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。     

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.     

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.     

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。     

钛合金绝热剪切带宽度的演变规律研究

根据梯度增强的Johnson-Cook模型,对Ti-6Al-4V绝热剪切带中心区域的宽度(绝热剪切带宽度w5%)随平均塑性剪切应变的演变规律进行了预测。结果表明,随着平均塑性剪切应变的增加,w5%先是快速减小,然后趋于稳定。当绝热剪切带总宽度为0.3235mm时,w5%的稳定值接近Ti-6Al-4V绝热剪切带宽度的上限(55μm);当绝热剪切带总宽度为0.0705mm时,w5%的稳定值接近Ti-6Al-4V绝热剪切带宽度的下限(12μm)。绝热剪切带宽度受多种因素影响,例如,材料特性、加载速度、环境温度、应力及应变状态。本文的分析是在绝热条件下进行的,不存在塑性功率与热传导达到平衡(稳态)的假定。     

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。     

Ti-6Al-4V绝热剪切带的厚度及应变率效应研究

通过引入与Batra及Kim类似的论点,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-6Al-4V绝热剪切带的厚度及应变率的影响.计算表明,在名义应变率的下限(800s-1)及上限(1400s-1)之间,当绝热剪切带的总厚度选取为0.3235及0.0705 mm时,计算结果非常接近于绝热剪切带宽度的上限(55 um)及下限(12 um).当应变率较低时,绝热剪切带较宽.随着应变率的增加,绝热剪切带宽度快速降低.在高应变率时,绝热剪切带宽度基本保持恒定.该理论结果与Dodd及Bai的理论结果有类似之处,与Weerasooriya及Beaulieu针对钨合金的实验结果非常一致,与Klepaczko及Rczaig的数值结果的前半部分相似.     

高应变速率下TC11钛合金动态剪切行为与性能

采用分离式霍普金森压杆（Hopkinson Bar）装置系统,对TC11钛合金进行室温高应变速率（700-2100s^-1）动态剪切试验,通过光学显微镜、显微硬度分析仪、扫描电镜研究了TC11钛合金动态剪切行为、绝热剪切带微观组织与性能。结果表明：TC11钛合金随应变速率的提高绝热剪切敏感性增加;绝热剪切带由过渡区域的变形拉长组织和中间部位的细小晶粒组织组成,具有清晰的剪切变形流线,宽度约为10μm;绝热剪切带内的显微硬度值高于基体组织,是,由应变速率强化和应变强化与热软化相互作用的结果。     

显微组织和应变速率对TC4合金动态力学性能和绝热剪切带的影响

通过热处理成功获得了等轴、片层和双态3种组织结构的TC4合金,并研究了其动态力学性能及变形机制。通过对比硬化-软化转变临界剪切应变率、最大抗剪切强度、萌发绝热剪切带的临界剪切应变速率和承载时间4个指标评估了3种组织合金的动态力学性能。结果表明,片层组织TC4合金具有较高的抗剪切强度和临界剪切应变速率以及最低的绝热剪切敏感性,其动态力学性能最佳。进一步的微观结构分析表明,3种组织合金中所形成的绝热剪切带均为脆性剪切带,且剪切带宽度随剪切应变速率的增大而减小。当剪切应变速率相同时,3种组织合金的剪切带宽度由大到小依次为：片层组织、双态组织、等轴组织。     

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.     

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.     

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。     

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。