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

Ti alloys such as Ti–6Al–4V belong to a group of materials that show tremendous potential in aircraft engine construction due to their mechanical properties and 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. The objective of this study is he characterization of the crack growth behavior under low-cycle fatigue conditions on a phenomenological and mathematical-analytical level. In-situ LCF fatigue measurements revealed that favorable crack initiation due to slip band formation and propagation sites are located within $\beta$ phases and at grain boundaries in Ti–6Al–4V. Crack closure and merging effects resulted in single but critical crack that lead to failure. In addition, the energetical investigation with elasto-plastic strain response showed a continuous increase in absorbed energy as consequence to formation of crack surfaces. The determination of crack growth rates completed the analytical investigation and enabled a full size modeling revealing advantages and potential of optimization of existing model.