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An important task in turbomachinery design is the assessment of material flaws in machine components. Such flaws may occur during manufacturing process. Despite the modern forging techniques, a typical problem is that forged rotors and discs still suffer from crack-like material flaws in the form of segregation, non-metallic inclusions, shrink holes, cracks and cavities. The focus of this work is to perform lifetime predictions for a forged expander impeller containing a cluster of crack like indication in the central part of the impeller disc. For this purpose, a representative crack geometry has been defined for the detected cluster of flaws. A numerical study of growing mixed-mode internal cracks in the impeller is undertaken with the help of a finite element simulation. The model enables us to predict the lifetime of the impeller and the crack paths due to steady state and transient stresses during operation, including start up and turn down. The propagation of the crack is governed by the principle of maximum driving force which is a direct consequence of the variational principle of a cracked body in equilibrium. This criterion considers the effect of all three stress intensity factors in mixed-mode condition, and without any ad hoc assumption, the crack growth rate is calculated using its thermodynamic duality with the local maximum driving force.