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Fracture behavior of VT22 near β titanium alloy produced by radial shear rolling and subsequent aging was studied in very high cycle fatigue tests with ultrasonic vibration frequency. This type of processing was shown to significantly increase the static and fatigue strengths of the alloy. According to fractographic analysis, the transition from very high cycle to high cycle fatigue failure occurs in the stress amplitude range of 850–900 MPa, which is almost two times higher than the fatigue limit of the conventionally produced VT22 titanium alloy. It was found that fatigue crack initiation in the very high cycle fatigue region is associated with the formation of a fracture facet inclined at an angle of less than 90° to the specimen axis. The facet exhibits a higher local concentration of interstitial impurities (carbon and oxygen) compared to the chemical composition of the surrounding material, which is due to diffusion mass transfer from the adjacent regions to the zone of highest compressive-tensile stresses.

Experiments on the dynamic loading of specimens from AMg6 alloy were conducted on a split Hopkinson–Kolsky bar and with target perforation. The thermodynamics of the deformation process was studied in situ by recording temperature fields using a CEDIP Silver 450M high-speed infrared camera in order to reveal the characteristic stages of strain localization. The temperature measurements in the localization zones do not confirm the traditional concept of the strain localization mechanism caused by thermoplastic instability. Dynamic tests of specimens made of Steel 3, AMg6, and D16 alloys of a special shape designed to study the localization of plastic deformation on a split Hopkinson–Kolsky bar were conducted using the StrainMaster noninvasive strain measurement system. The displacements and strain fields in these specimens are constructed. A comparison of the experimentally measured temperature and strain fields with the results of numerical simulation with allowance for the mesodefect accumulation kinetics in the material gives a satisfactory agreement within ~20%. In the specimens after the experiments, the surface relief was analyzed using a NewView-5010 optical surface profiler with subsequent processing of the 3D strain relief data and calculation of the scale invariant (Hurst index) and the spatial scale of the region on which the correlated behavior of mesodefects is observed. The data of the experiments and analysis of the surface relief of deformed specimens, as well as the data of numerical simulation taking the mesodefects accumulation kinetics in the material into account, suggest that one of the plastic strain localization mechanisms under high-speed loading is caused by jump-wise processes in the defect structure of materials.

The influence of dynamic preload on the fatigue endurance limit of AMg6 alloy has been experimentally studied in a range of 109 loading cycles. It is established that prestraining up to 10% deformation at 103 s–1 rate leads to a 22% decrease in the fatigue endurance limit. A mathematical model is proposed that can adequately predict fatigue fracture, including that after preload.

In the paper, we study the kinetics of fatigue crack growth in MMg6 and D16T aluminum alloys in the gigacycle fatigue mode under dynamic preloading. The relevance of the problem statement is determined by the critical applications—life estimation of materials and structural elements of aircraft gas turbine engines experiencing random dynamic effects under flight cycle conditions. Specimens were preloaded by dynamic tension using the split Hopkinson (Kolsky) pressure bar at strain rates up to ~103 s−1, with consecutive gigacycle loading on the Shimadzu USF-2000 ultrasonic testing machine. Quantitative fractography of fracture surfaces was performed using profilometry and scanning electron microscopy data. We propose an original form of the kinetic equation, which relates the fatigue crack growth rate to a change in the stress intensity factor. The scale invariance of defect structures responsible for the formation of the fracture surface relief under gigacycle fatigue loading is found to be related to the power exponent of the Paris law. The fracture surface morphology of an aluminum-magnesium alloy under consecutive dynamic and gigacycle loading is studied by the multifractal detrended fluctuation analysis method. It is found that a transition from the stage of formation of a fish-eye zone of localized deformation is caused by the generation of fracture sites and accompanied by a qualitative change of the nonlinear dynamics of the system—a transition from monofractal to multifractal dynamics characterized by broadening of a multifractal spectrum at the final crack growth stage, which leads to macrofracture.

Mechanisms of plastic strain localization under the dynamic loading of specially shaped specimens made of the AMg6 aluminum alloy and intended for tests under conditions close to a pure shear on a split Hopkinson–Kolsky pressure bar are studied theoretically and experimentally. The mechanisms of plastic flow instability are related to collective effects in the ensemble of microdefects in spatially localized regions visualized in situ using a CEDIP Silver 450M high-speed infrared camera. The calculation corresponding to the experimental loading scheme is implemented using wide range constitutive equations reflecting the dependence of structural relaxation mechanisms—manifestation of the collective behavior of microdefects—on the evolution of the localized flow shear instability. Microstructure analysis of deformed specimens included the study of the spatial relief (porosity) scaling by the data of a NewView-5010 microscope-interferometer in regions of plastic strain localization. An increase in the structural scaling exponent (Hurst exponent) reflected the degree of the multiscale correlated behavior of defects and the porosity induced by them in regions of localized plasticity. Infrared scanning of the strain localization region and numerical simulation followed by an estimate of the defect structure corroborated the hypothesis that effects of thermal softening do not play a decisive role in the process of plastic shear localization in the tested material under the considered loading regimes. A new, one of the possible ones, mechanism of plastic strain localization under dynamic loading is justified. It is caused by the multiscale collective behavior of mesodefects—structure-scaling transitions—and establishes the stadiality of the localized shear evolution.

