Localized shear in metals under impact loading

The results of studies on localized shear strain in high-strength steel, a titanium alloy, and mild sheet steel under impact loading are given. The analysis of experimental results, microstructural changes, and numerical simulation demonstrates that adiabatic shear bands formed at high-rate deformation are influenced by strain hardening and heating in plastic flow and phase transformations in a material. The distribution of temperatures in the regions of strain localization is responsible for the development of microstructural changes. Nonuniform deformation without intense strain localization develops at lower shear rates and small strain increments per loading cycle, eliminating considerable heating of a material. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 27–42, March–April, 2000.     

An approximate method of determination of maximum strain hardening levels in metals under nonproportional low-cycle loading

Using the findings of analysis of deformation curves for metallic materials under static and cyclic loading, an approximate method is put forward for the determination of maximum strain hardening levels in the case of non-proportional low-cycle loading with strain monitoring. Based on the correlation between strain hardening data obtained from the static and proportional and nonproportional cyclic deformation curves, an approximate analytical relationship is built up which allows for predicting maximum strain hardening levels under nonproportional low-cycle loading. __________ Translated from Problemy Prochnosti, No. 2, pp. 29–38, March–April, 2006.     

Cyclic hardening and fatigue behavior of stainless steel 304L

This article discusses cyclic hardening and fatigue behaviors of stainless steel 304L, the behavior of which is greatly influenced by prior loading. Effects of loading sequence, mean strain and mean stress, and pre-straining (PS) were investigated using constant amplitude as well as step and random loading tests. Contrary to common expectations, fatigue lives in strain-controlled mean strain tests were significantly affected by the mean strain, in spite of mean stress relaxation. PS induced considerable hardening and led to different results on fatigue life, depending on the test control mode. Secondary hardening was observed in some tests, characterized by a continuous increase in the stress response. Possible mechanisms for this behavior are also discussed. To correlate fatigue life data of a material such as stainless steel with strong deformation history effect, it is shown that a damage parameter with both stress and strain is required. The Fatemi–Socie (FS) parameter as such a parameter is shown to correlate the data under different control modes and loading conditions.     

Prediction of the fracture of metals in processes of cold plastic deformation. Part 2. Effect of anisotropic hardening and experimental verification of the model of plastic deformation and fracture

On the basis of the experimental investigation of the Bauschinger effect, in steels with developed prestrains, we generalize the model of plastic deformation to the case of anisotropic hardening. Within the framework of the model, we show that for high strains, the Bauschinger effect is caused by dislocations. We present the results of the experimental verification of the developed semiphenomenological model of the joint process of plastic deformation and fracture of metals under the conditions of cold deformation. It is shown that this model gives adequate predictions (in good agreement with the experimental data) of the probability of fracture of the metal caused by exhaustion of the plasticity margin in the processes of plastic deformation realized under the conditions of both simple and complex loading. Ufa State Aviation Technical University, Ufa, Russia. Translated from Problemy, Prochnosti, No. 2, pp. 74–84, March–April, 1999.     

Strain hardening of structural metals with metastable structure under complex loading

We investigated distinctive features of strain hardening under complex loading for quenched and low-tempered steels of ferritic-pearlitic (40KhN), martensitic (30Kh3NSMV), and maraging (Kh16N5D3) types and an Al-4Cu age-hardening alloy whose matrix phase is characterized by the metastable structure corresponding to supersaturated interstitial and substitutional solid solutions susceptible to disintegration in the process of plastic deformation. We analyzed structural indicators of the depletion of the matrix phase of supersaturating alloying elements forming carbides and intermetallic compounds and the mechanisms of hardening and softening influence of preliminary deformation (within reasonable ranges of its variation) on the value of the yield limit when repeated loading changes its sign. It is shown that, for the investigated type of metastable materials, in the analyzed range of preliminary deformations governed by alloying, the application of repeated biaxial tension leads to a uniform extension of the limiting yield curves. As the degree of supersaturation decreases (in the process of thermal treatment) and we observe the formation of equilibrium solid solutions of the matrix phase, the character of strain hardening evidently changes from predominantly isotropic to kinematic. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 85 – 94, January – February, 1998.     

