Strain path change effects in the local necking of aluminum sheet and in the tension of internally pressurized tubes

It is possible to increase significantly the uniform elongation achieved in the uniaxial tension of commercial purity aluminum by accelerating the testing rate. This effect is linked to a significant rate sensitivity of strain hardening. However, very little increase in the strains associated with the final stages of localization in sheet specimens were achieved by this means. The importance of the change in strain rate and path on local necking has been investigated by introducing path changes of appropriate magnitudes in tubular tensile specimens by internal pressurization. The path change led to a decrease in strain-hardening rate which was not compensated for by an increase in strain rate. The potential consequences of this effect upon ductility in sheets are significant and limit the potential usefulness of any rate sensitivity of strain hardening in increasing formability.     

The effects of combined strain-path and strain-rate changes in aluminum

The effect of changes in both the direction of tensile stress and the strain rate upon the plastic behavior and microstructure of aluminum has been investigated. The reduction in strain-hardening rate following a strain-path change was influenced by the strain rate prior to and after the change; higher strain rates in the first stage and lower rates in the second stage decreased the initial transient hardening rate. The introduction of a large path change significantly reduced the effect of strain-rate changes on the strain-hardening rate observed in testing without changing the tensile axis. Evidence of cell wall disruption following a path change was found, and this mechanism combined with effects of cell wall orientation can qualitatively explain the removal of the rate sensitivity of hardening rate because the usual recovery events in cell walls no longer have a dominant influence on flow stress development.     

The effects of combined strain-path and strain-rate changes in aluminum

The effect of changes in both the direction of tensile stress and the strain rate upon the plastic behavior and microstructure of aluminum has been investigated. The reduction in strain-hardening rate following a strain-path change was influenced by the strain rate prior to and after the change; higher strain rates in the first stage and lower rates in the second stage decreased the initial transient hardening rate. The introduction of a large path change significantly reduced the effect of strain-rate changes on the strain-hardening rate observed in testing without changing the tensile axis. Evidence of cell wall disruption following a path change was found, and this mechanism combined with effects of cell wall orientation can qualitatively explain the removal of the rate sensitivity of hardening rate because the usual recovery events in cell walls no longer have a dominant influence on flow stress development.     

Plastic behavior of dual phase steel following plane-strain deformation

Dual-phase, high-strength steel sheet has been prestrained in plane-strain tension. Residual hardening and ductility properties were evaluated by performing subsequent uniaxial tensile tests in either co-axial or noncoaxial principal strain axis orientations. In contrast to similar work on aluminum-killed 1008/1010 steel sheet, only minor changes were found in the subsequent flow behavior of dual-phase steel, and no significant difference was found between the two orientations. The small effect of an abrupt strain path change observed in this study is consistent with the low work hardening rate of this alloy.     

Effects of changes in strain path on work-hardening in CP aluminium and an AlCuMg alloy

Effects of abrupt changes in the direction of plastic deformation on work-hardening behaviour have been investigated in two-stage stretching of sheets of AA1050 and a heat-treated 2014 aluminium alloy aged at various temperatures up to 300°C. The results show that reorganisation of dislocation distribution after a change in strain path can result in transient changes in work-hardening behaviour of two kinds. Changes of the first kind, which tend to increase the hardening rate in early stages of the second mode of deformation, are associated with reorientation of internal stresses. Changes in the second kind, which tend to cause transient reductions in hardening rate, are believed to be associated with partial dissolution of the original dislocation substructure. The relative magnitudes and strain dependencies of these two kinds of change depend on the deformation sequence and on material variables. The change in hardening rate of CP aluminium after a change in strain path is dominated by changes of the second kind which, after moderate prestrains, cause reductions in the limits of stable elongation. In similar tests on overaged conditions of the 2014 alloy the overall changes in hardening rate are dominated by changes of the first kind, so that the limit of uniform elongation is increased by a change in strain path. When dynamic ageing is active in the 2014 alloy changes of the second kind can be suppressed so that reductions in the hardening rate do not occur.     

