Bauschinger Curves Research Articles

TECHNOLOGICAL PASSPORT STUDY PISTON AND CONNECTING ROD BARE MATERIAL AXIAL ROTARY PISTON PUMP

A technological passport of the alloy steel material has been formed, from which a piston-connecting rod pair of an axial-rotor piston pump is made in the form of functions: flow curve, plasticity diagram, Bauschinger curve, calibration graph. Using the approximation of the neck contour by the Gaussian function at any moment of deformation of a cylindrical specimen in the supercritical region, the main parameters used in engineering calculations are related: the minimum diameter of the specimen, the current diameter of the specimen with a given coordinate, the relative (absolute) elongation and contraction with the parameters referred to the moment of rupture sample. In addition, the elongation and contraction of the sample after fracture are unambiguously associated. The approximation avoids the inconvenient and relatively expensive video filming for studying the history of deformation. It is shown that the effect of heat treatment on the mechanical properties of these steels is not the same for different stress state schemes. The tensile strengths of these steels are close as delivered. The plasticity characteristics of steel 30Kh3MFA exceed the plasticity of steel 38Kh2MYuA by 1.5 times. The flow curves of the steels under consideration have the following tendency: so the coefficient of approximation of the flow curve for 38Kh2MYuA steel in the delivery state exceeds its value in relation to steel 30Kh3MFA by 1.23 times. After heat treatment of these steels, the value of the approximation coefficient increases: for steel 38Kh2MYuA by 1.11 times, for steel 30Kh3MFA by 1.46 times. The results obtained make it possible to control the mechanics of the technological operation of rolling the piston-connecting rod pair and create a process control mechanism in order to prevent rejects during the rolling process, which manifests itself in the form of going beyond the tolerance field for the axial clearance of the piston-connecting rod pair, as well as destruction during rolling of the inner part of the piston.

Evaluation of metal plasticity and research on the mechanics of pressure treatment processes under complex loading

A calculating apparatus has been developed to help evaluate the stress-strain state in the plastic forming processes accompanied by a complex loading in which the Bauschinger effect can be manifested. The developed methods are based on the phenomenological approach in which a material map is devised in the form of flow curves, Bauschinger curves, a function j characterizing the hereditary influence of the loading history, plasticity diagrams, and limiting strain surfaces. To evaluate the plasticity resource used, taking into account the non-monotonicity of loading, the relations for determining the main components of the guiding strain-rate tensor were obtained, which made it possible to simplify the calculation of the components of the damage tensor. With the help of the developed calculating apparatus, the applied plasticity resource was evaluated in the process of radial extrusion with the contour sag, which allowed setting the limiting parameters of the shaping. Also, as a result of the research, an increase in the plasticity of the metal was established by selecting rational loading paths in the space of the dimensionless coordinates h, m s , and e u . For example, in the process of radial extrusion with the contour sag due to a change in the nature of the deformation, it was possible to obtain a flange diameter of 44 mm, with the initial diameter of the cylindrical workpiece being 20 mm. The results obtained are important because in most cases the processes of metal pressure treatment are accompanied by non-monotonic deformation

Absence of yield points in iron on strain reversal after aging, and the Bauschinger overshoot

The yield phenomenon is absent in low carbon steel if the direction of straining after full aging is opposite to that just before aging. That this is not the consequence of macroscopic residual stresses is demonstrated by torsion experiments with thin-walled tubular specimens provided with a shallow circumferential groove, in which yield takes place by widening of a single circumferential Lüders band with the elimination of significant internal stresses. If the straining is interrupted after partial yielding of the annealed specimen and then stress is re-applied immediately without aging, the consequences are opposite. Yielding continues at the normal level if the direction of straining is not changed; if it is reversed, there is a normal Bauschinger effect in the previously yielded volume, but the Bauschinger stress–strain curve rises without discontinuity considerably above the former yield level. When the Lüders band begins to widen, the stress drops abruptly from the overshooting Bauschinger curve to the normal yield level and remains constant until the end of the yield. Repeated strain aging with straining in the same direction gives yield points rising with the total strain roughly as the strain hardening rises with the strain along the uninterrupted stress–strain curve. After each yielding the rate of strain hardening is roughly the same as it is in uninterrupted straining at the same strain, but the curve is displaced to higher stress levels which rise with the number of aging treatments. The “permanent strain-age-hardening” may reach 30% or more of the normal yield stress at the same strain.

Bauschinger effect and multiaxial yield behavior of stress-reversed mild steel

Thin-walled cylindrical specimens subjected to torsional prestraining are stress-reversed along the Bauschinger curve. The Bauschinger effect (BE), yield behavior, and flow behavior of the stress-reversed mild steel were examined by using combined loadings of axial load, internal pressure, and torsion. The results indicate that the stress-reversed steel has the same yield stress at 0.2 pct offset strain in reloading tests of forward and reverse torsion, when the reverse strain is \(\bar \varepsilon _1^p \)=0.77 pct. Furthermore, it is possible to cause the yield stresses in forward and reverse torsion to coincide in any offset strain. The yield locus of the stress-reversed steel is symmetric with respect to the tensile stress axis in a tension-torsion stress field. However, it has been found to be an anisotropy in the stress-reversed steel, and the magnitude of anisotropy is related to the offset strain. For example, there is a stronger anisotropy at 0.2 pct offset strain than at 2 pct offset strain, even though the BE is eliminated for the former. It is shown that the reduction in the BE by stress reversal is concerned with the relief of the long-range back-stress generated by prestraining. Besides, the roles of aging in the stress-aging process lie in the contributions to age hardening and development of directional back-stress.

