Influence Of Plastic Strain Research Articles

Influence of Plastic Strain on Transformation Behavior of Shape Memory Alloys

Influence of Plastic Strain on Transformation Behavior of Shape Memory Alloys

Evaluation of Hydrogen Embrittlement for High Strength Steel Sheets

Steel sheets for automobiles are usually formed into various parts by cold working. Therefore, plastic strain introduced by the cold working must be considered as a factor affecting the hydrogen embrittlement in addition to the applied stress and the content of diffusible hydrogen entered into steels, which are considered as factors in the studies of high strength steel bolts. However, there are few detailed reports investigating the influence of these factors on hydrogen embrittlement of steel sheets quantitatively. In this study, the influence of plastic strain, as well as stress and diffusible hydrogen content, on hydrogen embrittlement of steel sheet was quantitatively studied to evaluate the hydrogen embrittlement susceptibility of steel sheet by using an 1180 MPa grade cold rolled dual phase steel sheet. Plastic strain was introduced by U-shape bending, and stress was applied by tightening the bent specimen with a bolt. Then, hydrogen was introduced by dipping in hydrochloric acid, and the time to fracture and the content of diffusible hydrogen entered into steel during dipping were investigated. The fracture was promoted by severe deformation near the bending limit, and it seemed to be caused by the presence of micro cracks and/or micro voids. The hydrogen cracking conditions region of the steel sheet were mapped in the three-dimensional space with the axes of applied strain, applied stress and diffusible hydrogen content. It was considered that the evaluation of the risk of delayed fracture of automotive parts made of the steel sheet under service environment was possible by a comparison of the 3D map and the service conditions of the parts.

Generalized Coupled Thermoplasticity Taking into Account Large Strains: Part I. Conditions of Uniqueness of the Solution of Boundary-Value Problem and Bifurcation Criteria

The global and local conditions of uniqueness and the criteria excluding a possibility of bifurcation of the equilibrium state for large and small strains are derived. The conditions and criteria are derived analyzing the problem of uniqueness of solution of the basic incremental boundary problem of coupled generalized thermoplasticity. This paper is a continuation of some previous work, but contains new derivation of the global and local criteria excluding a possibility of bifurcation of the equilibrium state for a comparison body dependent on statically admissible fields of stress velocity. All the thermomechanical coupling effects, non-associated laws of plastic flow and influence of plastic strains on thermoplastic properties of a body were taken into account in this work. Thus, the mathematical problem considered here is not a self-conjugated problem.

Evaluation of Hydrogen Embrittlement for High Strength Steel Sheets

The risk of delayed fracture should be evaluated when applying ultra high strength steel sheets to automotive parts. Steel sheets for automobiles are usually formed into various parts by cold working. Therefore, plastic strain introduced by the cold working must be considered as a factor affecting the hydrogen embrittlement in addition to the applied stress and the content of diffusible hydrogen entered into steels, which are considered as factors affecting the hydrogen embrittlement of high strength steel bolts. In this study, the influence of plastic strain, as well as stress and diffusible hydrogen content, on hydrogen embrittlement of steel sheets was quantitatively studied by using an 1180 MPa grade cold rolled steel sheet. Plastic strain was introduced by U-shape bending. Stress was applied by tightening the bent specimen with a bolt. Then, hydrogen was introduced by immersing in hydrochloric acid. The time to fracture and the content of diffusible hydrogen entered into steel were investigated. The fracture was promoted by severe deformation, and it seemed to be caused by the presence of micro cracks and/or micro voids. The hydrogen cracking conditions region of the steel sheet was mapped in the three-dimensional space with the axes of applied strain, applied stress and diffusible hydrogen content. It was considered that the evaluation of the risk of delayed fracture of automotive parts made of the steel sheet under service environment was possible by a comparison of the hydrogen cracking conditions and the service conditions of the parts on the 3D space.

