In-situ investigation and analytical modeling of crack initiation and propagation in Ti-6Al-4V under low-cycle fatigue conditions on microscale

Abstract

Ti-Al alloys such as Ti-6Al-4V belong to a group of materials that show tremendous potential in aircraft engine construction due to their excellent mechanical properties and high corrosion resistance. However, not only the knowledge of the material properties are relevant, but also a comprehensive understanding of the damage mechanisms that occur. One of the most relevant types of damage mechanisms is caused by the cyclic loading, as this accounts for more than 70% of the economically relevant damages, not only in the construction and operation of gas turbines. In order to address this concern, the Institute of Test and Simulation for Gas Turbines at the German Aerospace Center is developing a test bench that is dedicated to perform in-situ research investigating these damage mechanisms. The focus in this project is on a micro-load frame that allows both microscopic examination within a scanning electron microscope and crack monitoring at macroscopic level. Automation and efficient data acquistion lead to the creation of a comprehensive research environment that allows holistic research into crack growth behaviour in Ti-6Al-4V. The objective of this thesis is the characterization of the crack growth behaviour under low-cyclic fatigue conditions on a phenomenological and mathematical-analytical level. For this purpose, the results of the previously carried out literature research are presented first. Afterwards the test equipment is presented and experiment planning and execution as well as simulation are discussed. In order to characterize the fatigue behaviour of Ti-6Al-4V, the material's mechanical properties and its microstructure is determined. The focus of this work is on the description of fatigue behaviour. In addition to the phenomenological investigation, which describes the crack development from crack initiation to the failure of the material qualitatively, the results of hysteresis loops analysis and image correlation are discussed. The combination of data and simulation, that enables the analytical modeling of the crack growth behaviour of Ti-6Al-4V concludes this thesis.