Prestrain Direction Research Articles

Strain hardening response and modeling of EDDQ and DP780 steel sheet under non-linear strain path

The anisotropic hardening behaviors of dual phase (DP780) and extra deep drawing quality (EDDQ) steel sheets under non-proportional strain paths were investigated. Two-step uniaxial tension tests, which consisted of the first loading in the rolling (RD) or transverse (TD) and the second loading in every 15° from the first loading axis, were conducted. For DP780 steel, a significant Bauschinger effect accompanied by a transient hardening behavior after reverse loading was the prominent phenomenon. In contrast, EDDQ exhibited stress overshooting followed by strain hardening stagnation with respect to the monotonic flow curve near cross-loading conditions. The extended HAH model combined with the Yld2000-2d yield function were used to reproduced the anisotropic hardening behavior of the two materials. For DP780, the extended HAH model could capture the Bauschinger effect and transient hardening behavior well for tension reloading at 0°, 15°, 75° and 90° from the RD or TD prestraining direction. However, the predictions at 30°, 45° and 60° were slightly different from the experiments. For EDDQ, this approach reproduced the strain hardening anisotropy well including flow stress overshooting followed by a stage of strain hardening stagnation.

Aging-induced anisotropy of mechanical properties in steel products: Implications for the measurement of engineering properties

Abstract Strain aging in low-carbon steels is a well-known strengthening phenomenon, the typical results of which are an increase in yield stress and/or an increase in the extent of discontinuous yielding. Aging effects are generally characterized through the use of results from mechanical tests in which the strain path prior to aging (prestrain) and the strain path after aging are in the same direction. However, these tests do not completely characterize the properties of aged materials, since the effects of aging are reduced when materials are tested in directions different than the direction of prestrain. The result is anisotropy of properties which can affect the performance of industrial products. In this paper, the effect is demonstrated in two examples of industrial products made from low carbon steels, the aging of which during processing results in performance changes that are not predicted through standard tensile testing of as-fabricated products. The first example compares the effect of aging on yield strength and dent resistance of stamped hood panels on an electro-galvanized, Al killed, bake hardenable sheet steel for auto body panel applications. The second example shows the effect of aging on the anisotropy of tensile data from two American Petroleum Institute (API) grade X100 pipe steels in the as-received condition. The data show that the performance gains realized from strain-aging in the tensile tests on base material are not apparent in the tensile data from the stamped panels after aging, but the dent resistance clearly demonstrated the beneficial effect of aging. The high degree of anisotropy in the yield strength and yielding behaviors between the circumferential and longitudinal tensile data in the two pipe steels demonstrates the effect of strain path on a materials response to aging, which may occur during downstream processing or in field service. The manifestation of material properties that are dependent upon the relationship between the pre-aging strain direction and the post-aging strain direction underscores the importance of correct evaluation of mechanical performance in the design of structural components in materials which undergo aging.

Structural Stability of Transparent Conducting Films Assembled from Length Purified Single-Wall Carbon Nanotubes

Single-wall carbon nanotube (SWCNT) films show significant promise for transparent electronics applications that demand mechanical flexibility, but durability remains an outstanding issue. In this work, thin membranes of length purified single-wall carbon nanotubes (SWCNTs) are uniaxially and isotropically compressed by depositing them on prestrained polymer substrates. Upon release of the strain, the topography, microstructure, and conductivity of the films are characterized using a combination of optical/fluorescence microscopy, light scattering, force microscopy, electron microscopy, and impedance spectroscopy. Above a critical surface mass density, films assembled from nanotubes of well-defined length exhibit a strongly nonlinear mechanical response. The measured strain dependence reveals a dramatic softening that occurs through an alignment of the SWCNTs normal to the direction of prestrain, which at small strains is also apparent as an anisotropic increase in sheet resistance along the same directio...

