Probabilistic Fracture Mechanics Framework Including Crack Nucleation of Rotor Forging Flaws

We present a probabilistic rotor life prediction framework that combines the forging flaw crack nucleation process and the subsequent crack growth to failure. Experimental fatigue tests of specimens including forging flaws show that the life cycle of a forging flaw can be described by a nucleation phase followed by a fatigue crack growth phase. These results demonstrate that the nucleation phase is a significant fraction of the whole life cycle to failure. However, as there is no engineering method available that describes reliably the nucleation phase, this portion is oftentimes neglected in engineering life prediction frameworks, therefore resulting in a conservative life quantification. In order to improve probabilistic life quantification methods, we introduce a rigorous scheme that convolutes the local crack nucleation probabilities and the local crack growth failure probabilities in order to provide a local failure probability. Integration over the whole component then yields the total probability of failure for the engineering part under a specific load spectrum. A specific direct simulation Monte Carlo numerical implementation is demonstrated. It is applied to fatigue crack nucleation from large gas turbine rotor disk forging flaws followed by crack growth to component failure. For different regions of the analyzed rotor components, the results show the probabilistic interplay of the different temperature and stress dependences of the applied empirical nucleation models and the fatigue crack growth models. The presented probabilistic approach is generic and not restricted to the discussed fatigue nucleation and subsequent crack growth process in large rotor forgings. The framework can be applied to a variety of sequential failure processes including static and fatigue loading phenomena, as well as a multiplicity of failure modes and sequences relevant for engineering components.