Ti-6Al-4V剪切带内部的应变、温度分布及演变研究

采用梯度塑性理论，考虑了峰值剪切应力之后的材料承载能力缓慢降低的过程及承载能力快速降低的过程，推导了剪切带内部的剪切变形、应变及温度分布的公式。计算了Ti-6Al-4V剪切带内部塑性剪切应变，温度的分布及演变。在剪切带内部，塑性剪切应变及温度分布是高度不均匀的，这种不均匀性随着施加的塑性剪切应变的增加而增加。随着流动剪切应力的降低，剪切带内部的最大塑性剪切应变线性增加，最高温度非线性增加。由于微结构效应，基于梯度塑性理论的剪切带内部的最大塑性剪切应变及最高温度的预测值高于经典理论的预测值。将Ti-6Al-4V剪切带内部的剪切变形及应变的理论结果与根据前人高速摄影实验图片的计算结果进行了对比，理论与实验结果的趋势非常吻合，在数值上，剪切带内部的最大剪切应变的理论值仍低于实测值。     

钛合金绝热剪切带出现时临界塑性剪切应变的估算方法

基于曲线的最小二乘拟合方法,计算了Ti-6Al-4V绝热剪切带出现时的临界塑性剪切应变。根据梯度塑性理论,获得了绝热剪切带内部的局部塑性剪切变形分布曲线的理论表达式,用于拟合Liao及Duffy的实验数据。在不同的绝热剪切带宽度取值条件下,估算了绝热剪切带出现时的临界塑性剪切应变。当绝热剪切带的宽度取值在1~2mm时,计算出的临界塑性剪切应变在0.1~0.47之间。绝热剪切带内部的局部变形分布曲线的理论表达式可以很好地描述流线的非线性特征。当绝热剪切带的宽度取值较大时,绝热剪切带内部的流线的理论结果在两端较弯曲;而当其取值较小时,这些流线比较平直,仅稍微弯曲。     

动态冲击条件下Ti-15Mo时效钛合金的变形机制

通过固溶时效处理Ti-15Mo合金获得片层组织,采用分离式霍普金森压杆（SHPB）研究应变速率对变形机制产生的影响,结合绝热温升、显微组织和硬度分析表明：由于位错与第二相的相互作用,导致流变应力曲线发生波动。提高应变速率,一方面造成应变速率强化;另一方面促进绝热升温软化。合金温度达到379K时,热软化效应超过应变硬化效应,变形方式由均匀塑性变形变为绝热剪切变形。绝热剪切带的宽度随切应变的增加而增大,通过亚晶旋转再结晶机制产生等轴晶粒。再结晶的界面强化导致组织硬度由高到低为：混合组织>条状组织>基体组织。时效处理抑制应力诱发孪生（TWIP）效应,造成合金较低的应变硬化能力,劣化材料的动态力学性能。     

基于实测剪切应力及局部应变的Ti-6Al-4V绝热剪切带的峰值温度估算

认为试样表面的变形场出现不连续性不是绝热剪切带出现的标志,而是形变绝热剪切带进一步发展的结果;在计算绝热剪切带内部的峰值温度时应从局部剪切应变中扣除弹性应变,因为弹性应变不会对塑性功有所贡献。以动态扭转的Ti-6Al-4V试样（TA-50）为例,计算了绝热剪切带内部的峰值温度,其被划分为3部分：环境温度、均匀和非均匀变形引起的温度。在两种条件下（从局部剪切应变中扣除弹性应变与否）,计算出的峰值温度分别为669和665 ℃,其在热回复和再结晶的温度范围之内,未达到相变的温度,比Liao及Duffy的理论计算值（630 ℃）要高。如果剪切应力-局部塑性剪切应变的关系不能完全确定,适当的近似是必要的。     

