Bauschinger Tests Research Articles

Magnetic evaluation of Bauschinger effect in marine engineering steels

In the present study, the association between the magnetic behavior of steels the Bauschinger effect was evaluated. For this reason, three different grades of steel were subjected to typical Bauschinger tests involving a single reversal from tension to compression loading, for the same amount of pre-strain. The reversal compression lasted until the opposite of the pre-strain's maximal stress was reached. As a nondestructive magnetic technique for determining the magnetic properties of the examined samples, magnetic Barkhausen noise was chosen. For the characterization of the Bauschinger effect, four indicators were used, in terms of strain, stress, and energy. Mechanical stresses alter material magnetization, but linking Bauschinger factor with magmatic noise parameters has not been widely studied. Comparing Bauschinger Effect parameter values and magnetic Barkhausen methods, results show consistency with ferromagnetic samples' mechanical behavior. As Bauschinger Effect becomes more forceful, disorder density and magnetic wall anchoring decrease. A new proposed parameter, the exponent ξ, is directly related to effect rate. The magnetic Barkhausen noise method and its peak position and full width half minimum parameters can be a promising complementary measure for evaluating BE contribution in cycle loading ferromagnetic steels. Finally, the applicability of the examined techniques for the non-destructive characterization of steel degradation was discussed.

{332} mechanical detwinning as a deformation mechanism in the β-metastable Ti-15Mo alloy during cyclic loading

The cyclic shear study of the Ti-15Mo (wt.%) alloy, with twinning induced plasticity deformation mechanism, shows that whereas conventional {332}<113> twinning is obtained during forward loading, room temperature backward loading in Bauschinger tests are able to cause twins reversion, suggesting that detwinning can act as a proper deformation mechanism. After reversion, the mechanical twins leave behind residual bands with low misorientation (∼2°) in the matrix, and secondary twins. When the backward deformation is continued, new twins of different {332}<113> systems favorable to backward stress are then produced in the microstructure. The formation of the various deformation products is rationalized by trace analysis and calculation of the resolved shear stress on the considered twinning systems.

Significant Bauschinger effect and back stress strengthening in an ultrafine grained pure aluminum fabricated by severe plastic deformation process

Bauschinger test in uniaxial tension-compression mode was carried out for the first time on the pure Al specimens having homogeneous ultra-fine grained (UFG) microstructures fabricated by equal-channel angular pressing (ECAP) and subsequent annealing processes. Significant Bauschinger stress (transient softening), Bauschinger energy parameter and their strong dependences on the tensile plastic pre-strain at the very early stage of the tensile deformation were measured in the UFG specimens, in sharp contrast to their coarse-grained (CG) counterpart. The grain size dependence of the Bauschinger effect in pure Al was qualitatively discussed in terms of the back stress arising from the formation of dislocation pile-up against the grain boundary during plastic deformation.

Bauschinger Effect in an Austenitic Steel: Neutron Diffraction and a Multiscale Approach

The generation of internal stresses/strains arising from mechanical deformations in single-phase engineering materials was studied. Neutron diffraction measurements were performed to study the evolution of intergranular strains in austenitic steel during sequential loadings. Intergranular strains expand due to incompatibilities between grains and also resulting from single-crystal elastic and plastic anisotropy. A two-level homogenization approach was adopted in order to predict the mechanical state of deformed polycrystals in relation to the microstructure during Bauschinger tests. A mechanical description of the grain was developed through a micro–meso transition based on the Kroner model. The meso–macro transition using a self-consistent approach was applied to deduce the global behavior. Mechanical tests and neutron diffraction measurements were used to validate and assess the model.

Microstructure characterisation of an X70 grade pipeline steel and its dislocation development during Bauschinger testing

The microstructure of an X70 alloy and its dislocation development during one cycle of compression/tension strain has been analysed. The microstructure of the as received material and the dislocation arrangements before, during and after the Bauschinger test have been characterised. Results for this particular alloy show no evidence of grain boundary dislocation pile-ups or of precipitates pinning dislocations. A comparison between the dislocation arrangement in the as received, compressed and compressed-tensioned samples shows the formation of small dislocation cells, implying that the conception of the Bauschinger effect comes from dislocation-dislocation interactions.

The Bauschinger effect in drawn and annealed nanocomposite Cu–Nb wires

To compliment recent diffraction based measurements of the internal stresses in heavily co-deformed Cu–Nb nanocomposite wires, Bauschinger tests have been performed and used to interpret bulk mechanical response. The role of microstructural heterogeneity has been highlighted and a Masing-type model developed to predict both forward and reverse Bauschinger behaviour. This model has been compared to recent physically based models that are used to describe the flow stress of the various microstructural components of these wires and shown to give good agreement when the role of initial residual stresses is considered. The model is also shown to give insight into the evolution of the strength and work hardening behaviour of the material on annealing based on the softening of the different constituent phases.

