13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fatigue life prediction based on a mechanism-based simulation of crack initiation and short crack growth in forged Ti-6Al-4V Hans-Jürgen Christ 1,*, Helge Knobbe1, Philipp Köster1,2, Claus-Peter Fritzen2, Martin Riedler3 1 Institut für Werkstofftechnik, Universität Siegen, D-57068 Siegen, Germany 2 Institut für Mechanik und Regelungstechnik - Mechatronik, Universität Siegen, D-57068 Siegen, Germany 3 Böhler Schmiedetechnik GmbH & Co KG, A-8605 Kapfenberg, Austria * Corresponding author: Hans-Juergen.Christ@uni-siegen.de Abstract Ti-6Al-4V was experimentally investigated in two different forged conditions with respect to fatigue crack initiation mechanisms and growth characteristics of microstructurally short fatigue cracks. The experimental findings show that the stage of short fatigue crack propagation is strongly affected by the microstructure and widely controls fatigue life under the testing conditions applied. The observations obtained were implemented into a mechanism-based short-crack model, which describes crack propagation as a partially irreversible dislocation glide on a crystallographic slip plane. The numerical model is based on dislocation dipole boundary elements. The non-uniform, oscillating propagation behaviour of short cracks is dealt with by defining grain boundaries and phase boundaries as obstacles to plastic slip and crack propagation. The model prediction in terms of short crack propagation behaviour allows for a quantitatively representation of the crack growth characteristics. Moreover, the simulation of short crack propagation was successfully used for the purpose of life assessment. Finally, by means of virtual microstructures, microstructural parameters such as grain size and volume fractions were systematically varied. Crack growth simulation calculations in these virtual microstructures are helpful to define those microstructural modifications which are desirable for improving the fatigue resistance. Keywords Ti alloy Ti-6Al-4V, fatigue crack initiation, short fatigue crack propagation, microstructural barriers, fatigue life assessment, boundary element method 1. Introduction The Ti6Al4V alloy is still by far the most common titanium material in use. It is used in many different fields, however the most important applications are probably found within the aerospace industry, where airframes and aero engine components are made of titanium alloys [1]. These employments are mainly driven by the superior structural efficiency of this alloy caused by an excellent combination of high strength and low density. Hence, a selection of high strength titanium forgings will always be found in the internal structure of planes. A proper fatigue assessment is of course a crucial requirement for all structural components in flight service, since fatigue loading conditions always occur in these assemblies. Many approaches only consider the propagation of long cracks (damage tolerant approach) and thus make use of phenomenological equations such as Paris Law in classical linear elastic fracture mechanics [2]. As an alternative, total life approaches are used with S-N curves as basis for a fatigue life prediction according to Basquin/Coffin/Manson [3-5]. These methods do generally not distinguish between
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