Traditional design fatigue analyses of pressure vessels and piping equipment have typically used linear-elastic stress analyses, where the stresses caused by various loads, such as thermal, pressure, bending moments, etc. are combined using the principle of linear superposition. Based on high stress locations, geometric and material discontinuities, and other engineering judgements, stress classification lines (SCLs) were defined for where fatigue usage factors would be calculated. It was then necessary to apply simplified elastic-plastic penalty factors, based on the through-wall linearized stresses, to the peak stress amplitudes, in order to account for the nonlinear behavior of materials. Nonlinear finite element analysis that directly calculates strains were not typically used, because of computing and material modeling limitations. However, such analyses, even for complex three-dimensional structures, have become much more practical today with advancements in computing speed and storage capacity. ASME Section III Subarticle NB-3200 includes a provision for performing nonlinear (or “plastic”) analysis (NB-3228.4(c)), but little to no guidance is provided for how to perform the analysis itself. In addition, the procedure for computing the strain range, as currently written in the Code, has been identified as being limited to a uniaxial stress condition and is fundamentally inconsistent with the traditional elastic methodology. This paper provides a proposal for an improved approach for computing fatigue usage and strain rates using nonlinear plastic analysis. Additional guidance for performing these analyses is provided, as they are expected to be used more frequently into the future.

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