Pre-strain direction effect on microstructure evolution and energy storage process during uniaxial tension of austenitic steel

In the present paper the influence of pre-strain direction on energy balance during deformation of austenitic steel was investigated and the analysis of microscopic phenomena responsible for this influence was performed. The specimens with different pre-strain directions were prepared and the ratio of the stored energy increment to plastic work increment, called energy storage rate, as a function of plastic strain was experimentally determined. At the initial stage of plastic deformation of annealed materials this quantity vs. plastic strain has a maximum. It has been shown that for specimens strained in the same direction as pre-strain the energy storage rate decreases monotonically with deformation while for specimens where strain path was changed, the maximum of the energy storage rate is observed (as in case of annealed material). The study of slip and microstructure evolution at meso- and micro-scales have shown that the change in pre-strain direction leads to the redistribution of internal stresses generated by incompatible slip in neighbouring grains of different orientation. Just after change in strain direction the accommodation of these stresses takes place not only by generation of geometrically necessary dislocations but also by micro-shear banding.

Large-strain Bauschinger effect in austenitic stainless steel sheet

Large-strain Bauschinger effect in cold-rolled austenitic stainless steel sheet is investigated after large amounts of prestrain. The material is prestrained in uniaxial tension, and the tensile properties of the prestrained material are measured in different angles with respect to the prestraining direction. By comparing the differences in the yield stresses in different orientations, the effect of prestraining on material anisotropy is studied. The method is applied to AISI 304-type stainless steel sheet. The test results are analyzed using a combined isotropic–kinematic hardening model. The results indicate that this kind of material shows a considerable Bauschinger effect. Transient and permanent softening is observed in the experiments. The experimental Young's modulus also seems to decrease with prestrain.

Effect of pre-strain direction on energy storage process during tensile deformation of polycrystal

Abstract The energy storage process is characterised by the energy storage rate d e s /d w p as a function of plastic work w p or of plastic strain e P ( e s is stored energy). At the initial stage of plastic deformation the dependence of d e s /d w p on e P has a maximum. The present work is devoted to the experimental verification of a hypothesis that the maximum of d e s /d w p can be related to the internal micro-stresses caused by incompatible slip in grains of different orientation. The verification needed two groups of specimens in which the influence of internal micro-stresses on the energy storage rate is different. This requirement was achieved using specimens pre-strained in different directions. Experiments that make it possible to estimate the influence of internal micro-stresses on the rate of energy storage are presented. The obtained results confirm the hypothesis.

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.

液体窒素温度と室温で予ひずみを与えた後の液体窒素温度と室温での繰返し変形挙動

Cyclic loading tests are performed by using thin-walled tubular specimens of SUS304 stainless steel at low temperature in liquid nitrogen (77K) and room temperature. Cyclic plastic deformation subsequent to pre-strain at 77K and room temperature is experimentally investigated under torsional, tensile and compressive loading conditions. At small cyclic numbers, the pre-strain perpendicular to cyclic loading direction results in large hardening, and cyclic plastic behavior subsequent to the pre-strain at 77K is more hardened than the pre-strain at room temperature. With increasing cyclic number, the effect of temperature on the hardening behavior becomes small. When the cyclic loadings are conducted in the same direction as the pre-straining direction, the effect of the pre-strain on hardening behavior is not remarkable at room temperature and 77K. The pre-strain at 77K influences the cyclic hardening at room temperature even for large cyclic number.

Experimental investigation of plastic deformation of stainless steel for cyclic loading at low temperature

Cyclic plastic deformation of stainless steel SUS 304 is experimentally investigated at low temperature of liquid nitrogen (77 K) under various cyclic loading conditions Thin walled tubular specimens are subjected to cyclic loading under constant strain ranges. At low temperature, the material shows remarkable hardening by cyclic loading comparing with cyclic loading at room temperature. The hysteresis curves of stress-strain relations by cyclic loading are saturated by increasing the cycle numbers. The saturation tendency depends on loading direction. The saturated stress values are related with cumulative plastic strain of cyclic loading. The prestraining is given at 77 K by axial and torsional loadings, and subsequent cyclic loading under constant strain range is conducted at 77 K. The cyclic stress-strain curves are saturated by increasing cyclic numbers. At small cyclic numbers, cyclic plastic deformation depends on the prestrain direction. The directional effect of pre-strain on cyclic loading becomes small with increasing number of cycles.

