Investigation of Variability in Fatigue Crack Nucleation and Propagation in Alpha+Beta Ti-6Al-4V

摘要

The understanding of fatigue variability in turbine engine materials is vital to permit a reduction of US Air Force sustainment costs through fatigue life extension and/or inspection interval extension. Additionally, the US Air Force is currently moving further towards the use of probabilistic damage tolerance design methods. These probabilistic models require not only a good understanding of the variability in specimen data, but also an understanding of the microstructural sources of variability to allow scaling to component analysis. The objective of this work was to study the fatigue variability of a common turbine engine alloy Ti-6Al-4V. Typical testing consisted of smooth bar fatigue tests at multiple stress ratios and stress levels in order to generate a fully populated stress-life curve. These tests, however, typically do not consist of many repeats. The approach of this work was to conduct a statistically significant number of repeated fatigue tests at several loading conditions. A similar approach has been performed on several other turbine engine material systems often revealing bimodal life distributions consisting of a number of low life specimens that may fail due to a separate mechanism. This paper discusses the Ti-6Al-4V life distributions and sources of variability. Crack propagation using small crack growth data was used to predict the lower tail of the life distributions.