This paper points out some relevant aspects of the simplified elasto-plastic fatigue analysis as addressed in the ASME Code Section III Subsection NB and its application to two structural components that are subjected to a slow or to a fast thermal transient. The structural components considered are a thick-walled pipe and a nozzle-to-vessel junction. For the case of the thick-walled pipe, a closed form analytical solution proposed by Albrecht for pipes subjected transient temperature loading was implemented and its results were compared to coupled thermal and mechanical finite element analyses using a commercial finite element software. The application of the analytical solution allows for an optimization of the time consumed to obtain the stresses that occur across the thickness of the pipe as a function of time, i.e. the membrane plus bending plus peak stress range, Sp. The analytical solution equally allows for the linearization of the stress components actuating along the pipe thickness for all time steps considered within the thermal stress solution. This yields the membrane plus bending stress range, Sn, and allows for a design code conforming plasticity correction by means of Ke factors. In the considered case of the nozzle-to-vessel junction, a finite element solution was used. It was one aim of the study to point out, that under fast transients loading situations the relevant stresses Sp and Sn do not necessarily coincide with each other.
In the ASME Code the alternating stress Sa is a function of the factor Ke and of the range of Sp, with Ke being a function of the range of Sn and of the material properties. Consequently, a non-conservative fatigue analysis may result in the case of performing cycle counting only based on the time history of the critical Sp values and simply assigning the corresponding Sn and Ke values. This paper exemplifies one of those cases and proposes a method to overcome this problem.