Effect of Pre-strain on Cyclic Loading of Stainless Steel SUS 304 at Low Temperature in Liquid Nitrogen.

Cyclic plastic deformation subsequent to pre-strain of SUS 304 stainless steel is experimentally investigated at low temperature in liquid nitrogen under torsional, tensile and compressive loading conditions. Thin-walled tubular specimens given pre-strain were subjected to cyclic loading under stress-controlled conditions. The pre-strain is conducted at 77 K, and subsequent cyclic lading under constant strain amplitude is given at 77 K. The cyclic stress-strain curves are saturated by increasing number of cyclic. At small number of cyclic, cyclic plastic deformation depends on the pre-strain direction. The pre-strain perpendicular to cyclic loading directions results in large hardening at the same accumulated plastic strain value comparing with the pre-strains of the same direction as cyclic loading directions. The directional effect of pre-strain on cyclic loading with increasing number of cycles becomes small.

Effects of changes in strain path on the anisotropy of yield stresses of low-carbon steel and 70-30 brass sheets

Uniaxial tensile tests in various directions following uniaxial extension, equibiaxial stretching or plane strain rolling have been performed to study the effects of changes in strain path on the anisotropy of yield stresses of aluminium-killed low-carbon steel and 70-30 brass sheets. The anisotropy could be predicted from the specimen textures, if dislocation structure were equiaxed, as in the case of equibiaxial stretching. However, elongated dislocation cell structures, developed in the steel specimens prestrained in uniaxial tension or plane strain rolling, gave rise to the second-stage yield stresses higher than predicted from textures in the directions different from the maximum prestrain direction. Planar dislocation structures in the brass specimens prestrained in uniaxial tension or plane strain rolling gave the second-stage yield stresses lower than predicted from the textures in the directions different from the maximum prestrain direction. The phenomena are discussed based on textures and dislocation structures.

Effect of stress components on fatigue crack growth rate under multiaxial stress condition: Yokobori, T.A., Isogai, T., Yokobori, T. and Koizumi, Y. J. Soc. Mater. Sci., Japan Aug 1990 39, 1106–1112 (in Japanese)

The anisotropic hardening behaviors of dual phase (DP780) and extra deep drawing quality (EDDQ) steel sheets under non-proportional strain paths were investigated. Two-step uniaxial tension tests, which consisted of the first loading in the rolling (RD) or transverse (TD) and the second loading in every 15° from the first loading axis, were conducted. For DP780 steel, a significant Bauschinger effect accompanied by a transient hardening behavior after reverse loading was the prominent phenomenon. In contrast, EDDQ exhibited stress overshooting followed by strain hardening stagnation with respect to the monotonic flow curve near cross-loading conditions. The extended HAH model combined with the Yld2000-2d yield function were used to reproduced the anisotropic hardening behavior of the two materials. For DP780, the extended HAH model could capture the Bauschinger effect and transient hardening behavior well for tension reloading at 0°, 15°, 75° and 90° from the RD or TD prestraining direction. However, the predictions at 30°, 45° and 60° were slightly different from the experiments. For EDDQ, this approach reproduced the strain hardening anisotropy well including flow stress overshooting followed by a stage of strain hardening stagnation.