The effect of interrupted aging on the yield strength and uniform elongation of precipitation-hardened Al alloys

The yield strength and elongation are both important properties of engineering alloys. In precipitation-hardenable alloys, these properties are directly related to the interaction between the precipitates and mobile dislocations and an inverse correlation is usually observed. It was recently reported that bimodal microstructures containing both shearable and shear-resistant precipitates, generated by interrupted aging schedules, can simultaneously increase both the yield strength and the uniform elongation in selected Al alloys. This is an effect of considerable technological significance but the physical origin is not understood. An explanation for the interrupted aging effect is offered in this contribution. The mechanical response of a model Al–Cu alloy after interrupted aging has been characterized using uniaxial tensile tests and tension–compression Bauschinger tests. The undeformed and deformed microstructures have been characterized using transmission electron microscopy, scanning transmission electron microscopy and differential scanning calorimetry. It is shown that dissolution of the shearable Guinier–Preston (GP) zones occurs during uniaxial deformation, and that the repartition of solute from the GP zones to the matrix can provide a positive contribution to strain hardening that is quantitatively capable of explaining the observations of enhanced elongation. The result depends on the effect of the repartitioned solute on the plasticity of the Al matrix and, using a series of Al–X (X=Cu, Mg, Si, Zn) binary solid solutions, the effect of solute in solution on the mechanical response is quantified. This information is incorporated into models for the yield strength and strain hardening of precipitate-containing microstructures. It is suggested that simultaneous increases in both strength and elongation due to interrupted aging may be expected in 2xxx series alloys but are less likely to be significant in the 6xxx series alloys and are unlikely in most 7xxx series.

The influence of precipitation on plastic deformation of Al–Cu–Li alloys

The plastic behaviour of two recently developed Al–Cu–Li alloys with different Cu/Li ratios, AA2198 and AA2196, was studied as a function of precipitation state by tensile tests, strain hardening rate analysis, Bauschinger tests, slip line imaging and atomic scale characterization of precipitate shearing by advanced electron microscopy. For early ageing times where δ′ and/or solute clusters are present, plasticity occurs heterogeneously, the strain hardening rate is high, and the Bauschinger effect is moderate. When T1 precipitates are present, plasticity becomes more homogeneous, and the strain hardening rate shows a slower decrease compared with other aluminium alloys containing shearable precipitates. In addition, these ageing conditions show a high reversibility of plastic strain, and it is observed that precipitates are sheared only by single steps. The possibility of a specific shearing mechanism of the T1 phase is discussed in light of these results.

Influence of dispersoids on microstructure evolution and work hardening of aluminium alloys during tension and cold rolling

The influence of dispersoids on work hardening of aluminium during tension and cold rolling has been studied by comparing Al–Mn alloys containing similar amounts of solutes but various dispersoid densities. The microstructure evolution with deformation strain was examined in transmission and scanning electron microscopy. It is found that a high density of fine dispersoids strengthens the materials significantly, but their strengthening effect diminishes as the strain increases. From a series of Bauschinger tests, it is found that the internal stress, due to particles, increases rapidly at the initial stage of deformation, but saturates at strains larger than 5%. It is concluded that the internal stress makes a small contribution to the work hardening and contributes to less than 10% of the total flow stress during monotonic loading at strains larger than 5%. The work-hardening behaviour has been correlated to the corresponding microstructure, and the strengthening mechanisms are discussed.

High Strain Rate Torsion and Bauschinger Tests on Ti6Al4V

An accurate isotropic and kinematic hardening model and description of the strain rate dependent material behaviour is necessary for simulation of fast forming processes. Consequently, the material model parameter identification requires experiments where large strains, high strain rates and strain path changes can be attained. Usually, quasi-static tension-compression Bauschinger tests are used to assess the materials kinematic hardening. Hereby it’s important to have the same specimen geometry and boundary conditions in the forward and reverse loading step which is not easily achieved in high strain rate testing techniques. In this work, high strain rate split Hopkinson bar torsion experiments on Ti6Al4V are carried out to study the constitutive material behaviour at large plastic strain and strain rate. In torsion experiments, due to the absence of cross sectional area reduction, higher strains than in tensile tests can be obtained. In addition, a modified torsional split Hopkinson bar setup is developed to perform dynamic Bauschinger tests. A shear reversed-shear load is applied instead of the classical tension-compression load cycle. The test results are analysed to find out if the technique can be used for characterisation of the kinematic material behaviour. Digital image correlation and finite element simulations are used to improve the interpretation of the experimental results.