Influence of plastic strain on deformation-induced martensitic transformations

The effects of plastic strain on deformation-induced martensitic transformations have been investigated experimentally. Austenitic metastable stainless steel samples were heated to a temperature at which the transformation is suppressed and were plastically strained to different amounts. The resulting pre-strained material was cooled to room temperature and a tensile test was conducted during which transformation was monitored via a magnetic sensor. Results of these tests are discussed concerning the existing theories that describe the mechanically induced martensitic transformation phenomenon.

Influence of the plastic strain on the hydrogen evolution reaction on polycrystalline nickel electrodes in H 2S0 4

The influence of plastic strain on the hydrogen evolution reaction (HER) on polycrystalline nickel, in H 2SO 4 electrolyte at 293 K, is studied a cathodic potential range. Linear polarisation curves and electrochemical impedance spectra show that the HER is described by a mechanism of Volmer–Heyrovsky, which parameters do not depend on plastic strain. The plastic strain modifies the density and the distribution of dislocations. These evolutions are well understood and lead to changes in the number of hydrogen additional adsorption sites, and then to changes in the current density developed on pre-strained samples. From 0 to 2.4% of plastic strain, the current density increases to reach a maximum value at 2.4%. This current density increase is directly linked to the dislocation density increase. Over 2.4%, the distribution of dislocations is modified and the current density decreases but keeps higher than the current density on an unstrained sample.

Studies of SCC and Hydrogen Embrittlement of High Strength Alloys Using Fracture Mechanics Methods

Fracture mechanics based test and evaluation techniques are used to gain insight into the phenomenon of stress corrosion cracking (SCC) and to develop guidance for avoiding or controlling SCC. Complementary to well known constant load and constant deflection test methods experiments that are based on rising load or rising displacement situations and are specified in the new ISO standard 7539 – Part 9 may be applied to achieve these goals. These are particularly suitable to study cases of SCC and hydrogen embrittlement of high strength steels, aluminium and titanium alloys and to characterise the susceptibility of these materials to environmentally assisted cracking. In addition, the data generated in such R-curve tests can be used to model the degradation of the material caused by the uptake of atomic hydrogen from the environment. This is shown for the case of a high strength structural steel (FeE 690T) where in fracture mechanics SCC tests on pre-cracked C(T) specimens a correlation between the rate of change in plastic deformation and the crack extension rate due to hydrogen embrittlement was established. The influence of plastic strain on the hydrogen diffusion was additionally studied by electrochemical permeation experiments. By modelling this diffusion based on the assumption that trapping of the hydrogen atoms takes place at trap sites which are generated by the plastic deformation, a good agreement was achieved between experimentally obtained data and model predictions.

Variations of work function and corrosion behaviors of deformed copper surfaces

Surface work function (WF) and the corrosion behavior of copper under influence of plastic strain were investigated using experimental and computational approaches. It was observed that both the corrosion potential (Ecorr) and the WF decreased while the corrosion rate increased with an increase in plastic strain, indicating that the strained surface layer became more electrochemically active. Ecorr and WF eventually became stable when the plastic strain reached a certain level. However, the corrosion rate continuously increased. It was demonstrated that this continuous increase in corrosion rate could be dominated by the dislocation density rather than the corrosion potential. The study has shown that the WF is closely related to the corrosion potential and could thus be a sensitive parameter for investigating mechanisms responsible for corrosive wear. The investigation of the effects of plastic deformation on the corrosion behavior would help to fundamentally understand the synergism of wear and corrosion.

On structural sensibility of work function

Work function of metals decreases under the influence of plastic strain and fatigue tests. The higher the degree of strain, the larger the work function drops. The phenomenon is related to motion of dislocations on the surface. Creation of charged steps on a surface is a precursor to crack nucleation.