基于梯度依赖的JOHNSON-COOK模型的韧性绝热剪切敏感性

以基于梯度塑性理论提出的绝热剪切带内部的局部塑性剪切变形分布的理论表达式为基础,研究10个参数对绝热剪切敏感性的影响。对LIAO及DUFFY给出的Ti-6Al-4V绝热剪切带内部的1条流线的实验结果进行最小二乘曲线拟合。估算绝热剪切带宽度取值不同时的临界塑性剪切应变。理论曲线很好地反映了绝热剪切带内部流线的非线性变形特征。利用不同的临界塑性剪切应变值反算了JOHNSON-COOK模型中的一些参数。研究发现,绝热剪切敏感性随着初始静态屈服应力、功热转化因子和应变率参数的降低而降低,这与密度、热容、环境温度及应变硬化指数的影响刚好相反。所提出的模型可以预测绝热剪切带的宽度由高至低的演变过程,直到达到一个稳定值,这一点DODD和BAI模型做不到。     

Effects of constitutive parameters on adiabatic shear localization for ductile metal based on JOHNSON-COOK and gradient plasticity models

1Introduction Adiabatic shear band(ASB)is a very narrow zone with a high concentration of shear strain.It is believed that ASB is formed by a process of thermo-mechanical instability.ASB can be observed in the process of dynamic deformation of various fer…     

Quantitative calculation of local shear deformation in adiabatic shear band for Ti-6Al-4V

JOHNSON-COOK（J-C） model was used to calculate flow shear stress-shear strain curve for Ti-6Al-4V in dynamic torsion test. The predicted curve was compared with experimental result. Gradient-dependent plasticity（GDP） was introduced into J-C model and GDP was involved in the measured flow shear stress-shear strain curve, respectively, to calculate the distribution of local total shear deformation（LTSD） in adiabatic shear band（ASB）. The predicted LTSDs at different flow shear stresses were compared with experimental measurements. J-C model can well predict the flow shear stress-shear strain curve in strain-hardening stage and in strain-softening stage where flow shear stress slowly decreases. Beyond the occurrence of ASB, with a decrease of flow shear stress, the increase of local plastic shear deformation in ASB is faster than the decrease of elastic shear deformation, leading to more and more apparent shear localization. According to the measured flow shear stress-shear strain curve and GDP, the calculated LTSDs in ASB are lower than experimental results. At earlier stage of ASB, though J-C model overestimates the flow shear stress at the same shear strain, the model can reasonably assess the LTSDs in ASB. According to the measured flow shear stress-shear strain curve and GDP, the calculated local plastic shear strains in ASB agree with experimental results except for the vicinity of shear fracture surface. In the strain-softening stage where flow shear stress sharply decreases, J-C model cannot be used. When flow shear stress decreases to a certain value, shear fracture takes place so that GDP cannot be used.     

Analysis of localized shear deformation of ductile metal based on gradient-dependent plasticity

1　INTRODUCTIONFailureprocessofmaterialsisparticularlycom plex ,whichisaprobleminvolvedinmulti scaleandmanydisciplines .Thoughscientistsfrommanycoun trieshavecontributedsomeimportantresultsfortheprobleminrecentyears ,furtherinvestigationsbyme chanicalscientists ,physicistsandmaterialscientistsarenecessarytoobtainafullunderstandingofthefailuremechanisms .Especiallyinlast 2 0 years ,asamechanismofprogressivefailure ,theproblemoflocalizationhasat tractedtopicinterest .Asaconsequenceofsofteni…     

Temperature distribution in adiabatic shear band for ductile metal based on JOHNSON-COOK and gradient plasticity models

1 Introduction Adiabatic shear localization is one of the most important deformation and failure mechanisms in some titanium alloys subjected to moderate and high shear strain rates. Adiabatic shear band(ASB) can be observed in various applications, such as metal forming, perforation, impact on structures, ballistic impact, machining, torsion, explosive fragmentation, grinding, interfacial friction, powder compaction and granular flow[1?15]. The formation of ASBs is often followed by ductile…     

Localized shear deformation during shear band propagation in titanium considering interactions among microstructures

