RMS Fatigue Curves for Random Vibrations

Abstract

Fatigue usage factors for deterministic or constant amplitude vibration stresses may be calculated with well-known procedures and fatigue curves given in the ASME Boiler and Pressure Vessel Code. However, some phenomena produce nondeterministic cyclic stresses which can only be described and analyzed with statistical concepts and methods. Such stresses may be caused by turbulent fluid flow over a structure. Previous methods for solving this statistical fatigue problem are often difficut to use and may yield inaccurate results. Two such methods examined herein are Crandall’s method and the “3σ” method. The objective of this paper is to provide a method for creating “RMS” fatigue curves” which accurately incorporate the requisite statistical information. These curves are given in Appendix A and may be used by analysts with the same ease and in the same manner as the ASME fatigue curves.