Experimental determination of strain-induced anisotropy during nonproportional straining of an A1/Mg alloy at room temperature

A servohydraulic, computer controlled MTSn axial-torsion testing machine with a bi-axial clip-on extensometer is used to test thin-walled tubes of an A1/Mg alloy under strain control. A plastic offset strain of 10−4 determines the yield surfaces. Straining and yield surface probing is governed by a computer program which also controls digital data acquisition. Yield surfaces in stress and in strain space as well as the axial and shear stress-strain diagrams can be reconstructed from the digitally recorded data. The specimens were subjected to a strain path in the form of a regular 16-sided polygon which was followed on some specimens by a square path. The total inelastic strain path length can exceed 15% while the equivalent strain excursion is less than 2%. It is shown that yield surfaces measured on specimens withclose initial stress-strain diagrams are very consistent and that yield surface probing has an insignificant effect on subsequent yield surfaces. Yield surfaces are shown to translate, changein shape and size and to exhibit a cross effect. A post processor which includes a least square smoothing routine calculates the area and the centroid of each yield surface. The size increase is initially rapid but the rate of increase decreases as a saturation is approached. After strining for less than 1% in a fixed direction a characteristic yield surface shape is established. Yield surfaces obtained at the same point in strain space with identical prestrain direction of at least 1% but with increased amounts of accumulated plastic strain have the same shape but show an increase in size. The yield surfaces differ in shape and size when the same strain point is reached from different directions. The centroid of the yield surface in stress space moves almost in a circular path for a polygonal strain path. All stress space yield surfaces contain the origin but this is not the case for the surfaces in strain space.

Plastic behavior of nuclear structural metals after cyclic prestraining

Abstract The plastic behavior of metals (SUS 304 stainless steel, 2 1 4 Cr-1Mo steel and Al-alloy) after cyclic prestraining is systematically and experimentally investigated in a state of plane stress that consists of axial and torsional stress (strain) components. A practical biaxial extensometer for use in cyclic and subsequently large strain tests is also developed. The stress level in reloading after cyclic prestraining is never below the corresponding initial stress-strain curve, regardless of the loading direction, even after cyclic softening. Initial plastic anisotropy does not disappear or become small due to cyclic prestraining, and may become strong, depending on the prestrain direction. The larger the cyclic strain hardening, smaller the work hardening coefficient in reloading. This suggests initial stress-strain relations should and can be used as basic data on materials when analyzing the strength and safety of a structure undergoing normal cyclic loading followed by abrupt overloading such as that occuring during an accident or earthquake.

The effect of anisotropy on creep and creep crack growth in cold-worked CMn steel at 360°C

A study has been made of the anisotropy of creep and creep crack growth properties at 360°C of cold-worked C-Mn pipe steels. For prestrain in the direction of the pipe axis, crack growth rates in the plane parallel to the prestrain direction and perpendicular to the rolling plane are faster than in the plane perpendicular to the prestrain axis. Similarly, creep rates are higher in the axial than in the transverse direction. The creep rate anisotropy is thought to be a consequence of the dislocation structure developed during prestrain. The anisotropy in crack growth then results both from the directionality of the creep strength and from the presence of inclusion stringers along the prestrain axis. For cold-worked C-Mn steels showing a high creep crack growth susceptibility, the effect of anisotropy is small by comparison. However, for less susceptible casts the two effects may be comparable in magnitude.

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.

An Experimental Investigation of the Yield Loci of 1100-0 Aluminum, 70:30 Brass, and an Overaged 2024 Aluminum Alloy After Various Prestrains

The multiaxial yield behaviors of 1100-0 aluminum, 70:30 brass, and an overaged 2024 aluminum alloy (2024-T7) have been investigated for a variety of prestress histories involving combinations of normal and shear stresses. Von Mises effective prestrains were in the range of 1.2–32%. Prestress paths were chosen in order to investigate the roles of prestress and prestrain direction on the nature of small-strain offset (ε = 5 × 10−6) yield loci. Particular attention was paid to the directionality, i.e., translation and distortion, of the yield locus. A key result, which was observed in all three materials, was that the final direction of the prestrain path strongly influences the distortions of the yield loci. Differences in the yield locus behavior of the three materials were also observed: brass and the 2024-T7 alloy showed more severe distortions of the yield locus and a longer memory of their entire prestrain history than the 1100-0 aluminum. In addition, more “kinematic” translation of the subsequent yield loci was observed in brass and 2024-T7 than in 1100-0 aluminum. The 2024-T7 differed from the other materials, showing a yield locus which decreased in size subsequent to plastic straining. Finally, the implications of these observations for the constitutive modeling of multiaxial material behavior are discussed.

