A Holistic Method for Fatigue Durability and Reliability Evaluations of Engineering Machinery and Structures

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Contemporary damage tolerance methods concerned with fatigue durability of engineering objects require two-stage analysis, i.e. the estimation of fatigue crack initiation and propagation lives. Such approaches require using two different theories and an arbitrary definition of the fatigue crack initiation period. The methodology, developed as a result of research activities carried out within the current Grant 2007-2012, eliminates some of those inconsistencies. However, one of the basic elements of the approach is the material property Rho* assumed to be the effective radius of the crack tip or the size of the smallest material imperfection which could be consider as a crack. This material constant is at present determined indirectly from experimental fatigue crack growth data. It is proposed, as a part of the requested Discovery Grant 2013-2018, to undertake systematic microstructural studies of selected materials, with the aim of determining the Rho* constant without the knowledge of any preliminary fatigue data. Our own up-to-date studies have shown that the Rho* constant can be evaluated based on the fracture toughness, KIC, and the theoretical strength. The up-to-date studies of the fatigue crack growth based on the UniGrow model and associated software indicate that it is possible to estimate the total fatigue life based solely on the fatigue crack growth analysis. Therefore it is necessary to develop a model for simulating fatigue growth of cracks comparable in size with the Rho* parameter. If the proposed research activities are supported, a universal method of fatigue analysis will be developed. However, the deterministic analyses are not sufficient when reliability and safety decisions need to be carried out while estimating remaining fatigue lives of engineering objects or when applying the Damage Tolerance concept, because all input data are in reality scattered. Therefore, it is proposed to combine the deterministic Fatigue Crack Growth model with the Monte Carlo simulation method and to run multiple fatigue crack growth analyses based on the input data sampled according to their prescribed probability distributions. When successful the analysis would provide information on safe fatigue lives of various engineering objects.