The paper presents the results of fatigue-testing ultrafine-grained and coarse-grained Ti-45 wt.% Nb alloy samples under very high cycle fatigue (gigacycle regime), with the stress ratio R = −1. The ultrafine-grained (UFG) structure in the investigated alloy was formed by the two-stage SPD method, which included multidirectional forging (abc–forging) and multipass rolling in grooved rollers, with further recrystallization annealing. The UFG structure of the Ti-45 wt.% Nb alloy samples increased the fatigue limit under the high-cycle fatigue conditions up to 1.5 times compared with that of the coarse-grained (CG) samples. The infrared thermography method was applied to investigate the evolution of temperature fields in the samples under cyclic loading. Based on numerical morphology analysis, the scale invariance (the Hurst exponent) and qualitative differences for UFG and CG structures were determined. The latter resulted from the initiation and propagation of fatigue cracks in both ultra-fine grained and coarse-grained alloy samples under very high-cycle fatigue loading.

An experimental and theoretical study of plastic shear localization mechanisms observed under dynamic deformation using the shear–compression scheme on a Hopkinson–Kolsky bar has been carried out using specimens of AMg6 alloy. The mechanisms of plastic shear instability are associated with collective effects in the microshear ensemble in spatially localized areas. The lateral surface of the specimens was photographed in the real-time mode using a CEDIP Silver 450M high-speed infrared camera. The temperature distribution obtained at different times allowed us to trace the evolution of the localization of the plastic strain. Based on the equations that describe the effect of nonequilibrium transitions on the mechanisms of structural relaxation and plastic flow, numerical simulation of plastic shear localization has been performed. A numerical experiment relevant to the specimen-loading scheme was carried out using a system of constitutive equations that reflect the part of the structural relaxation mechanisms caused by the collective behavior of microshears with the autowave modes of the evolution of the localized plastic flow. Upon completion of the experiment, the specimens were subjected to microstructure analysis using a New View-5010 optical microscope–interferometer. After the dynamic deformation, the constancy of the Hurst exponent, which reflects the relationship between the behavior of defects and roughness induced by the defects on the surfaces of the specimens is observed in a wider range of spatial scales. These investigations revealed the distinctive features in the localization of the deformation followed by destruction to the script of the adiabatic shear. These features may be caused by the collective multiscale behavior of defects, which leads to a sharp decrease in the stress-relaxation time and, consequently, a localized plastic flow and generation of fracture nuclei in the form of adiabatic shear. Infrared scanning of the localization zone of the plastic strain in situ and the subsequent study of the defect structure corroborated the hypothesis about the decisive role of non-equilibrium transitions in defect ensembles during the evolution of a localized plastic flow.

The objective of this work is to provide the link between the fatigue behaviour of pre-strained aluminium alloys and the scaling properties of damage induced on the fracture surface. Fatigue tests performed on pre-strained aluminium alloys revealed a large difference in their residual fatigue resistance linked to the material: the Al–Cu alloy demonstrated a sharp decrease of HCF life-time due to the pre-straining whereas the insensitivity of the Al–Mg alloy was clear. For the Al–Cu alloy, the investigations made at a ‘mechanical’ scale allow us to associate the strain energy absorbed during the prior loading with the aspect of the surface and the residual HCF life-time. The statistical characterization of the fatigue damaged zone was done from the measurement of the surface roughness. Scaling properties were established that allowed the conclusion of the universality of HCF damage kinetics as the mechanism controlling the sensitivity of Al–Cu alloy whatever the pre-straining history.

The role of the collective behavior of defect ensembles at the crack tip and the laws of fatigue crack propagation in aluminum- magnesium alloy have been studied under conditions of symmetric tension–compression gigacycle loading at 20 kHz using ultrasonic fatigue testing machine (Shimadzu USF2000). 3D New View 5010 interferometer profiler high resolution data of defect induced roughness in the crack process zone under fatigue crack path revealed the existence of two characteristic scales: the scale of the process zone and the correlation length that is the scale when the correlated behavior of defect induced roughness has started.