Issue 49

M. Bannikov et alii, Frattura ed Integrità Strutturale, 49 (2019) 383-395; DOI: 10.3221/IGF-ESIS.49.38 384 assessing fatigue strength. Special attention should be paid to the foreign object damage (FOD) research section, which deals with the destruction of engine parts after preliminary dynamic loading [7-11]. At present, after several decades of research, it is becoming clear that fatigue damage is the result of accumulation of microplastic deformations, which lead to irreversible microstructural and topological changes [12] that determine critical states of damage and the transition to destruction. General knowledge of the mechanisms of microstructural changes leading to fatigue failure allows us to associate these critical states with the development of an ensemble of microcracks originating in localized shifts. The interest to fundamental problems of fatigue damage has sharply increased in connection with the possibility of attaining a fatigue resource corresponding to the so-called very high (gigacycle) fatigue [1-10]. The results of these studies raised, in particular, the question of the existence of a fatigue limit in the transition from high cycle to gigacycle loading conditions. The formation of fatigue damage is traditionally associated with microplastic deformations [1-2, 12], which are occur under cyclic loading conditions, initiating various microstructural mechanisms that control durability and depend on the nature and initial structure of the material. For ductile metals, a special type of fatigue (cyclic) deformation localization (persistent slip bands - PSB) is observed, the development of which traditionally leads to the initiation of cracks in the near-surface zone. The second common case, for example, in the fatigue fracture of high-strength steels, is the onset of damage (microcracks), in the vicinity of inclusions at low load amplitudes corresponding to very high cycle fatigue. A characteristic feature of destruction under gigacycle fatigue conditions is the decisive effect of the initiation stage of a fatigue crack on the fatigue life. At the same time, a qualitative difference is the formation of a fatigue crack in the bulk of the material, which decisively changes the formulation of the problem of assessing the fatigue life, methods for studying the stages of fracture development. In contrast to the established traditions in the field of high-cycle fatigue, where central attention is paid to the crack propagation stage, a fundamental problem arises about formation of the fatigue crack during multiscale damage development processes associated with defects of various nature (inclusions, localized plastic shear bands, microcracks, pores). The staging of destruction is characterized by the effects of “irreversibility”, initiated by the formation of localized shifts that play a key role in the initiation of a fatigue crack [13-15], which can manifest itself in signs of nonlinearity of the elastic behavior of materials, “anomalies of elastic compliance” of fatigue samples. The role of the initiation stage is especially important for gigacyclic loading regimes, which are characterized by "fish-eye" crack formed in the bulk of the material [13-14]. This article presents a method for monitoring the kinetics of accumulation of defects by analyzing the amplitude of the second harmonic of oscillations of the free end of the sample. With the appearance of internal defects, nonlinearity effects arise, which significantly increase with their correlated interaction and the formation of a fatigue crack. M ATERIAL AND EXPERIMENT CONDITION n this research were investigated samples of aluminum alloy AMG-6, the chemical composition and mechanical characteristics of which are presented in Tabs. 1 and 2. The material was subjected to dynamic deformation performed by split Hopkinson pressure bar set-up at the strain rates of ~10 3 s -1 [16]. The material was pre-deformed in order to specify a certain degree of initial damage to assess further of the defects development kinetics. Samples with a geometry corresponding to Fig. 1. Al Cu Mg Mn Si Fe Zn Be Ti 91.1-93.68 0.10 5.8-6.8 0.5-0.8 0.4 0.4 0.20 0.0002-0.005 0.02-0.1 Table 1: The chemical composition of AMg6 (as a percentage) Elastic modulus (GPa) Yield strength (MPa) Tensile strength (MPa) Maximum elongation (%) 71 180 355 25 Table 2: Quasi-static tensile properties of AMg6 I