PSI - Issue 41

Aleksandr Inozemtsev et al. / Procedia Structural Integrity 41 (2022) 510–517 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The fundamental aspects of physics and mechanics of fatigue damage in aircraft materials sequentially subject to dynamic and fatigue loads are the key problem in ensuring the reliability of aircraft gas turbines and predicting the catastrophic events caused by the damage of fan blades under conditions of Foreign Object Damage (FOD). Progress in reliability prediction is achieved mainly through the use of multifunctional materials for designing structures with high tolerance at all stages of damage-failure transition under consecutive dynamic, High Cycle Fatigue (HCF) and Very High Cycle Fatigue (VHCF) loads by Nicolas (1999, 2006), Peters (2000), Froustey (2009), Spanrad (2011). The stage character of the failure process is determined by the relationship between the multiscale correlated behavior of defects with the mechanisms of structural relaxation and damage-failure transition revealing the features of critical phenomena by Naimark (2003). To gain insight into the critical phenomenon of damage-failure transition in materials under consecutive dynamic and fatigue (HCF and VHCF) loads by Froustey (2010), Oborin (2016, 2019) it is necessary to state and solve two fundamental problems: the problem of damage localization due to multiscale nucleation and growth of defects and the problem of fatigue crack growth in the damaged material. The definition of stages of fatigue crack initiation and propagation is based on the formulation of a fundamental problem concerning the description of the multiscale kinetics of damage localization, fatigue crack initiation and crack propagation. The duration of these stages largely depends on the state of the material structure and nonlinear damage kinetics. The characteristic feature of damage-failure transition under conditions of VHCF is a decisive influence of the stage of fatigue crack initiation on the fatigue life. In contrast to the conventional way of relating the HCF life time with the stages of crack propagation, the fundamental problem formulated by Bathias (2005), Sakai (2009), McDowell (1996) interprets the initiation of a fatigue crack as a multiscale damage localization associated with defects (slips, microcracks, pores) in the bulk of the specimen. It changes radically the formulation of the fatigue problem, the formation of critical conditions for damage-failure transition, experimental and structural methods for estimating the stages of failure. Mughrabi (2010, 2013) has also been noted that the stages of failure are characterized by the effect of slip irreversibilities, which contributes much to fatigue crack initiation. According to Bannikov (2014, 2016), the idea of using the concept of nonlinearity of damage-failure transition as the basis of experimental technique for identification of stages (including the cases of consecutive dynamic and fatigue loading) seems to have considerable promise. The role of the initiation stage is important for VHCF regimes, which are characterized by the emergence of an area of specific damage localization ("fish-eye") in the material by Bathias (1999), Mughrabi (2006). Summarizing the results of studying HCF and VHCF, one can identify the following promising areas of research: study of the role of irreversibility initiated by localized slips, whose collective behavior determines the stages of fatigue damage localization; structural identification of stages of fatigue crack initiation and crack propagation in the HCF and VHCF regimes; study of the effects of nonlinear elasticity in materials under HCF and VHCF loading conditions as the basis for the development of "in-situ" methods for evaluating the fatigue fracture stages and non destructive testing under conditions of successively applied dynamic and fatigue loads.

Nomenclature N

number of cycles applied stress

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l sc scale related to the correlated behaviour in the ensemble of defects L pz scale associated with the process zone K(r) correlation function H Hurst exponent (surface roughness index) z(x) surface relief height r window of size