【作者】 肖大武；

【导师】 胡时胜； 李英雷；

【作者基本信息】 中国科学技术大学 ， 工程力学， 2008， 博士

【摘要】 本文采用了MTS材料试验机和分离式霍普金森压杆（SHPB）装置对纯锆在不同温度（298K-1073K）和应变率(10^-4s^-1-2800s（-1）)下的压缩力学性能进行了系统的研究。进行高应变率实验时采用了小尺寸试样SHPB实验技术。通过数值模拟和部分实验对小尺寸试样实验涉及的二维效应、偏心压缩效应以及杆弯曲对SHPB测量精度的影响等问题进行了详细的分析。其中,横截面不匹配引起的二维效应主要表现为端面凹陷,而端面凹陷对于小变形下应变的测量精度影响很大,进入塑性变形后端面凹陷带来的影响迅速减小。偏心压缩效应可以通过对称贴片或靠后贴片的办法予以消除。采用小尺寸试样SHPB技术对纯锆高应变率下力学行为进行探索时,首次在纯锆中发现了绝热剪切失稳现象;对弯曲杆的直撞有限元模拟结果表明,随弯曲程度的增大,波形失真程度呈幂函数形式增长,提出了霍普金森杆实验中杆允许的弯曲程度,并进行了实验验证。通过在试样和压杆之间添加A95陶瓷块建立了一套组合杆式高温SHPB装置,采用有限元模拟对添加陶瓷块及温度梯度场对测试精度的影响进行了分析。基于以上实验技术得到的纯锆不同温度和应变率下的实验结果表明:常温下不同应变率纯锆应力～应变曲线流动应力段呈发散状,流动应力和加工硬化随应变率提高而增大,随温度升高而降低。常温下动态压缩试验中大应变时试样发生与载荷方向约成45°沿对角线的宏观剪切破坏,单次大变形和重复加卸载大变形动态压缩应力～应变曲线材料的破坏应变几乎相同,但随变形增大,单次大变形的加工硬化明显要低。微观金相观测结果表明,孪晶密度随应变率提高而增大,随温度升高而锐减。在小变形段,孪晶密度随应变增大而增大;当变形进一步增大,孪晶之间及孪晶与晶界及位错滑移带之间发生交互作用,试样内部可见许多微小孪晶簇;大应变试样中可见绝热剪切带的萌生,剪切带附近的孪晶弯成弓形靠拢。破坏试样SEM断口形貌呈典型的剪切破坏特征。宏观应力-应变曲线的加工硬化行为与孪晶的演化发展相对应,绝热剪切带可能是由微孪晶簇内孪晶取向的不一致诱发的。结合试验结果,在位错动力学的基础上建立了描述纯锆力学性能温度和应变率相关的本构方程,采用该本构方程拟合了纯锆在不同温度和应变率下的应力应变曲线,结果发现除了大应变和温度较低时孪晶影响比较显著外,拟合的结果和试验结果符合很好。由于在微观分析中发现孪晶变形主要出现在常温条件下,因此通过考虑孪晶引起的硬化效应在本构方程中引入修正,经过修正后的本构方程能够很好地描述孪晶变形和位错滑移对纯锆温度和应变率相关力学性能的影响。采用LS-DYNA接口技术,将得到的本构模型嵌入LS-DYNA中,对金属圆管动态破坏SHPB实验及爆炸载荷下柱壳的变形和破坏过程进行了数值模拟,给出了材料内部的应力、温度、损伤的时程曲线。最后对全文的工作进行了总结和展望。更多还原

【Abstract】 Compression mechanical properties of Zirconium under different temperature and different strain rates were studied by MTS and split Hopkinson pressure bar（SHPB） apparatus.Small SHPB tests was used to investigate mechanical properties under high rates.Two-dimensional effects would be brought into small specimen SHPB tests,and this effect was mainly elastic indentation of the bar’s interface with the specimen.The two-dimensional effects decreased rapidly,when the specimen developed into plastic deformation stage.The influence could be eliminated efficiently by mounting strain gauges in series or far away from the specimen-bar interfaces in the eccentric compression tests.Also,a distortional signal would be recorded when a bent bar was used in test.It’s found that the amplitude of distortion increases with the increasing bending.A limiting condition for bent bar in Hopkinson bar tests was presented.Also, these difficulties were evaluated and testified by numerical simulations and some experiments.The experiment system of high temperature split Hopkinson pressure bar （SHPB）by adding ceramic platens between bars and specimen was investigated in this paper,which is suitable for measuring the dynamic response of metals at 1073K. The influence of ceramic platens and temperature field was analyzed.Compression experiments on the HP-Zr were carried out in a temperature range of 298K-1073K and a strain rate range of 10^-4s^-1-2800s^-1.Results indicated that:The stress～strain curves show an unusual strain hardening behavior,flow stress and strain hardening increased with the increasing strain rate,and decreased with the increasing temperature.Macroscopic ductile cracking at 45°to the compression axis is seen for large deformation tests.The failure strain of single large deformation test was the same as that of small strain reloading tests.Optical micrographs show that the twinning increased with an increase in strain and strain rates,and decreased rapidly with the increasing temperature.The cylindrical compression specimens failed by adiabatic shearing bands（ASB）.In small deformation stage,the twinning density increased with the increasing strain;further more,the interactions between twins, grain boundry and slip band,many micro-twins were seen;adiabatic shearing bands were seen in large deformation specimens,the twins bowed toward to the shearing bands.SEM graphs of failure surface appeared typical shearing rupture characteristics. The strain hardening of stress～strain curves coincided with the twinning development in specimen during deformation,the ASB might be caused by in-homogeneity of twinning orientations.A constitutive model based on the dislocation kinetics was used to describe the stress-strain relationship of HP-Zr at the investigated temperatures and strain rates.Furthermore,a modification incorporating the twinning effect was applied to the model.The correlations between the simulated curves and the experimental results are in good agreement,except for the conditions in which developing twins play an important role.With the interface of LS-DYNA and the theory of plastic constitutive relation,the constitutive model based on the dislocation kinetics was used to study the dynamic failure of metallic tube of SHPB tests and the process of deformation and failure of cylinder shells subjected to internal sliding explosion.The time curves of the stress, temperature and damage were given,which would gave reference to the test.The final part is the summary of the paper and the prospect of the future work.更多还原