Thermomechanical characterization of NiTiCu and NiTiSMA actuators: influence of plastic strains

The focus of this study is the thermomechanicalcharacterization and comparison between two different shapememory alloys (SMAs) quantifying the effect of plastic strain onthe transformation characteristics of SMA actuators. In thisstudy, the thermomechanical response and transformationcharacteristics of a NiTiCu and a NiTi SMA are studied as afunction of the induced plastic strain for four differentloading paths: (1) an elastic-plastic loading of the austeniticphase, (2) a stress-induced martensitic phase transformation, (3)an elastic-detwinning-plastic loading of the martensitic phaseand (4) a thermally-induced phase transformation under a constantapplied stress. Each loading path is repeated multiple times,with an incremental change of the total applied strain, todetermine the effect of accumulated plastic strain on the phasetransformation characteristics of the two SMA material systems.The effects of plastic strain are quantified by measurements ofrecoverable strain during a thermal cycle under zero appliedstress, measurements of heat of transformation from adifferential scanning calorimeter and microstructuralevaluations.

A Study of the Influence of Plastic Strain upon Fractal Dimensional Increment of an Aged Al-Si-Ge Alloy Based on Synthetic Fractal Profiles and Dimple Pattern of Fracture

A theoretical model was developed to correlate the fractal dimension, D, of ductile tension fracture surfaces of an aged Al-0.57Si-2.03Ge alloy with the macroscopic fracture strain, e f , based on the general definitions of D and the roughness parameter, R, as well as on a linear relation developed between e f and the natural logarithm of the scaling factor, (1/n), irrespective to the self-similar system to which it is referred. Two examples of synthetic self-similar profiles were used to show the general connection between D, R and e f . The higher the scaling factor, the higher the fracture strain and the lower the fractal dimension. This result suggests an increase of the shear stress component as the ductility increases, which generates an increase of the fracture surface flatness.

Magnetism and internal stresses : Concept of magneto-plastic anisotropy

This paper deals with the influence of plastic strains and correlated internal stresses on the magnetic properties of nonoriented 3%Si-Fe alloy. We first studied the mechanical behaviour of the material and evaluated the internal stresses which occur with the strengthening. The alloy exhibits an isotropic mechanical behaviour and three stages of straining. The intemal stresses are evidenced thanks to tensile / compressive tests: a high initial increase followed by a gradual one is observed. The internal stresses are linked to the drop of stress at the first stages of the deformation and to the heterogeneous structures of dislocations at higher plastic strain levels. The effect of the internal stresses on the magnetic properties is then discussed. A suitable magnetic frame has been manufactured. Magnetic characteristics are investigated along either the tensile or cross directions. The magnetic properties strongly decrease from the beginning of the plastic strain. Crossing experiments indicate an attenuated effect of the plastic strain on the magnetic properties compared to the colinear measurements. This directionnal effect, called magnetoplastic anisotropy seems to be directly linked to the internal stresses.

An iterative approach to mechanical properties of MMCs at the onset of plastic deformation

The current work presents a generalized Eshelby model allowing interaction among reinforcing particles under a Mori-Tanaka like scheme. Different aspect ratios and geometries are studied in the elastic and incipient elasto-plastic regime for a model SiC Al composite. The fibers are taken as purely elastic and the matrix is regarded elastic perfectly plastic responding to a Von Mises yield criterion. The phenomenon of plastic localization in the vicinities of the inclusions is carefully described for different reinforcement volume fractions and thermo-mechanical loading. Equivalent stress, hydrostatic pressure and elastic and plastic strains are depicted as contour levels around a representative inclusion. Effective coefficients of thermal expansion of composites are calculated both under purely elastic composite response and at the onset of plastic localized deformation. The influence of plastic strain over those effective coefficients is shown to be detectable. The simulated stress-strain curves show the influence of interaction stresses over macroscopic yield stress by isolating this phenomenon from matrix hardening. Accumulated elastic energies and plastic work are calculated to show the different nature of purely thermal and mechanical loads.

