Abstract
Paper presents the “in situ” method for determining of irreversible fatigue damage accumulation, based on the analysis of nonlinear manifestations of the feedback signal in a closed system of an ultrasonic fatigue system. During very high cycle (gigacycle) fatigue, the anomalies of the elastic properties of the material are appear, which leads to a nonlinearity effect in the amplitude of oscillations. This effect increases with the initiation and growth of fatigue cracks. The technology was applied to samples of AMG-6 alloy with preliminary dynamic deformation at various levels of average stress to determine the moment of initiation and growth of the fatigue crack in very high cycle fatigue regime. This method is applicable for the early detection of fatigue cracks both on the surface and inside the material under cyclic loading in the ultrasonic mode. On the basis of wide-range defining relations for a deformable solid body with mesoscopic defects, a mathematical model has been proposed that can adequately describe behavior of the material during fatigue failure. The results of mathematical modeling are in good agreement with the experimental data
Highlights
The determination of fatigue cracks origin nature is one of the most important fundamental problem for various application areas, especially speaking about ultrahigh-cycle [1,2,3] fatigue, when crack forms inside the material
The technology was applied to samples of AlMG-6 alloy with preliminary dynamic deformation to determine the moment of initiation and growth of the fatigue crack in very high cycle fatigue regime
The series of catastrophes caused by the fatigue destruction of gas turbine engines [4,5,6], combined with the high cost of resource assessment and the potential cost of developing new structures, stimulated promising concepts of national programs in the field of high and very high cycle fatigue, based on the use of new fundamental results in
Summary
The determination of fatigue cracks origin nature is one of the most important fundamental problem for various application areas, especially speaking about ultrahigh-cycle (gigacycle) [1,2,3] fatigue, when crack forms inside the material. Βrelative increases slightly at the initial stage, which is usually attributed to the effect of heating the sample and, as a result, changing its linear dimensions [17,18,19], most of the time of the experiment, the amplitude of the second harmonic remains constant and increases significantly with the formation of a fatigue crack and its growth (Fig. 4, a). In one of the samples, which were destroyed with the formation of a surface crack, a monotonic decrease in the amplitude of the second harmonic uncharacteristic for others was found during the testing process (Fig. 4.b) This is supposed to be due to the fact that the initial structure of the material was saturated with defects, since the samples were pre-deformed. The complete system of constitutive equations looks as follows:
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