Closed-form analytical solutions of plastic shear strain and relative plastic shear displacement during shear band propagation are proposed under dynamic loadings based on gradient-dependent plasticity considering the effect of microstructures due to heterogeneous texture of Ti. According to the differences in shear stress levels, Ti specimen is divided into three regions: residual region, strain-softening region and elastic region. Well-developed shear band is formed in the residual region and the relative plastic shear displacement no longer increases. In the normal and tangential directions, the plastic strain and the displacement are nonuniform in the strain-softening region.At the tip of shear band, the shear stress acting on the band is increased to shear strength from the elastic state and the shear localization just occurs. Prior to the tip, Ti remains elastic. At higher strain rates, the extent of plastic strain concentration is greater than that under static loading. Higher strain rate increases the relative plastic shear displacement. The present analytical solution for evolution or propagation of shear localization under nonuniform shear stress can better reproduce the observed localized characteristics for many kinds of ductile metals.     

Ti2AlNb合金高应变率下流动应力特征与本构关系

利用电子万能试验机和分离式Hopkinson压杆得到Ti_2AlNb合金准静态拉伸曲线及高应变率下动态压缩应力-应变曲线,观察分析变形后试样的微观组织,研究其高应变率下的流动应力特征。结果表明:在应变率2500～7500 s-1范围内,Ti_2AlNb合金的流动应力对应变率有较强的敏感性,且具有应变强化、应变率增强及增塑效应;应变率为5500、6500、7500s-1的3组试样中观察到了与加载方向约成45°的绝热剪切带。改进Johnson-Cook本构模型,拟合实验数据得到Ti_2AlNb合金室温下的动态塑性本构关系,与实验对比,改进后的模型能够较好地描述Ti_2AlNb合金在高应变率下的流动应力。     

辽宁工程技术大学，辽宁 阜新 123000

提出了利用梯度塑性理论计算Ti-6Al-4V绝热剪切带的局部剪切应变新方法.绝热剪切带的最大局部塑性剪切应变依赖于临界塑性剪切应变、试样的标定长度、绝热剪切带总厚度、绝热剪切带的平均塑性剪切应变.计算表明,随着绝热剪切带总厚度的增加,绝热剪切带的最大局部塑性剪切应变以非线性方式下降.当绝热剪切带总厚度的取值接近1 mm时,尽管确定临界塑性剪切应变的方法不同,但是,绝热剪切带的最大局部塑性剪切应变的计算值差别很小.当绝热剪切带总厚度取值在0.335～1 mm之间时,绝热剪切带的最大局部塑性剪切应变的计算值位于Liao及Duffy(1998)实验结果的下限(75%)和上限(350%)之间.     

30MnCrNiMoB低合金钢绝热剪切带的形成机制与微结构

研究了３０ＭｎＣｒＮｉＭｏＢ低合金钢在穿甲弹冲击下绝热剪切带（ＡＳＢ）的形成机制。指出在高速变形（冲击）条件下ＡＳＢ的形成是应变硬化、热软化和位错动态发时导致的应变软化等因素相互耦合产生的塑性失稳现象。进一步确认ＡＳＢ内具有周期性微观结构，并说明ＡＳＢ中周期性开裂与微结构的关系。     

TC4合金绝热剪切动态演变过程数值模拟研究

对TC4合金强迫剪切过程进行了数值模拟,获得了绝热剪切过程中变形局域化区域内的von-Mises应力、有效塑性应变以及温度的分布规律。通过分析发现：大量应变集中于帽形试样变形局域化区域内,且在试样两拐角处最大;变形局域化区域内温度明显高于基体温度,且在试样两拐角处最高。结果表明,绝热剪切带内温度达到了TC4合金的再结晶温度,这为TC4合金绝热剪切带内微观组织变形机制的研究提供了依据     

钛合金绝热剪切的敏感性分析及环境温度的影响

通过引入与Batra及Kim类似的观点,将绝热剪切带宽度定义为绝热剪切带的中心区域的宽度(W5%),在该区域上温度比其峰值小5%,利用Johnson-cook模型及梯度塑性理论分析Ti-6A1-4V绝热剪切带的厚度随环境温度的演变规律.计算表明,随着环境温度的升高,绝热剪切带宽度增加,这与许多实验观测结果一致.当绝热剪...     