Influence of plastic strain on magnetic behaviour of non-oriented Fe–3Si and application to manufacturing test by punching

The prediction of core losses in a rotating electrical machine generally requires a numerical approach (finite element or integral method). However, the magnetic characteristics obtained through a standard Epstein facility and inserted in the numerical models predict losses which are quite different from those obtained experimentally. This difference can be partly explained by the presence of plastic strain close to the cutting edge of manufactured motor sheets. In the present paper, numerical functions are first established to predict the change in the magnetisation and loss curves with respect to the plastic strain level. Second, a microhardness test is used to measure the local plastic strain directly on the manufactured sheet. Finally, relations are obtained between the main magnetic properties and the position on the sheet: the resulting magnetic property profiles allow the numerical approach to be improved.MST/2039

Influence of plastic strain on magnetic behaviour of non-oriented Fe–3Si and application to manufacturing test by punching

AbstractThe prediction of core losses in a rotating electrical machine generally requires a numerical approach (finite element or integral method). However, the magnetic characteristics obtained through a standard Epstein facility and inserted in the numerical models predict losses which are quite different from those obtained experimentally. This difference can be partly explained by the presence of plastic strain close to the cutting edge of manufactured motor sheets. In the present paper, numerical functions are first established to predict the change in the magnetisation and loss curves with respect to the plastic strain level. Second, a microhardness test is used to measure the local plastic strain directly on the manufactured sheet. Finally, relations are obtained between the main magnetic properties and the position on the sheet: the resulting magnetic property profiles allow the numerical approach to be improved.MST/2039

On the influence of plastic deformation on discontinuous precipitation in MgAl

The influence of a prestrain on intragranular precipitation is well known: the dislocations created by plastic deformation act as nucleation sites for heterogeneous precipitation. The influence of a similar prestrain on discontinuous precipitation is much less documented. There appears to be no general rule: depending on the systems considered, the kinetics of discontinuous precipitation can either be accelerated or slowed down. Moreover, in certain alloys, discontinuous precipitation is associated with recrystallization phenomena. In this paper, the authors results on the influence of plastic strain at room temperature on discontinuous precipitation in a Mg-8.5 wt% Al alloy. The interesting feature of this alloy is that plastic deformation induces both dislocation glide and twinning.

Birefringence of plastically deformed poly(methyl methacrylate)

The birefringence of poly(methyl methacrylate) (PMMA) has been studied for samples deformed plastically in plane strain compression. The results have been analysed in a semi-quantitative manner based on the theory of birefringence for rubbers. The results suggest that the molecular chains are bound by cohesion points which are dissociated by increasing temperature or under the influence of plastic strain. It is suggested that these cohesion points can be attributed to interactions between the dipoles of the ester side groups.

The influence of plastic strain upon the aging characteristics of alloys

The conditions under which dislocations can influence the decomposition of supersaturated solid solutions with different degrees of supersaturation are discussed. It is shown that the major effects are associated with the formation of intermediate precipitates and with low degrees of super saturation. Examples are then given of the effects of prior plastic strain upon aging reactions in aluminum, nickel and copper based alloys. The changes in mechanical properties are related to the observed changes in microstructure. It is argued that with martensitic steels the tempering reactions are not influenced by prior plastic strain, since the dislocation density is already high as a result of the martensitic reaction; however plastic strain after initial temper, followed by a further temper can produce marked changes in structure and properties. It is shown that in general prior plastic strain can considerably shorten the time cycle for heat treatment and it may at the same time induce improved mechanical properties.

On the influence of plastic strain on magnetomechanical damping in iron and nickel

Abstract The influence of plastic strain on the room temperature magnetomechanical damping, δm, was studied in pure nickel and iron. Strains up to ∼2% produced similar reductions in the damping of both metals. At ∼6% strain the magnetic peak in nickel was barely perceptible, but in iron it had stabilized at ∼⅕ of the value in the annealed specimen, and was still evident after 19% strain. This anomalous behaviour is thought to be due to the higher crystalline anisotropy of iron, such that K 1 is always greater than the stress anisotropy γσi.

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