Determination of the energy of electroplastic deformation of metals

On the basis of the well-known mechanism of electroplastic deformation, we suggest a method for determination of the fraction of energy of pulses of the electric current directly spent on the work of plastic deformation. The experimental data presented in the work confirm the validity of the proposed approach. The present work was partially financially supported by the International Science Foundation and the Ukrainian Government (Grant UBW 200). Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 38–43, July–August, 1997.     

The nature of the electroplastic effect in metals

On the basis of an analysis of the known regularities of the electroplastic effect (EPE) the article suggests a mechanism of plastic deformation of metallic materials when electric current pulses (ECP) act; in accordance with this mechanism the energy of conduction electrons is transferred directly to dislocations and is realized in the form of plastic work. The plastic strain rate at the instant when an ECP acts increases in consequence of the increased density of the moving dislocations and the mean speed at which they move. The article shows that the suggested mechanism makes it possible to describe all the known regularities of the EPE. With the aid of theoretical modeling of electroplastic deformation by the suggested mechanism new regularities of the EPE were established: when ECP act, the temperature of the deformed material is lower than that calculated by the Joule law of electric heating; the threshold value of the current density determining a jump of the load becomes lower with increasing degree of plastic deformation of the material, etc. The established regularities of the EPE were experimentally confirmed.Translated from Problemy Prochnosti, No. 1, pp. 47–51, January, 1991.     

On evolution equations for elastic deformation and the notion of hyperelasticity

For elastic-plastic and elastic-viscoplastic materials it is possible to introduce an evolution equation for an elastic deformation measure. Also, it is possible to develop constitutive equations for which the stress and strain energy are functions of elastic deformation only, the stress is determined by a derivative of the strain energy function and the associated material response is rate-independent and non-dissipative in the absence of the rate of inelasticity. Yet, these equations do not necessarily exhibit hyperelastic response in the elastic range. The objective of this paper is to emphasize the importance of satisfying an additional condition that requires the work done between two configurations to be insensitive to the history and rate of total deformation. This work condition places restrictions on the evolution equation which ensure that the integrated elastic deformation measure is a function of total deformation only. Also, it is argued that there is no need to complicate the evolution equation for elastic deformation to accommodate alternative strain measures since the nonlinearities of these strain measures can be absorbed into the form of the strain energy function.     

On nonlocal regularization in one dimensional finite strain elasticity and plasticity

The purpose of this article is to explore in details the theoretical and numerical aspects of the behavior of spatial trusses, undergoing large elastic and/or elastoplastic strains. Two nonlocal formulations are proposed in order to regularize the problem, avoiding the mesh dependence of the numerical response. The classical example of a simple bar in tension is chosen to explore the various features of the proposed models and to highlight the interplay between material and geometrical nonlinearity in the localization.Aknowledgement The financial support of FAPESP, a Brazilian research funding agency and of the italian miur, project prin 2003082105, are greatly appreciated.     

Effect of electroplastic rolling on the ductility and superelasticity of TiNi shape memory alloy

The brittle TiNi shape memory alloy strips were processed by electroplastic rolling (EPR) without intermediate annealing. The maximum thickness reduction of the alloy strip in the individual EPR pass can be 21.6% and the total deformation in terms of thickness reduction can reach 74% in the seven passes of EPR processing. The ductility of the TiNi alloy is enhanced by electropulse due to the dynamic recrystallization in the process. The rolling separating force in EPR drops compared to the cold rolling. Furthermore, the EPR processed TiNi after annealing exhibits an excellent superelasticity and the recoverable strain of 6% is obtained.     

On the behaviour of a particulate metal matrix composite subjected to cyclic temperature and constant stress

The paper describes a method of characterising the behaviour of an idealised particulate metal matrix composite composed of elastic particles and an elastic–perfectly plastic matrix subjected to constant macro stress and a cyclic temperature history. The computational method, the Linear Matching Method, was originally developed for structural life assessment studies, and allows a direct evaluation of the load ranges for which differing modes of behaviour occur in the steady cyclic state; shakedown, reverse plasticity and ratchetting. A simple homogenised model is considered, consisting of spherical particles embedded in a cubic matrix array. The resulting solutions are presented as non-dimensional equations derived from numerical solutions for two composites, alumina and silicon carbide particles embedded in an aluminium matrix.     

