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。     

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.     

钛合金绝热剪切带的局部剪切应变估算的新方法

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.     

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。     

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。     

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     

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。     

纯锆的动态力学行为及绝热剪切带

使用分离式霍普金森压杆装置(SHPB)对纯锆在室温下进行动态压缩实验,应变速率为800～4000 s-1。采用金相显微镜和Quanta200型扫描电子显微镜对绝热剪切带及压缩断口进行观察。结果表明,纯锆具有较低的应变速率敏感性,由于高应变条件下产生了高密度的孪晶,材料硬化显著增强,真应力-真应变曲线随应变增加呈上升趋势,最大抗压强度为843 MPa。纯锆试样中观察到明显的绝热剪切带,且沿剪切带出现裂纹,压缩断口呈韧性断裂。绝热剪切带的出现和发展是材料失效的主要原因。对剪切带内部的温度进行了估算,结果表明,纯锆剪切带内部最高温度为1564℃。     

高碳硅锰钢的动态力学性能

通过扫描电镜、霍普金森压杆和拉伸试验机等仪器研究了不同温度淬火高碳硅锰钢的组织、变形程度和动态力学性能。研究表明,随淬火温度提高,试验钢中残留碳化物数量减少,在880 ℃时,试验钢的残留碳化物全部溶解。动态力学试验后,880 ℃淬火试样因残留碳化物固溶,动态强度增幅较小,对应变速率不敏感,由均匀变形转为破碎。绝热剪切带内和远离绝热剪切带的区域均存在孔洞,远离剪切带区域的孔洞无序分布,且伴随有均匀分布的碳化物。靠近绝热剪切带区域的碳化物存在分布不均匀的情况,且绝热剪切带内部的孔洞沿着热量传播方向扩展,形成沿剪切带分布的裂纹。     

温度对TB6钛合金动态压缩性能的影响

利用分离式高温Hopkinson压杆试验系统,对TB6钛合金在不同温度(550～850℃)下的动态冲击压缩力学性能进行测试,研究环境温度对试验材料在高应变率下的应力-应变行为和绝热剪切带生成机制的影响。试验结果表明,在高应变率条件下,随着温度的升高,绝热升温热软化作用增强,材料表现出较强的应变率增塑效应和一定程度的应变率增强效应。同时发现,材料在650～750℃表现出较强的绝热剪切敏感性,承载时间越长,剪切带长度越长。材料在750℃时,剪切带出现分叉,导致材料强度异常降低。随着环境温度的增加,剪切断口韧窝面积增大,韧窝数量增加,材料塑性增强。     

Calculation of temperature distribution in adiabatic shear band based on gradient-dependent plasticity

A method for calculation of temperature distribution in adiabatic shear band is proposed in terms of gradient-dependent plasticity where the characteristic length describes the interactions and interplaying among microstructures. First, the increment of the plastic shear strain distribution in adiabatic shear band is obtained based on gradient-dependent plasticity. Then, the plastic work distribution is derived according to the current flow shear stress and the obtained increment of plastic shear strain distribution. In the light of the well-known assumption that 90% of plastic work is converted into the heat resulting in increase in temperature in adiabatic shear band, the increment of the temperature distribution is presented. Next, the average temperature increment in the shear band is calculated to compute the change in flow shear stress due to the thermal softening effect. After the actual flow shear stress considering the thermal softening effect is obtained according to the Johnson-Cook constitutive relation, the increment of the plastic shear strain distribution, the plastic work and the temperature in the next time step are recalculated until the total time is consumed. Summing the temperature distribution leads to rise in the total temperature distribution. The present calculated maximum temperature in adiabatic shear band in titanium agrees with the experimental observations. Moreover, the temperature profiles for different flow shear stresses are qualitatively consistent with experimental and numerical results. Effects of some related parameters on the temperature distribution are also predicted.     

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…     

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…     

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.     

AZ31镁合金中绝热剪切带的组织演变规律

为了深入了解镁合金绝热剪切带与裂纹的关系,进而揭示镁合金在高速冲击载荷作用下局部变形绝热剪切的组织演变规律,采用分离式Hopkinson压杆对AZ31镁合金的帽状式样进行冲击压缩实验,而后利用光学显微镜,扫描电镜和维氏硬度计分别对冲击后的AZ31试样进行分析。结果表明,绝热剪切带形成于最大剪应力方向,随着冲击载荷的不断增加,沿着切应力方向上的微孔洞和微裂纹不断长大,直至彼此相互连接成裂纹,最终导致材料的断裂。经对剪切带及周围组织维氏硬度的测量发现,剪切带内细小晶粒区的硬度明显高于周围组织。     

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.     

A method for calculating damage evolution in adiabatic shear band of titanium alloy

A method for calculating the evolution of the local damage variable at the adiabatic shear band (ASB) center was proposed. In the present method, the JOHNSON-COOK model and the nonlocal theory were adopted, and the damage variable formula applicable for the bilinear (linearly elastic and strain-softening) constitutive relation was further generalized to consider the plastic deformation occurring in the strain-hardening stage. Aiming at Ti-6Al-4V, the effect of strain rate on the evolution of the local damage variable at the ASB center was investigated. In addition, a parametric study was carried out, including the effects of strain-hardening exponent, strain rate sensitive coefficient, thermal-softening exponent, static shear strength, strain-hardening modulus, shear elastic modulus, work to heat conversion factor, melting temperature and initial temperature. The damage extent at the ASB center in the radial collapse experiment was assessed. It is found that at higher strain rates the damage in the ASB becomes more serious at the same average plastic shear strain of the ASB.     

细晶93W-4.9Ni-2.1Fe合金动态压缩下绝热剪切带的形成及其特征

利用分离式Hopkinson动态压缩装置对添加0.03%Y2O3(质量分数, 下同)的细晶93W-4.9Ni-2.1Fe合金试样进行动态力学性能测试,观察分析了动态压缩后合金试样的显微组织。结果表明：在应变速率为1900 s-1下,合金沿着与冲击方向成45o的方向形成了明显的绝热剪切带,宽度10~25 μm。说明该合金对局部绝热剪切的敏感性大大提高且能在相对较低的应变速率下发生绝热剪切。同时位于剪切带中心区域的钨颗粒沿着其扩展方向被剧烈拉长成纤维状,表现出塑性流动局域失稳的特征