高体积分数TiB2/Al复合材料的动态压缩性能

采用分离式霍普金森压杆(SHPB)研究高体积分数TiB2/Al复合材料(55%-65%)的动态压缩性能,并对其在高应变率下的损伤机制进行分析。结果表明,相比应变率不敏感的基体合金,高体积分数TiB2/Al复合材料表现出明显的应变率敏感性;且随着体积分数的增加,其应变率敏感系数和流动应力值表现出先升后降趋势。在高应变率冲击下,复合材料表现为脆性断裂,增强相含量越高复合材料越易剪切破坏。复合材料断面局部呈现熔融铝团,直径为50～200μm,这与绝热温升效应有关。绝热温升使得基体合金软化,有效缓解高体积分数复合材料的瞬态失稳破坏。     

Phenomenon of transformed adiabatic shear band surrounded by deformed adiabatic shear band of ductile metal

The coexistent phenomenon of deformed and transformed adiabatic shear bands（ASBs） of ductile metal was analyzed using the JOHNSON-COOK model and gradient-dependent plasticity（GDP）. The effects of melting point, density, heat capacity and work to heat conversion factor were investigated. Higher work to heat conversion factor, lower density, lower heat capacity and higher melting point lead to wider transformed ASB and higher local plastic shear deformation between deformed and transformed ASBs. Higher work to heat conversion factor, lower density, lower heat capacity and lower melting point cause higher local plastic shear deformation in the deformed ASB. Three reasons for the scatter in experimental data on the ASB width were pointed out and the advantages of the work were discussed. If the transformed ASB width is used to back-calculate the internal length parameter in the GDP, undoubtedly, the parameter will be extremely underestimated.     

铝基复合材料的绝热剪切失效机理分析

采用分离式霍普金森压杆(SHPB)研究了TiB2/Al复合材料在高应变率压缩下的绝热剪切失效行为,并对其动态失效机理进行了分析。结果表明,高体积分数TiB2/Al复合材料在高应变率压缩下表现出流动应力软化现象,这与带温度软化系数的Johnson-Cook模型预测值一致,而采用弹线性硬化Cowper-Symonds模型的预测值远高于本测试值。TiB2/Al复合材料试样在动态压缩下呈45o剪切断裂或劈裂,在绝热剪切面上发现大量熔融铝相变带。分析表明,绝热剪切带的形成在复合材料内部形成局部低强度区域,从而诱发了材料的瞬间失稳破坏。     

TC6钛合金动态压缩变形中“应力塌陷”的形成原因

采用分离式Hopkinson Bar技术和一种新型的中断动态试验方法对TC6钛合金进行了动态压缩试验,通过光学显微镜、扫描电子显微镜分析了TC6钛合金变形到不同应变量时所产生绝热剪切带的微观形貌,通过与宏观力学响应相对应,研究了TC6钛合金在动态压缩变形中,绝热剪切带的形成过程及造成应力快速下降的原因。结果表明:在高应变率下,材料绝热剪切带的形成是一个由萌生、扩展、完全发展组成的过程;在应变率为2.5×10~3s~(-1)的动态压缩过程中,"应力塌陷"现象是由于材料内产生了大于一定尺寸的微裂纹所致。     

Adiabatic shear critical condition in the high-speed cutting

An adiabatic shear critical condition in the high-speed cutting was built by linear perturbation analysis. The effects of material properties and deformation conditions on the onset of adiabatic shear were quantitatively investigated. The results show that the adiabatic shear is likely to form under the conditions of smaller strain hardening index, smaller strain rate hardening index, larger thermal softening index, larger viscosity, larger strain, larger strain rate, larger compression stress, lower initialization temperature, smaller density and smaller thermal capacity. The effect of thermal conductivity on the onset of adiabatic shear can be neglected.