Dynamic plastic behaviour of annular plates with transverse shear effects

The dynamic response of a rigid-plastic annular plate clamped at the outer edge and free at the inner edge is considered.     

On yield criteria in plasticity coupled with damage

The coupling of elasto-plasticity and damage is still the subject of many papers, namely in the construction of evolution laws for internal parameters. In this paper we propose to partly overcome some difficulties by using the only basic fact that a plastic-damaged body is made up of two physical constituents, namely the matrix material and the micro-defects. The elasto-plastic matrix material is the resisting skeleton connecting the elements of the body and the micro-defects bring their own contributions to reversible and irreversible strains. Naturally strong couplings exist between the stress states of the body and the matrix material. These considerations will lead to generalisations and new formulations of the so-called equivalence principles and a new equivalence principle will be proposed. Finally, concerning the irreversible strains due to both plasticity of the matrix and growing of micro-defects, we will prove that yield conditions must be used simultaneously on the body as well as on the matrix, leading to some non-smooth resultant yield-damage surface. The modified Gurson model for porous material is then analysed in order to illustrate this last point. In this paper, large strains are considered, but time-dependant plasticity and thermal effects are excluded.     

电轧AZ31镁合金在模拟海水中腐蚀行为研究

为对比研究高能电脉冲轧制工艺和冷轧工艺对AZ31镁合金腐蚀性能的影响,采用腐蚀形貌观察、动电位极化测试、电化学阻抗谱与腐蚀速度测试等方法系统地研究了高能电脉冲轧制和冷轧AZ31镁合金带材在模拟海水(3.5%NaCl)中的腐蚀行为.结果表明:在同样变形量下,与冷轧AZ31镁合金相比,电轧AZ31镁合金的耐腐蚀性略有提高.这与电轧AZ31镁合金再结晶比例大,位错密度小,具有较低能态的位错组态及能形成较稳定的腐蚀产物膜有关.     

Overload effects in fatigue crack growth by crack-tip blunting

A basic mechanisms for fatigue crack growth in ductile metals is that depending on crack-tip blunting under tensile loads and re-sharpening of the crack-tip during unloading. In the present paper, the effect of an overload in one of the cycles is studied based on this mechanism. In a standard numerical analysis accounting for finite strain, it is not possible to follow the blunting/re-sharpening process during many cycles, as severe mesh distortion at the crack-tip results from the huge geometry changes developing during the cyclic plastic straining. Here, based on an elastic-perfectly plastic material model, crack growth computations are continued up to 700 full cycles by using remeshing at several stages of the plastic deformation. Crack growth results for purely cyclic loading are compared with predictions for cases where an overload is applied, and it is shown how crack growth slows down after the overload. Different load amplitudes, and an overload at different cycle numbers are considered.     

On amorphization as a deformation mechanism under high stresses

In this paper we review the work related to amorphization under mechanical stress. Beyond pressure, we highlight the role of deviatoric or shear stresses. We show that the most recent works make amorphization appear as a deformation mechanism in its own right, in particular under extreme conditions (shocks, deformations under high stresses, high strain-rates).     

Generalized additivity rule and isokinetics in diffusion-controlled growth

Validity of traditional additivity rule in diffusion-controlled growth is discussed. This process has a memory of thermal history due to temperature-dependent interface concentrations. When the thermodynamics is involved, the application of additivity rule should be carefully considered. By introducing a thermal history-related function, generalized isokinetic hypothesis and additivity rule involving the thermal history-dependent instantaneous reaction rate are proposed. According to the exact solutions of diffusion-controlled growth, the generalized additivity rule is analyzed, discussed, and applied well.     

The influence of electroplastic rolling on the mechanical deformation and phase evolution of Bi-2223/Ag tapes

Electroplastic rolling (EPR) of Bi-2223/Ag superconducting wires was performed, where pulse currents were applied during rolling to introduce an electroplastic effect. It was found that the rolling force decreased significantly compared with the traditional rolling process. Furthermore, EPR favorably minimized the sausage effect. It is revealed that the electroplastic effect can facilitate the mechanical deformation of Bi-2223/Ag composites. Segments of the Bi-2223/Ag tapes were heat treated at 830 °C for different time periods. The phase assemblies of these samples suggest that current pulses contribute to faster transformation kinetics from the Bi-2212 phase to the Bi-2223 phase. In addition, a preliminary improvement of 28% of critical current density has been achieved in a fully processed tape with EPR.