Strain hardening of metastable austenitic steel under repeated simple loading at different temperatures

On the basis of experimental data obtained under repeated simple loading of 18-10 steel at temperatures of 77, 123 and 293 K, we discovered that the direction of maximum hardening in the space of stresses does not coincide with the direction of preloading. This effect becomes more pronounced if preloading and repeated loading are performed at different temperatures. It is shown that the maximum degree of hardening is attained after repeated loading preceded by uniaxial preloading in the direction of trajectories close to uniform biaxial tension. It is also noted that the yield surface of structurally unstable steel undergoes complicated transformations. Translated from Problemy Prochnosti, No. 6, pp. 59–65, November–December, 1997.     

A novel high manganese austenitic steel with higher work hardening capacity and much lower impact deformation than Hadfield manganese steel

To tackle the problem of poor work hardening capacity and high initial deformation under low load in Hadfield manganese steel, the deformation behavior and microstructures under tensile and impact were investigated in a new high manganese austenitic steel Fe18Mn5Si0.35C (wt.%). The results show that this new steel has higher work hardening capacity at low and high strains than Hadfield manganese steel. Its impact deformation is much lower than that of Hadfield manganese steel. The easy occurrence and rapid increase of the amount of stress-induced ε martensitic transformation account for this unique properties in Fe18Mn5Si0.35C steel. The results indirectly confirm that the formation of distorted deformation twin leads to the anomalous work hardening in Hadfield manganese steel.     

Analysis of the stressed state of a circular thin-walled cylinder under complex loading and with nonlinear hardening of the material

The generalized differential equations of plastic flow for a material with nonlinear hardening are derived using the Prager kinematic model. An example of numerical analysis for stress variation under elastoplastic deformation of a thin-walled cylinder of a structural carbon steel is given for different elastoplastic material models. Translated from Problemy Prochnosti, No. 3, pp. 58–65, May–June, 2009.     

An experimental study of failure and softening in sand under three-dimensional stress condition

This paper describes an experimental study of failure and softening behaviour in dense Toyoura sand. A true triaxial apparatus equipped with three pairs of rigid loading platens is used to test sand sample under three-dimensional stress condition. The testing results demonstrate that the rigid boundary around the sand samples cannot prevent formation of shear localization. Shear localization are observed to emerge in the hardening or the softening regime in the loading depending on the magnitude of intermediate principal stress. Uniform deformation for the whole strain range is obtained only in triaxial compression tests. The peak stress state obtained from tests of sand samples of the same initial density can be described with good approximation by the Matsuoka–Nakai criterion.     

Localized shear in sheet materials

Procedures and results of numerical simulations and experimental investigations of sheet materials under localized plastic deformation in shear are presented. Under dynamic loading these materials are considered to be viscoelastoplastic solids. Localized deformation and stress-strain distributions resulting from plastic shear are examined as a one-dimensional problem with account for nonlinear viscosity, damage, and temperature effects. Mild steel and high-strength aluminum alloy strips were tested under multiple impact loading with a small strain increment per impact (which corresponds to isothermal loading with sufficient accuracy). The experimental relations of localized shear deformation under such loading allow one to make the conclusion of a prevailing stress concentration effect on the kinetics of localized shear at the edge of the strip. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 53–63, September–October, 1999.     

Evolution of mechanical properties for a dual-phase steel subjected to different loading paths

The evolution of the mechanical properties of a dual-phase (DP590) steel sheet after being prestrained by uniaxial tension, plane strain and equal biaxial stretching was investigated. Specimens were first loaded using the three prestraining modes. Then, from the prestrained specimens, a few sub-sized samples were machined along the rolling direction and the transverse direction for further uniaxial tension testing. Six loading paths were provided. Equal biaxial stretching was performed using a cruciform specimen. The evolution of work hardening performance, elastic modulus, yield stress and tensile stress under the six loading paths were discussed in detail. The results indicate that loading paths can affect the latent work hardening performances, strain hardenability, yield stress and tensile stress evolution as well as the elastic modulus decrease during plastic deformation. The uniaxial tension–uniaxial tension path results in a cross-softening phenomenon, the largest yield stress enhancement and a mild maximum tensile stress increase. The equal biaxial stretching-uniaxial tension path leads to a cross-hardening phenomenon, the least yield stress enhancement and the largest tensile strength increase maximum tensile strength. The elastic modulus of DP590 steel not only changes with the accumulated plastic strain but also varies with the loading paths. The largest decrease of the elastic modulus equal biaxial stretching–uniaxial tension can reach 12.7% beyond 8% equivalent strain, which is 5.2% greater than that in the monotonic uniaxial tension path.     