Microstructure-based modelling of isotropic and kinematic strain hardening in a precipitation-hardened aluminium alloy

This paper presents a study of the strain hardening of a 7000 series aluminium alloy. A wide variety of microstructural states are tested both in uniaxial tension and by performing Bauschinger tests. Their microstructural features are evaluated quantitatively in terms of precipitate size and volume fraction using small-angle X-ray scattering. A physically-based model for strain hardening, using a modified version of the Kocks–Mecking–Estrin formalism, is presented for the precipitation states that exhibit precipitate bypassing. The model takes explicitly into account the microstructural information and describes in detail the mechanisms governing the storage, stability and annihilation of Orowan loops around the precipitates. It describes both the uniaxial and the strain-reversal tests, by evaluating separately the kinematic and isotropic contributions to strain hardening and using an appropriate mixing law. In addition, the transient of strain-reversal Bauschinger tests is discussed with respect to the effect of precipitates on strain reversibility.

Experimental Investigation of Isotropic and Kinematic Hardening in Al-3Cu-0.05Sn (wt%)

The work hardening behavior of an Al-3Cu-0.05Sn (wt %) alloy was studied using tensile tests and Bauschinger tests. Emphasis is placed on the influence of the precipitation state (number density, size distribution and volume fraction) and separating the isotropic and kinematic components of the work hardening.

Finite Element Investigation of Bauschinger Effect in High-Strength A723 Pressure Vessel Steel

The Bauschinger effect has been evaluated in high-strength pressure vessels. A simple initial test suggested that a biaxial Bauschinger effect was present and that it was different from previously published uniaxial Bauschinger results. The difference was believed to be significant, so further investigation was undertaken. Several full-size A723 steel gun sections were heavily overstrained and subjected to slit tests in order to measure opening angles and displacements. These geometries were then modeled with finite element (FE) analysis using both ideal autofrettage stresses and Bauschinger modified stresses, which were based on previously published uniaxial Bauschinger test results. Because techniques available for predicting reverse yielding for overstrained pressure vessels were limited, a simple methodology for predicting the yield surface upon reverse yielding from a series of uniaxial Bauschinger test data was developed and is presented. This methodology, when used in the FE predictions, compares favorably with analytical predictions made previously. Comparisons of slit-opening results measured from pressure vessel sections with FE calculations using uniaxial Bauschinger data are made. The opening displacements comparison between the uniaxial predictions and those measured from the heavily overstrained sections with biaxial stresses are so subtle (&lt;1mm) that the tests appear to be inconclusive.

Effect of strain and strain rate on the bauschinger effect response of three different steels

The Bauschinger behavior after a strain reversal was evaluated for samples with microstructures representative of production sheets for a low-carbon (LC) steel, a high-strength low-alloy (HSLA) steel, and a dual-phase (DP) steel. The microstructures were produced in the samples by laboratory hot rolling and heat treatment. Bauschinger tests were run at strain rates of 0.0001, 0.001, and 0.01 s−1, with tensile prestrains between 1 and 7 pct. After the reversal, the samples were strained 2 pct in compression. The Bauschinger effect is described by a Bauschinger effect parameter (BE), which is the difference between the steel strength at reversal and the 0.05 pct offset yield strength on the reversal, normalized by the steel strength at reversal. It is found that the Bauschinger effect is a continuous increasing function of the strength of the steel, provided the steel is prestrained at least 2.5 pct or beyond the yield point elongation. A single trend line describes the Bauschinger effect variation with steel strength, for all three steels in the present study and for an aluminum-killed drawing quality (AKDQ) steel from a previous investigation. No strain rate influence on the BE was found, due to the limited strain rate range and data uncertainty.

Modelling of Bauschinger effect by various constitutive relations derived from thermodynamical formulation

Bauschinger effect in an AISI 316L austenitic stainless steel at room temperature was studied as a function of the plastic prestrain. Typical uni-axial Bauschinger tests in tension–compression and in compression–tension were carried out. Internal and effective stresses were evaluated from the experimental stress–strain curves using the method proposed by Kuhlmann-Wilsdorf and Laird. Four constitutive relations derived from a thermodynamical formulation were identified using an inverse identification method. The modelled stress–strain curves were then compared to the experimental ones. Bauschinger effect was characterised using Bauschinger indicators proposed in the literature in terms of strain, stress and energy, respectively. These indicators were evaluated from the experimental stress–strain curves on the one hand, from the modelled ones on the other hand. Their variations as functions of prestrain were then compared. The best constitutive relation uses two non-linear kinematic work-hardening terms.

