An experimental methodology was developed for estimating a very high cycle fatigue (VHCF) life of the aluminum alloy AMG-6 subjected to preliminary deformation. The analysis of fatigue damage staging is based on the measurement of elastic modulus decrement according to “in situ” data of nonlinear dynamics of free-end specimen vibrations at the VHCF test. The correlation of fatigue damage staging and fracture surface morphology was studied to establish the scaling properties and kinetic equations for damage localization, “fish-eye” nucleation, and transition to the Paris crack kinetics. These equations, based on empirical parameters related to the structure of the material, allows us to estimate the number of cycles for the nucleation and advance of fatigue crack.
The paper presents an experimental methodology for assessing the ultra-high-cycle resource as applied to titanium VT1-0 in the submicrocrystalline and nanostructured states. The program for testing very-high-cycle loading (number of cycles 10 7 -10 9 ) has been experimentally implemented. An "in situ" technique for determining the accumulation of irreversible fatigue damage was used. This technique is based on the analysis of nonlinear manifestations of the feedback signal in a closed system of an ultrasonic fatigue unit. This makes it possible to establish a connection between microscopic mechanisms of fatigue and model concepts and to consider the stages of damage development based on the nonlinear kinetics of defect accumulation during cyclic loading in the regimes of high-and very-high-cycle fatigue. A mathematical model of a deformable solid based on wide-range constitutive relations of the statistical theory of defects is presented. The proposed model contains a structural scaling parameter that allows one to describe the deformation behavior and fracture of the material under study in various structural states. The effect of damage accumulation under gigacycle loading is described. Numerical calculations predict well the experimental Wöhler curves. In an axisymmetric formulation, a boundary value problem is solved -the process of emergence of a crack originating inside the material is modeled.
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