Plastic behavior of metals after cyclic prestraining (3rd report, Deformation properties of reloading after cyclic prestraining)

The plastic behavior of metals (SUS 304, 2-1/4Cr-1 Mo steel Al alloy) after cyclic prestraining is systematically and experimentally investigated in a stress (strain) plane that consists of axial and torsional stress (strain) components. A useful multiaxial deformations sensor for both a cyclic test and a following large straining test is developed. The stress level in reloading after cyclic prestraining is never below the initial stress-strain curve, independently of the loading direction, even after cyclic softening, but may become strong depending on the prestraining direction, even after cyclic softening has occurred. An initial plastic anisotropy is never weak and never vanishes due to cyclic prestraining, but may become strong depending on the prestraining direction. The larger the cyclic strain hardening, the smaller the coefficient of work hardening in reloading. Therefore, it is suggested that the initial stress-strain relations should/can be used as basic material data for a strength and safty analysis of a structure that is subjected to both an usual cyclic load and an abrupt overload due to accident, earthquake and so on.

Effects of Prior Plasticity on Subsequent Creep of Type 316 Stainless Steel at Elevated Temperature

Interaction between creep and plastic deformation was studied experimentally for type 316 stainless steel at 650°C, with special emphasis on creep behavior subsequent to plastic prestraining. In combined creep-plasticity experiments, thin-walled tubular specimens were first prestrained plastically in the axial tensile direction, and were subsequently subjected to constant stress creep under various multiaxial stress states with an identical effective stress. Furthermore, the variation in creep resistance due to the plastic prestrain was compared with that due to the same amount of creep prestrain. From the experimental results, it was found that creep resistance was markedly enhanced by the plastic prestrain and that the increase in the creep resistance depended on the amount and relative direction of the plastic prestrain. The creep resistance was increased more markedly by creep prestrain than the same amount of plastic strain.

Anisotropy Induced in an Uncrosslinked Elastomer Via Large Strain Deformation

Abstract The anisotropy induced in an ungelled SBR elastomer upon large scale deformation and the rate at which the deformed elastomer returns to its original state upon removal of the deforming force were investigated. In these experiments, a standard birefringence technique was unsuitable for measuring the extent of orientation after release, however, tensile stress-strain measurements successfully showed the presence of anisotropy. After prestraining and then releasing, samples have an initial resistance to deformation which is the same both parallel and perpendicular to the prestraining direction. However, testpieces cut parallel with the prestraining direction show stress-strain curves that lie above those for cross-cut specimens at high elongations, while cross-cut samples have stress-strain curves remarkably similar to those of the isotropic controls. With increasing time after prestraining or for larger prestrains, the normalized anisotropy, at intermediate elongations, becomes negative; that is, in this region, it becomes easier to deform “parallel” specimens than to deform “perpendicular” specimens. This phenomenon is proposed to be the result of two opposing entanglement networks—an original one, which remains in tension, and a compressed one, which was formed by chains re-entangling while the sample was extended. Although the shapes of the “parallel” and “perpendicular” stress-strain curves may be quite different, the total energy required to rupture the samples in both cases is similar. Finally, for a lightly crosslinked sample, it is demonstrated that after various prestrains, hold times, and relaxation times before testing after prestraining, the normalized anisotropy is a unique function of the residual extension at the moment when the specimens were tested.