Anisotropic work-hardening behaviour of structural steels and aluminium alloys at large strains

Sheet metal forming processes may often involve intense forming sequences, leading to large strains and severe strain-path changes. Optimizing such technologies requires a good understanding and description of the anisotropic plastic behaviour of the deformed material, in connection with the evolution of its texture and microstructure. In this paper, we present the predictions provided by a model involving isotropic and kinematioc hardening and by a physically-based microstructural model, which introduces additional internal variables taking into account the directional strength of dislocation structures and their polarity. These models have been identified by using sequences of uniaxial traction and simple shear experiments, carried out on various steels (DC06, DP600, HSLA340) and aluminium alloys (AA5182-O, AA60l6-T4). The microstructural model proved able predict the complex hardening behaviour displayed, especially by the ferritic steels, namely the transient work-hardening stagnation during reversed deformation in Bauschinger tests, the temporary work-softening during orthogonal tests, and the grain fragmentation at large monotonic strains.

Effect of martensite content, its dispersion, and epitaxial ferrite content on Bauschinger behaviour of dual phase steel

Dual phase microstructures were produced in a low carbon steel, in which the martensite volume fraction was kept constant at two levels, of 18 and 25%, while the epitaxial ferrite content was varied independently. The microstructures were produced with two dispersions of martensite, a relatively coarse dispersion by intercritical annealing of a ferrite/pearlite starting microstructure and a finer dispersion from an initial martensitic microstructure. Bauschinger tests were conducted using prestrains in both tension and compression of 0.4, 1, and 2.2%. Prestrain direction had no measurable effect on plastic flow behaviour after strain reversal. Mean back stresses increased with increasing strain and martensite content, and were higher for the finer martensite dispersion. They decreased significantly with increasing epitaxial ferrite content in the case of the finer dispersion, but less significantly in the coarser dispersion. These effects of microstructure are discussed in terms of stress transfer to martensite, work hardening, and tensile properties. It is concluded that about half of the mean back stress developed during early plastic deformation was generated by stress transfer to the martensite, the remainder arising from strain hardening of the matrix. A small permanent softening in the Bauschinger test resulted from a reduction of effective stress in the ferrite matrix when the strain path was reversed.

Effects of alloy additions and tempering temperature on the sag resistance of Si–Cr spring steels

The effects of Mo, W, and V additions and tempering temperatures on the sag resistance in relation to the microstructural evolution during tempering of Si–Cr spring steels were investigated by performing TEM examination, torsional Bauschinger tests, hardness tests and tensile tests. The hysteresis loop area measured in torsional Bauschinger tests, which is closely related with the sag resistance, is directly influenced by the distribution of precipitates. It increases and then decreases with tempering temperature, after reaching its maximum value at 350°C. The additions of Mo and/or W result in a finer distribution of tempered carbide particles due to the decrease in the rate of the spheroidization and of the coarsening of carbide particles above 400°C. However, they do not affect the behavior of tempered carbides at low tempering temperatures. Accordingly, the additions of Mo and/or W increase the hysteresis loop area at tempering temperature above 400°C. The V addition increases the hysteresis loop area as well as hardness due to the precipitation of vanadium carbonitrides, regardless of tempering temperature. In considering industrial application of spring steels, a new parameter, the ratio of the hysteresis loop area in the torsional Bauschinger test to hardness, is suggested.

Microstructural influence on sag resistance of Cr containing and Cr free spring steels

The effects of Cr addition and tempering temperature on mechanical properties, especially sag resistance, with respect to the microstructural evolution during tempering in 1.5%Si spring steels, were investigated by performing hardness tests, tensile tests, torsional Bauschinger tests, and examination by transmission electron microscopy. The results obtained showed that the sag resistance depended on the distribution of tempered carbides. With increasing tempering temperature, sag resistance increased to a maximum value at 350°C and then decreased. The Cr addition did not affect the behaviour of tempered carbides at low tempering temperatures, while at high temperatures, above 400°C, the Cr addition reduced the coarsening rate of the cementite, resulting in finer particle distribution of the cementite. The Cr addition improved the sag resistance as well as the hardness at tempering temperatures above 400° C, but it decreased the ratio of loop area in the torsional Bauschinger test (the sag resistance) to hardness, regardless of tempering temperature.

Improvement of sag resistance by the addition of tungsten in Si[sbnd]Cr[sbnd]Mo[sbnd]V steels

The sag resistance, which is represented by loop area in the torsional Bauschinger test, is directly influenced by the distribution of precipitates, especially tempered carbides in the present steels. Although the W addition does not change the behavior of tempered carbides at low temperatures, such as the precipitation of ɛ-carbide and the conversion of ɛ-carbide to cementite, it suppresses the coarsening rate of cementite at higher temperatures of 400–500 °C. Thus, the W addition to 0.6C–1.5Si-0.5Cr-0. lMo-0.2V (in wt.%) spring steels improves the sag resistance for tempering temperatures of 400–500 °C.