Prediction of Strain Hardening and Fatigue Life of Cyclically Unstable Materials

The data on cyclic deformation of materials with different cyclic properties are analyzed. The relationship between the level of additional strain hardening and the mechanical characteristics of a material is established. The proposed models take into account the influence of the form of stressed state on strain hardening and durability under conditions of nonproportional cyclic deformation. The models are constructed on the basis of the analysis of the experimental data on cyclic deformation of 304 stainless steel. The prediction of strain hardening and durability is performed with separate analysis of the influence of shape of the cycle and the form of stressed state. To take into account the influence of shape of the cycle, we use the coefficient of disproportionality of the cycle and the parameter of sensitivity to disproportionality. To take into account the form of stressed state, we use the coefficient of the form of stressed state and the corresponding parameter of sensitivity to the form of stressed state. The results of prediction of both strain hardening and durability of materials on the basis of the proposed phenomenological models reveal their fairly high efficiency. __________ Translated from Problemy Prochnosti, No. 5, pp. 118 – 130, September – October, 2005.     

Corrosion-mechanical characteristics of the materials of nonmagnetic shroud rings of turbogenerators. I. 8Mn−8Ni−4Cr and 18Mn−4Cr steels

On the basis of the comprehensive analysis of the corrosion-mechanical characteristics of austenitic nonmagnetic 8Mn−8Ni−4Cr steels, we show that this type of steel is susceptible to corrosion cracking even in low-aggressive aqueous solutions, including humid working environments at a temperature of 80°C. The application of aluminum coatings and mechanical surface treatment increase the time to failure of specimens. Only deep drying of hydrogen used to cool the shrouds eliminates the causes of corrosion cracking of these types of steel. The procedure of cold tensile hardening cannot be applied to 40Kh4G18 steel because it is characterized by high susceptibility to the formation of α-martensite in the process of plastic deformation due to the insufficient stability of austenite. The optimal vanadium contents of steel guaranteeing its high strength and plastic characteristics lie within the range 1.3–1.5%. In the absence of water, 18Mn−4Cr steel does not exhibit any noticeable susceptibility to corrosion cracking. For this reason, the main condition that should be realized to prevent corrosion cracking and corrosion fatigue of shroud rings is the maintenance of the dry state of working environments in the processes of manufacturing, storage, and transportation of these rings, in service, and during downtimes. The presence of Cl⁻ ions, hydrogen sulfide, halides, and nitrates in the working environments significantly enhances the fatigue crack growth rates in the entire investigated range. These ions also promote crack growth under the conditions of long-term static loading. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 4, pp. 127–138, July–August, 1997.     

Computer thermal model for hardening grinding

A mathematical model of heat transfer in steel parts under force grinding is considered that allows one to determine the grinding parameters at which a steel layer of prescribed thickness is heated up to hardening temperatures. Results of numerical simulation of the process of hardening grinding are compared with experimental data. The internal structure of the material after hardening grinding is analyzed. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 2, pp. 430–438, March–April, 2000.     

Yield strength enhancement of martensitic steel through titanium addition

Yield strength enhancement for martensitic steel fabricated by vacuum induction melting is investigated. It is found that the addition of Ti can improve the yield strength property of the martensitic steel, which can be attributed to increase in precipitation hardening from formation of TiC precipitates in the martensitic matrix. Moreover, the yield strength can be further enhanced by tempering and reheat quenching process, which can be ascribed to the formation of a superfine sized (~8 μm) grains and large amount of freshly nano-sized (1–10 nm) precipitates in the final martensitic structure for martensitic steel containing Ti. The experimental and theoretical results on the contribution of TiC precipitates to hardening of the martensitic steel are in excellent agreement, showing that the precipitation hardening of 188 MPa caused by TiC precipitates is the main reason why the yield strength for martensitic steel is enhanced via titanium addition.     

