FCP Report No. 178

The Competing Roles of Microstructure and Flaw Size on the Fatigue Limit of Metals

by

Timothy Edward McGreevy

Abstract

Slip band and crack formation, propagation and arrest are the active mechanisms in the fatigue process as was first observed in 1903. However, engineers relied on macroscopic properties such as hardness and tensile strength to predict fatigue limits since analytical tools to model the process did not exist. Many empirical modifications to the fatigue limit have since been made to account for variables such as surface roughness, state of stress, inclusion content, environmental effects, etc. A method is proposed to qualitatively and quantitatively predict the effects of several of these parameters on the fatigue limit of metals, specifically steels.

Research includes the development and verification of an analytical model that addresses the fatigue process, namely the threshold condition of non-propagating cracks. Two parameters are identified to govern the fatigue resistance: non-propagating crack size and crack barrier strength. The concept of three defect types associated with three different flaw dominated fatigue regimes is introduced. Furthermore, application of the model to fatigue mechanisms in high strength steels, synergistic effects of surface finish and intergranular cracks, competition between surface and subsurface fatigue nucleation, and unexplained observations and scatter in fatigue behavior is demonstrated. Overall, the model is proven as a simple and robust tool for qualifying and statistically quantifying material behavior. In addition, the model can be implemented in material screening, selection, and processing as well as a guide for future material research and design.

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