Fatigue Analysis Methods: Their Performance Evaluation for a Class of Pressure Vessels With Welded Joints

This paper provides a basis for evaluating the performance of a fatigue analysis method for a selected class of welded pressure vessels. After the allowable number of cycles that the method permits for a member of the class is calculated, the question is addressed whether that number of cycles can be regarded as conservative or unconservative. To answer that question, a performance evaluation is proposed that requires a mean life estimate of a representative sample of the selected vessel class from tests, and the predicted mean life and the permissible number of cycles from the method. The performance is based on the ratio of two factors: (1) Achieved Factor — test mean life cycles divided by the permissible number of cycles, and (2) Assigned Factor — predicted mean life cycles divided by the permissible number of cycles. The ratio between the former and the latter defines a Performance Index, which indicates whether, and to what extent, the permissible number of cycles can be regarded conservative or unconservative. For an illustration of the performance evaluation, it is applied to three methods for a class of pressure-cycled carbon steel vessels with welded-on flat plates.