Dynamic mechanical behavior of ultra-high strength steel 30CrMnSiNi2A at high strain rates and elevated temperatures

During high speed machining in the field of manufacture,chip formation is a severe plastic deformation process including large strain,high strain rate and high temperature.And the strain rate in high speed cutting process can be achieved to 105 s~(-1).30CrMnSiNi2Asteel is a kind of important high-strength low-alloy structural steel with wide application range.Obtaining the dynamic mechanical properties of30CrMnSiNi2Aunder the conditions of high strain rate and high temperature is necessary to construct the constitutive relation model for high speed machining.The dynamic compressive mechanical properties of30CrMnSiNi2Asteel were studied using split Hopkinson pressure bar(SHPB)tests at 30-700°C and3000-10000s~(-1).The stress-strain curves of 30CrMnSiNi2Asteel at different temperatures and strain rates were investigated,and the strain hardening effect and temperature effect were discussed.Experimental results show that 30CrMnSiNi2Ahas obvious temperature sensitivity at 300°C.Moreover,the flow stress decreased significantly with the increase of temperature.The strain hardening effect of the material at high strain rate is not significant with the increase of strain.The strain rate hardening effect is obvious with increasing the temperature.According to the experimental results,the established Johnson-Cook(J-C)constitutive model of 30CrMnSiNi2Asteel could be used at high strain rate and high temperature.     

The effect of grain size, strain rate, and temperature on the mechanical behavior of commercial purity aluminum

Commercial purity aluminum AA1050 was subjected to equal channel angular extrusion (ECAE) that resulted in an ultrafine-grained (UFG) microstructure with an as-received grain size of 0.35 μm. This UFG material was then annealed to obtain microstructures with grain sizes ranging from 0.47 to 20 μm. Specimens were compressed at quasi-static, intermediate, and dynamic strain rates at temperatures of 77 and 298 K. The mechanical properties were found to vary significantly with grain size, strain rate, and temperature. Yield stress was found to increase with decreasing grain size, decreasing temperature, and increasing strain rate. The work hardening rate was seen to increase with increasing grain size, decreasing temperature, and increasing strain rate. The influence of strain rate and temperature is most significant in the smallest grain size specimens. The rate of work hardening is also influenced by strain rate, temperature, and grain size with negative rates of work hardening observed at 298 K and quasi-static strain rates in the smallest grain sizes and increasing rates of work hardening with increasing loading rate and grain size. Work hardening behavior is correlated with the substructural evolution of these specimens.     

The effect of hydrogen on the multiaxial stress-strain behavior of titanium tubing

The influence of internal hydrogen on the multiaxial stress-strain behavior of commercially pure titanium has been studied. Thin-walled tubing specimens containing either 20 or 1070 ppm hydrogen have been tested at constant stress ratios in combined tension and internal pressure. The addition of hydrogen lowers the yield strength for all loading paths but has no significant effect on the strain hardening behavior at strains ε ≥ 0.02. Thus, the hydrogen embrittlement of titanium under plain strain or equibiaxial loading is not a consequence of changes of flow behavior. The yielding behavior of this anisotropic material is described well by Hill’s quadratic yield criterion. As measured mechanically and by pole figure analysis, the plastic anisotropy changes with deformation in a manner which depends on stress state. Hill’s criterion and the associated flow rule do not describe the multiaxial flow behavior well because of their inability to account for changes of texture which depend on multiaxial stress path. Hence, a strain dependent, texture-induced strengthening effect in equibiaxial tension is observed, this effect having the form of an enhanced strain hardening rate. Formerly with Michigan Technological University     

高强IF钢的高速应变行为

利用拉伸实验装置研究了高强IF钢在高应变速率下的变形特性。结果表明：高强IF钢是应变速率敏感性材料,在应变速率10-4～103/s的范围内,应变速率对高强IF钢的应变硬化率与屈服强度的影响具有2阶段性。在第一阶段,应变速率较低,应变硬化率与屈服强度对应变速率的敏感性较小;在第二阶段,应变速率较高,随应变速率的增加,应变硬化率迅速降低,屈服强度迅速增加。     

Dynamic strain aging and ductility minima in zirconium

Tensile tests using coarse grained zirconium specimens were conducted at two strain rates, differing by 3 orders of magnitude, between 77° and 1032°K. At each strain rate, peaks were observed when the flow stress was plotted against the temperature. The temperature corresponding to a given peak was observed to rise with increasing strain rate. A pronounced minimum in the strain rate sensitivity of zirconium near 675°K can be explained in terms of the strain rate dependence of these peaks. At each strain rate, the zirconium tensile specimens also showed a minimum elongation at the hardening peak temperature. Since the reduction in area did not pass through a corresponding minimum, the elongation minima do not reflect a true ductility loss. What actually takes place is an increased tendency to neck at the hardening peak temperature. This tendency to promote a neck can be rationalized in terms of variations in the strain rate sensitivity caused by dynamic strain aging. Former Graduate Student now Post Doctoral Worker and Professor     