Yield behavior of a mild steel after prestraining and aging under reversed stress

Tubular specimens of mild steel are subjected to tensile prestraining and then aged under compressive stresses along the Bauschinger curve, at various temperatures below 175 °C. The compressive (reversed) stress imposed on the specimen during aging, called here the aging stress, is employed up to a maximum level of 90 pct of the prestress. Reloading tests in tension and in compression are carried out at room temperature, and the critical aging stress at which the yield stresses in the two reloading directions become equal is determined as a function of aging temperature. The critical aging stress decreases with increasing aging temperature and becomes constant at aging temperatures above 100 °C. Under such critical aging conditions, the aging index, i.e., the ratio of yield stress to prestress, rises with increasing aging temperature and approaches a saturation value of 0.86 at 150 °C. It is shown that stress aging is an effective means of reducing the Bauschinger effect while preserving the work hardening induced by the prestrain.

Multiple-slip work-hardening model in crystals with application to torsion-tension behaviors of aluminium tubes

Abstract Torsion-tension behaviors of aluminium tubes are simulated numerically using the finite-element polycrystal model. As a multiple-slip work-hardening model in crystals, the backlash model and the maximum forest dislocation model are proposed to account for Bauschinger effect in reverse loading and cross effect (latent hardening) in non-proportional loading, respectively. The simulations with the new microscopic constitutive law predict well Bauschinger curves, cyclic loading curves and cross loading curves.

Effect of temperature variation on plastic behavior of S25C mild steel

Plastic behavior of S25C mild steel was investigated at constant and variable temperatures in the range from room temperature to 400°C. The combined loading tests were carried out with the thin walled tubular specimens by applying the axial and torsional loads. The initial yield condition of the material obeyed Tresca's law, but the stress-strain relation after yielding followed neither Tresca's nor Mises's laws. Bauschinger's curves in the torsional loading conditions were different from those in the axial loading conditions. The stress-strain relations subsequent to the tensile pre-strain were obtained at various temperatures. The experimental results were compared with the isothermal stress-strain relation.

Influence of loading sequence on the Bauschinger curve.

In case of some kinds of material, distinct difference between pre-tensed and pre-compressed Bauschinger's stress-strain curves occurs. Experiments of copper test pieces were conducted in pre-strain region to about 30% in this paper. From the test data, the ratio of two stress values in the narrowest point between simple stress-strain curve in pre-strain direction and reversely loaded stress-strain curve which is shifted by pre-strain value and shown in pre-strain direction is approximately 0.94, not depending on kinds of pre-strain and pre-strain value. Therefore the above said difference of Bauschinger's curve is derived from difference between simple tensile and compressive stress-strain curves in some kinds of material. And also this phenomenon can be explained by observation of slip bands.

A Description of Bauschinger Curves Based on the Random Barriers Theory

A new approach to describing the Bauschinger curves for metals is presented on the concept of the field of the resisting force against the movement of dislocations. In a previous paper, it has been clarified that the cyclic plasticity model, proposed by the authors based on this concept, would be applicable to the quantitative descriptions of various cyclic deformation phenomena in the steady state, and the tensile stress-strain curves with a different type of yielding. In this paper, the Bauschinger curve can be expressed by applying this concept with some assumptions of the changes in the field of the resisting force caused by pre-loading. This has provided a satisfactory agreement with each experimental curve for a carbon steel and a brass with a wide range of strains.

A description of Bauschinger curves based on the random barriers theory.

A new approach to describe the Bausuchinger curves for metals is presented by means of the concept of the field of resisting force against the movement of dislocations. In previous papers, it has been clarified that the cyclic plasticity model, proposed by the authors based on this concept, could be applicable to the quantitative descriptions of the various cyclic deformation phenomena in the steady state, and the tensile stress-strain curves with a different type of yielding. In this paper, the Bauschinger curve can be expressed by applying this concept with some assumptions of the changes in the field of resisting force caused by pre-loading. This has provided satisfactory agreement with each experimental curve for carbon steel and brass with a wide range of strain.

Bauschinger Curves at Large Strain

Torsional tests of aluminium pipe specimens were carried out to obtain reliable data on Bauschinger curves at large strain. The experimental results revealed some features of Bauschinger curves, which suggest an interpretation that Bauschinger effect is a kind of back lash phenomenon.

Bauschinger Curves at Large Strain

Bauschinger Curves at Large Strain

The Bauschinger Effect of Aluminum Single Crystals Tested under Plane-Strain Compression

A plane-strain compression test was made to investigate the Bauschinger effect in aluminum single crystals which deform by multiple slip. First, the testing method, especially as to the clearance and the lubrication between crystal and die, was discussed in comparison with torsion test. The Bauschinger curves of crystals with various non-symmetrical orientations were represented by the shear stress and the shear strain determined from the forward and the reverse compression data using the analysis of Taylor-Bishop-Hill. Here, in the reverse compression the free plane of crystal in the forward compression was compressed without changing the restricted plane. The analysis did not always produce satisfactory results in the permanent softening. This is due to the lattice rotation during deformation. Nevertheless, the average curve of single crystals was found to be in good agreement with the Bauschinger curve of polycrystal. Furthermore, the dislocation structure was examined by transmission electron microscopy and also an examination of slip system derived from the analysis was tried.