A study of deformation and damage accumulation processes in 10GN2MFa steel under low-cycle loading

The paper presents results of the experimental investigation of deformation behavior and damage accumulation kinetics in a heat-resistant reactor pressure vessel steel at high stress levels under stress-controlled (pulsating), low-cycle loading up to 10⁴ cycles. Hardness measurements have been performed upon various operating time periods under elastoplastic deformation. It has been found that hardness and strength of the steel exhibit qualitatively different behaviors throughout the lifetime. The damage accumulation process in steels under static and cyclic loadings has been studied by means of the LM-hardness method. The curves of Weibull homogeneity coefficient vs. cycling stress and accumulated strains are obtained. __________ Translated from Problemy Prochnosti, No. 2, pp. 5–10, March–April, 2008.     

Evaluation of the Elastic-Plastic Mixity Parameters on the Base of Different Crack Propagation Criteria. Part 2. Solution and Results

We propose a new scheme of mixed-mode problem solution based on the deformation theory of plasticity with a power-law hardening stress-strain response and on application of elastic and plastic mixity parameters. Depending on the mixed-mode loading conditions and the initial crack line direction, this approach allows one to analyze a wide range of possible crack propagation paths controlled by shear and tensile mechanisms. The equilibrium equation with Airy function is used for a two-dimensional problem in the polar coordinate system. The Ramberg-Osgood model is applied to a material with power-law hardening behavior. Using the finite difference method we obtained a numerical solution of the mixed-mode loading problem with boundary conditions corresponding to two cases of crack propagation. Within the framework of the proposed approach we estimated the dependencies between mixity parameters and various loading parameters and crack inclination angle for a range of strain hardening exponent values, which dependencies closely fit the experimental data. __________ Translated from Problemy Prochnosti, No. 4, pp. 46 – 63, July – August, 2005.     

Plastic anisotropy in aluminum and copper pre-strained by equal channel angular extrusion

The mechanical response of as-processed equal channel angular extrusion materials is anisotropic, depending on both direction and sense of straining. The stress–strain curves exhibit hardening characteristics different from the usual work hardening responses, e.g., Stages I–IV, expected in annealed fcc metals under monotonic loading. In this work, the anisotropic flow responses of two pure fcc metals, Al and Cu, processed by route Bc are evaluated and compared based on pre-strain level (number of passes), direction of reloading, sense of straining (i.e., compression versus tension), and their propensity to generate subgrain microstructures and to rearrange, should the slip activity change. In most cases, either macroscopic work softening or strain intervals with little to no work hardening are observed. Application of a crystallographically based single-crystal hardening law for strain-path changes [Beyerlein and Tomé, Int. J. Plasticity (2007)] incorporated into a visco-plastic self-consistent (VPSC) model supports the hypothesis that suppression of work hardening is due to reversal or cross effects operating at the grain level.

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Analytical study of the elastic–plastic stress fields ahead of parabolic notches under antiplane shear loading

An analytical study is carried out on the elastic–plastic stress and strain distributions and on the shape of the plastic zone ahead of parabolic notches under antiplane shear loading and small scale yielding. The material is thought of as obeying an elastic-perfectly-plastic or a strain hardening law. When the notch root radius becomes zero, the analytical frame matches the solutions for the crack case due to Hult–McClintock (elastic-perfectly-plastic material) and Rice (strain hardening material). The analytical frame provides an explicit link between the plastic stress and the elastic stress at the notch tip. Neuber’solution for blunt notches under antiplane shear is also obtained and the conditions under which such a solution is valid are discussed in detail by using elastic and plastic notch stress intensity factors. Finally, revisiting Glinka and Molski’s equivalent strain energy density (ESED), these factors are used also to give, under antiplane shear loading, the increment of the strain energy at the notch tip with respect to the linear elastic case.