Strain localization in the diffuse neck in sheet metal

The onset of diffuse instability in sheet metals is associated with initially small, but grad-ually increasing, changes in the strain rate,é, and in the ratio of the minor to the major principal strains,p = ε₂/ε₁in the plane of the sheet. The hardening and softening contri-butions from such changes have been considered to obtain a condition for partial flow sta-biliity in the neck. The effect of the change inè on the uniaxial stress was determined from measurements of ε changes in the neck and from the strain rate sensitivity of the materials. Similar evaluation of the change in the axial stress due to a change in p was made by means of an approximate analysis. The combination of these effects, along with the basic strain hardening of the materials, is used to explain the slowness or the rapid-ity of the process of necking in several materials, exhibiting different normal anisotropy, strain hardening and strain rate hardening behaviors. These results can also be used to explain the size of a diffuse neck and the strain distribution within it. Correlation has also been obtained between necking extensions to failure and the forming limits.     

Flow localization in sheet specimens with pairs of holes

The deformation localization behavior of sheet specimens containing geometric perturbations in the form of pairs of through-thickness holes is examined. Both experiments and computational modeling are performed in either uniaxial or equal-biaxial tension in order to examine the effect of applied loading path on the far-field strain needed to initiate localized necking in the ligament between the hole pairs. The models also examine the influence of hole spacing and matrix strain hardening on ligament localization. The far-field strain needed to cause the localization of the ligament is shown to increase as the biaxiality of the loading path increases, the hole spacing increases, and the strain-hardening exponent increases. The present study also indicates that the onset of localized necking can be predicted by employing the Hill criterion, if the local strain states within the ligament are taken into account.     

Effects of strain path changes on the formability of sheet metals

The effects of a change in strain path on the deformation characteristics of aluminum-killed steel and 2036-T4 aluminum sheets have been studied. These sheets were pre-strained various amounts in balanced biaxial tension and the resulting uniaxial proper-ties and forming limits for other loading paths were determined. In comparison to uni-axial prestrain the steel was found to suffer a more rapid loss in uniform strain upon the strain path change from biaxial to uniaxial. In contrast, the uniform strain in aluminum does not drop as rapidly after the same change. In keeping with this behavior, the form-ing limit diagram of steel is found to decrease with prestrain at a much faster rate than that of aluminum. Such effects can be explained in terms of the transition flow behavior of the metals occurring upon the path change. Thus, the path change produces strain soften-ing and premature failure in steel, while causing additional strain hardening and consequent flow stabilization in aluminum. AMIT K. GHOSH, formerly with General Motors Research Laboratories     

Biaxial path dependence of deformation substructure of type 304 stainless steel

Although martensitic transformations in austenitic stainless steels have been studied rather thoroughly for uniaxial monotonie and cyclic loading, data are scant for biaxially loaded specimens. In particular, recent nonproportional straining experiments have indicated a significant increase in cyclic hardening beyond that observed in uniaxial tests at equivalent strain levels. In this paper, a link is made between the additional hardening and microstructural uniformity of transformation product. This link is expressed through a micromechanical viewpointvia increased latent hardening associated with rotation of the principal stress and plastic strain rate directions.     

1300 MPa级Nb微合金化DH钢的组织性能

设计了不同相构成的超高强DH钢,抗拉强度均大于1300 MPa,组织由铁素体、马氏体、残留奥氏体和极少量碳化物构成。对比了不同相构成对超高强DH钢力学性能和应变硬化行为等的影响,并深入研究了残留奥氏体在超高强度DH钢中的作用机制。结果表明:随着马氏体和残留奥氏体体积分数的增大,铁素体体积分数的减小,实验钢屈服和抗拉强度同时升高,而延伸率呈先增大后减小趋势。软韧相铁素体体积分数的减小和硬相马氏体体积分数的增大导致屈服强度和抗拉强度增加。相对于回火马氏体,淬火马氏体对强度的提升更显著,在拉伸过程中转变的残留奥氏体的量是引起延伸率变化的主要原因,组织中显著的带状组织会造成颈缩后延伸率的明显降低。通过对应变硬化行为的分析表明,随着真应变的增大,应变硬化率呈减小的趋势,在真应变大于2%后的大范围内,对于应变硬化率,DH1>DH2>DH3,主要与铁素体体积分数有关;在真应变大于5.73%后,DH2钢的应变硬化率高于DH1钢和DH3钢,主要与DH2钢中更显著的TRIP效应有关。除了残留奥氏体体积分数,残留奥氏体中的碳含量对TRIP效应同样有显著的影响。较高比例的硬相马氏体组织结合适当比例的软韧相铁素体和残留奥氏体有助于DH2钢获得最良好的强塑积13.17 GPa·%,其中屈服强度达880 MPa,抗拉强度达1497 MPa,均匀延伸率为6.71%,总伸长率为8.8%,颈缩后延伸率为2.09%,屈强比0.59。      

