Vladimir Oborin

Experimental investigation of crack initiation and propagation in high- and gigacycle fatigue in titanium alloys by study of morphology of fracture

Fatigue (high- and gigacycle) crack initiation and its propagation in titanium alloys with coarse and fine grain structure are studied by fractography analysis of fracture surface. Fractured specimens were analyzed by interferometer microscope and electronic microscope to improve methods of monitoring of damage accumulation during fatigue test and verify the models for fatigue crack kinetics. Fatigue strength was estimated for high cycle fatigue (HCF) regime using the Luong method [1] by “in-situ” infrared scanning of the sample surface for the step-wise loading history for different grain size metals. Fine grain alloys demonstrated higher fatigue resistance for both HCF and gigacycle fatigue regimes. Fracture surface analysis for cylindrical samples was carried out using optical and electronic microscopy method. High resolution profilometry (interferometerprofiler New View 5010) data of fracture surface roughness allowed us to estimate scale invariance (the Hurst exponent) and to establish the existence of two characteristic areas of damage localization (different values of the Hurst exponent). Area 1 with diameter ~300 ?m has the pronounced roughness and is associated with damage localization hotspot. Area 2 shows less amplitude roughness, occupies the rest fracture surface and considered as the trace of the fatigue crack path corresponding to the Paris kinetics.

Structural and mechanical investigation of the estimating reliability of aluminum alloys with consecutive dynamic and gigacycle loading

In this paper we investigate the influence of consecutive dynamic and gigacycle fatigue loads on the lifetime of the aluminum-magnesium alloy AlMg6. Preloading of specimens is achieved during dynamic tensile tests in the split Hopkinson bar device, and quasi-static loadings are carried out on a servo hydraulic tensile machine (Biss Bi-00-100 series). Fatigue tests are conducted on a Shimadzu USF-2000 ultrasonic fatigue testing machine. This machine provides 109–1010 loading cycles with the amplitude from 1 to several dozens of microns and frequency of 20 kHz; this reduces dramatically the testing time in comparison with the classical fatigue testing machines. A New-View 5010 interferometer–profiler of a high structural resolution (0.1 nm) is used for qualitative fracture surface analysis, which provides the data allowing us to find a correlation between mechanical properties and scale-invariant characteristics of damage-induced roughness formed under dynamic and gigacycle fatigue loading conditions.

Study of plastic strain localization mechanisms caused by nonequilibrium transitions in mesodefect ensembles under high-speed loading

The behavior of specimens dynamically loaded during split Hopkinson (Kolsky) bar tests in a regime close to simple shear conditions was studied. The lateral surface of the specimens was investigated in-situ using a high-speed infrared camera CEDIP Silver 450M. The temperature field distribution obtained at different time allowed one to trace the evolution of plastic strain localization. The process of target perforation involving plug formation and ejection was examined using a high-speed infrared camera and a VISAR velocity measurement system. The microstructure of tested specimens was analyzed using an optical interferometer-profiler and a scanning electron microscope. The development of plastic shear instability regions has been simulated numerically.

Scaling invariance of fracture in aluminum alloy under fatigue and dynamic loading

The role of the collective behavior of defect ensembles at the crack tip and the laws of fatigue crack propagation in aluminum-magnesium alloy AMg6 (5056) 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.

Experimental and numerical study of plastic shear instability under high-speed loading conditions

The behavior of specimens dynamically loaded during the split Hopkinson (Kolsky) bar tests in a regime close to simple shear conditions was studied. The lateral surface of the specimens was investigated in a real-time mode with the aid of a high-speed infra-red camera CEDIP Silver 450M. The temperature field distribution obtained at different time made it possible to trace the evolution of plastic strain localization. The process of target perforation involving plug formation and ejection was examined using a high-speed infra-red camera and a VISAR velocity measurement system. The microstructure of tested specimens was analyzed using an optical interferometer-profilometer and a scanning electron microscope. The development of plastic shear instability regions has been simulated numerically.

Numerical simulation and experimental investigation of strain and damage localization in metals under dynamic loading

Simulation of high-strain rate loading of thin-walled cylindrical specimens are represented using the model linking characteristic stages of strain and damage localization and multiscale defects (microshears) kinetics. Shear compression the Kolsky bar test, “In-situ” infrared framing of strain localization areas and the following structural analysis of shear transformation areas are carried out to support the constitutive models and results of numerical simulation.

Experimental study of crack initiation and propagation in high- and gigacycle fatigue in titanium alloys

Fatigue (high- and gigacycle) crack initiation and its propagation in titanium alloys with coarse and fine grain structure are studied by fractography analysis of fracture surface. Fractured specimens were analyzed by interferometer microscope and SEM to improve methods of monitoring of damage accumulation during fatigue test and to verify the models for fatigue crack kinetics. Fatigue strength was estimated for high cycle fatigue regime using the Luong method [1] by “in-situ” infrared scanning of the sample surface for the step-wise loading history for different grain size metals. Fine grain alloys demonstrated higher fatigue resistance for both high cycle fatigue and gigacycle fatigue regimes. Fracture surface analysis for plane and cylindrical samples was carried out using optical and electronic microscopy method. High resolution profilometry (interferometer-profiler New View 5010) data of fracture surface roughness allowed us to estimate scale invariance (the Hurst exponent) and to establish the exist...

Structure and properties of proton exchange waveguides on Z cut of lithium niobate crystal fabricated in molten benzoic acid with the addition of lithium benzoate

Methods of chemical etching, non-contact profilometry, X-ray analysis and mode spectroscopy enabled to find out that proton exchange channel waveguides formed on Z cut of lithium niobate crystal in the molten benzoic acid with the addition of 3 mol.% of lithium benzoate at the temperature of 225°C were composed of κ2-phase and a thin layer of α-phase. Proton exchange on +Z and -Z faces of the lithium niobate crystal in benzoic acid (with or without the addition of lithium benzoate) occurs at the same rate.

Multifractal analysis of morphology of AlMg6 alloy fracture surface under consecutive dynamic and gigacycle loading

The influence of consecutive dynamic and gigacycle fatigue loads on the lifetime of aluminum-magnesium alloy AlMg6 (5083) has been studied to analyze the fracture surface morphology (roughness scaling) and generalized fatigue crack kinetics, taking into account the correlated behavior of defects at the crack tip area (process zone). The samples were dynamically preloaded by a split-Hopkinson bar device. Fatigue tests of the preloaded samples were conducted on a Shimadzu USF-2000 ultrasonic fatigue testing machine. A New-View 5010 interferometer-profiler of high structural resolution (resolution of 0.1 nm) was used for quantitative fracture surface analysis. The analysis provided data for finding the correlation between mechanical properties and scale-invariant characteristics of damage-induced roughness formed under dynamic and gigacycle fatigue loads.

Localized instability of plastic deformation at dynamic loading caused by nonequilibrium transitions in defect ensembles

Mechanisms of plastic shear localization in a material under dynamic loading are studied theoretically and experimentally. These mechanisms are associated with collective effects occurring in the ensemble of microdefects in spatially localized regions. In-situ infra red scanning of the instability zone and a subsequent analysis of the dislocation substructure lend support to the supposition that nonequilibrium transitions in defect ensembles play a decisive role in the development of localized plastic flow. The equations relating nonequilibrium transitions to the mechanisms of structural relaxation and plastic flow are used to simulate localizations of plastic deformation.