must already be captured in the stress analysis via stress concentration factors; the query on weld seam distance less than three times the wall thickness controls the treatment of adjacent welds.
Apply the modulus-of-elasticity correction
Since the design fatigue curves are valid for a fixed reference modulus of elasticity, the entry stress is corrected by the ratio E_curve/E_actual: S_entry = S_alt · E/E_actual. Only this value may be entered into the curve.
Select the fatigue curve
Based on the material group, ultimate tensile strength and yield strength, the applicable design fatigue curve is selected; for high-strength materials, if the condition (PL + Pb + Q) < 188 MPa is satisfied, the more favorable curve without full mean stress correction may be used where applicable.
Read the allowable number of cycles and evaluate the damage
From the curve follows the maximum allowable number of load cycles N for the corrected stress amplitude. With several cycle types, the partial damage n/N per type is summed per the linear damage accumulation rule (Miner's rule); the sum must remain less than or equal to 1.
Frequently asked questions
Why has this method been withdrawn, and what applies today?
With the 2007 Edition, ASME VIII-2 was completely restructured; the fatigue evaluation of the old Appendix 5 was absorbed into Part 5 (today Section 5.5), supplemented by modern methods such as the structural stress method for welds. For new designs, the current Division 2 Part 5 must be used. The old Appendix 5 method remains governing for the re-analysis of existing equipment built to the previous edition.
Why must the stress be corrected by the modulus-of-elasticity ratio?
The design fatigue curves were derived from strain-controlled tests and plotted as pseudo-stresses (strain times reference modulus of elasticity). If you calculate with the component's actual, temperature-dependent modulus, the amplitude must be scaled by E_curve/E_actual, otherwise the allowable number of cycles is overestimated at high temperatures.
Do the fatigue curves already contain safety margins?
Yes. The ASME design curves lie well below the mean curves of the tests: factors of 2 on the stress amplitude or 20 on the number of cycles (whichever is more conservative) are built in, covering scatter, size effect and surface/environment influences. Additional blanket safety factors are therefore not to be applied.
What is the role of the query (PL + Pb + Q) < 188 MPa?
This limit decides how the mean stress is treated for certain material curves: if the primary-plus-secondary stress range stays below the limit, no cyclic plastification with mean stress relaxation occurs, and the curve variant including mean stress influence (e.g. for high-strength bolt/steel curves) must be used. Answering this query incorrectly leads to non-conservative fatigue lives.
Technical content source: LV-Solved-Problems-Content 628d313247e7