Influence of Strain Rate,Temperature, Plastic Strain,and Microstructure on the Strain Rate Sensitivity of Automotive Sheet Steels

This work identifies the influence of strain rate, temperature, plastic strain, and microstructure on the strain rate sensitivity of automotive sheet steel grades in crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 10^?3–200 s^?1 and in the temperature range 233–373 K. The dynamic flow curves have been tested by means of servohydraulic tensile testing. The strain rate sensitivity decreases with increasing plastic strain due to a gradual exhausting of work hardening potential combined with adiabatic softening effects. The strain rate sensitivity is improved with decreasing temperature and increasing strain rate, according to the thermally activated deformation mechanism. The m‐value is reduced with increasing strength level, this decrease being most pronounced for steels with a yield strength below 400 MPa. Solid solution alloying with manganese, silicon, and especially phosphorous elements lowers the strain rate sensitivity significantly. Second phase hardening with bainite and martensite as the second constituent in a ferritic matrix reduces the strain rate sensitivity of automotive sheet steels. A statistical modeling is proposed to correlate the m‐value with the corresponding quasistatic tensile flow stress.     

The effect of the strain path on the work hardening of austenitic and ferritic stainless steels in axisymmetric drawing

The work-hardening characteristics of metals deeply affect the analytical and numerical analyses of their forming processes and especially the end mechanical properties of the products manufactured. The effects of strain, strain rate, and temperature on work hardening have received wide attention in the literature, but the role of the strain path has been far less studied, except for sheet-metal forming. Strain-path effects seem to have never been analyzed for bulk-forming processes, such as axisymmetric drawing. In the present work, drawn bars were considered as composed of concentric layers strained along varying strain paths. The tensile von Mises effective stress, effective-strain curves of two layers and of the full cross section of the drawn material, were experimentally determined. The flow behavior of these regions was compared to that resulting from pure monotonic-tensile processing. The AISI 420 and 304 stainless steels revealed a strain path and a material effect on their work-hardening characteristics. Higher or lower hardening rates were observed in axisymmetric drawing, as compared to pure tension. These phenomena were interpreted by considering the dislocation arrangements caused by initial drawing straining and their subsequent restructuring, associated with the strain-path change represented by tension after drawing. The analytical and numerical analyses of the tensile behavior of metals following axisymmetric drawing must consider the strain-path effects on the constitutive equations laws and on the hardening behavior of the material. The redundant deformation factor in axisymmetric drawing (φ) plays a central role in the analysis of the process and on the prediction of the mechanical properties of the final products. This parameter was evaluated considering (a) the strain distribution in the bar cross section caused by drawing or (b) the mechanical properties of the drawn bars. The comparison of the results from these two approaches allowed an unexplained interpretation of a material effect on this parameter.     

Influence of Pre‐Straining and Bake Hardening on the Strain Rate Sensitivity of Automotive Sheet Steels

The present investigation deals with the influence of pre‐straining with or without bake hardening on the strain rate sensitivity of automotive sheet steels in typical crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 0.005‐1000 s^?1 and in the temperature range 233‐373K. A bake hardening heat treatment at 170 °C for 20 min without pre‐straining does not influence the m‐value in comparison to the base material condition. A small pre‐straining near plane strain condition, as commonly found in outer door panels, or a 10% uniaxial, plane strain and biaxial pre‐straining, as typically used in formed automotive crash components, without bake hardening does not affect the m‐value of sheet steels in comparison to the base material condition. Uniaxial 2% to 10% pre‐straining, longitudinal or transverse to rolling direction with subsequent bake hardening, does not clearly change the m‐value in comparison to the base material condition either. Small differences in the strain rate sensitivity behaviour are rather attributed to experimental scattering without real physical background.