冲击相变对FeMnNi合金层裂行为影响研究

通过逆向加载和等厚对称碰撞实验相结合的方法,确定了FeMnNi合金完整的加卸载物理过程和相变层裂特征。采用修正的Boettger模型与非平衡两相相变理论模型,成功模拟再现了实验过程,解释了冲击相变、卸载逆相变及稀疏冲击波形成的物理机理。实验和数值模拟相结合,从应力波相互作用的角度定量分析了层裂发生的原因,指出层裂发生的机制正是由于受载样品发生了冲击相变和卸载逆相变。      

低相变阈值金属高压加载下的相变层裂特性

采用激光速度干涉仪（VISAR）和X光联合测试技术,在冲击压力远高于相变应力加载下,实验研究低相变阈值金属FeMnNi合金的相变层裂特性,结果发现等厚对称碰撞加载下FeMnNi合金出现反常的层裂行为。针对该反常的层裂行为,利用塑性波、相变波、稀疏波和逆相变引发的稀疏冲击波的相互作用过程进行分析,结果表明,该实验状态下FeMnNi合金样品存在相变和逆相变行为,逆相变引发的稀疏冲击波是导致反常层裂行为的主要因素。     

HR2钢及几种铁基材料的冲击相变行为

利用双灵敏度VISAR测量了抗氢钢HR2、工业纯铁DT2和铁锰镍合金FeMnNi在一维应变冲击载荷下的自由面速度历史,结合受载样品的回收分析对其动载行为和断裂表现进行了分析。研究表明,在实验加载压力范围内DT2和FeMnNi样品的自由面速度历史呈现包括相变波在内的典型三波结构,而对HR2钢,尽管金相分析显示其加载前后样品的相组织已发生变化,但速度剖面呈现的仅是典型的弹塑性双波结构。分析认为溶质材料成分和初始相组织是无相变波的主要原因。从冲击相变和卸载逆相变角度解释了在等厚靶碰撞时DT2和FeMnNi材料中出现的多重层裂、浅表层裂现象。     

FeMnNi合金的冲击相变和层裂特性的实验研究

对FeMnNi合金进行了轻气炮平板撞击实验研究,实验中材料发生了相变和层裂。得到的FeMnNi合金的相变阈值压力为6.3~6.9 GPa,远低于纯铁的相变阈值压力13 GPa,说明Mn、Ni合金元素的加入会极大的降低相变阈值。回收试样观测表明,当应力高于FeMnNi合金的相变阈值时,样品中可能产生二次层裂现象和浅表层裂新现象。实验结果还表明该合金相变后层裂强度没有明显的提高。     

Identification of damage mechanism and validation of a fracture model based on mesoscale approach in spalling of titanium alloy

The subject of this paper is identification of the physical mechanisms of spalling at low impact velocities for Ti–6Al–4V alloy and determination of the macroscopic stress of spalling via meso-macro approach. Spalling is a specific mode of fracture which depends on the loading history. The aspects of the initial microstructure and its evolution during plastic deformation are very important. In order to identify the spalling physical mechanisms in titanium alloy, numerous pictures by the optical microscopy of the spall surfaces created by plate impact technique have been taken. The scenario of failure observed is in complete agreement with known physical micro-mechanisms: namely nucleation, propagation and coalescence by adiabatic shearing of micro-voids. The most interesting point in spall fracture of Ti–6Al–4V alloy is the nucleation of micro-voids. A significant amount of small micro-voids in the region of the expected spall plane has been observed. It appears that microstructural effects are important due to dual α–β phase microstructure, called Widmanstätten structure. The orientation of microstructure has a direct influence on nucleation mechanism by means of distribution of nucleation sites and decohesion between the softer particles (α-phase lamellae) and the harder lattice (β-phase). According to these observations, a fracture model has been developed. This model is based on the numerous post-mortem microscopic observations of spall specimens. The goal is to determine the macroscopic stress of spalling in function of loading time and damage level via a meso-macro approach.     

基于相变松弛法则的水下爆炸空化模型研究

水下爆炸过程中存在着大量的空化现象,空化的产生、演化及其溃灭过程对于水下冲击波传播、爆炸气泡运动以及水下结构物冲击损伤都会产生重要影响。本文基于多相可压缩流体理论模型,考虑空化发生过程中汽-液两相流体亚平衡状态下两相之间发生的热力学-化学平衡机制,分析汽-液两相介质之间的质量和热量交换,从而实现对相变过程的自动捕捉。该系统的控制方程采用分步法处理,首先利用二阶MUSCL-Hancock格式和HLLC黎曼求解器来求解齐次双曲型方程,再采用牛顿迭代法求解相变方程。数值测试结果表明,本文的计算模型对于空化相变过程具有较好的捕捉能力。最后将该模型应用到水下近水面爆炸空化的数值模拟当中,研究发现空泡的溃灭压力峰值约为冲击波压力峰值的15%,有效作用时间是冲击波载荷有效作用时间的2倍以上。本文的空化相变模型能够为水下爆炸空化现象的机理研究提供重要支撑。     

温度对冲击相变波传播的影响

冲击加载下,相界面的传播是一热力耦合过程。相变波阵面不仅是力学和物质间断面,也是温度界面。为考虑温度对相变波传播的影响,本文首先建立了相界面上的热传导方程和热力耦合的相变本构方程,然后采用一维特征线理论和有限差分数值计算相结合的方法,分析了温度界面和相变波的基本相互作用规律,进而给出了连续温度梯度下和绝热冲击下相变波传播规律。结果表明,温度对相变波传播的作用主要体现在两个方面,一方面是作为温度界面将与各类间断面相互作用,另一方面冲击相变波阵面后区域热力学状态变化影响卸载波结构。其原因在于相变方式（可逆、不可逆）和相变阈值应力具有强烈的温度相关性。     

On the structure of the Hugoniot relation for a shock-induced martensitic phase transition

The Hugoniot curve relates the pressure and volume behind a shock wave, with the temperature having been eliminated. This paper studies the Hugoniot curve behind a propagating sharp interface between two material phases for a solid in which an impact-induced phase transition has taken place. For a solid capable of existing in only one phase, compressive impact produces a shock wave moving into material, say, at rest in an unstressed state at the ambient temperature. If the specimen can exist in either of two material phases, sufficiently severe impact may produce a disturbance with a two-wave structure: a shock wave in the low-pressure phase of the material, followed by a phase boundary separating the low- and high-pressure phases. We use a theory of phase transitions in thermoelastic materials to construct the Hugoniot curve behind the phase boundary in this two-wave circumstance. The kinetic relation controlling the evolution of the phase transition is an essential ingredient in this process.      

相变柱壳的径向冲击特性

采用改进的SHPB (split Hopkinson pressure bar)装置和高速CCD (charge-coupled device)相机对TiNi合金圆柱壳进行了径向冲击实验研究,得到了动态载荷-位移曲线,并采用数字边界提取技术和纯弯曲假定对结构的动态全场变形过程进行了定量分析。结果表明,相变柱壳经受动态冲击后可以恢复,其加、卸载非线性行为与相变状态和相变铰的演化相关。同时,该结构具有良好的横向抗冲效应,对冲击加速度和冲击载荷的衰减量达到95%以上,耗能率为42.4%,可望在可重复使用的抗冲装置中得到应用。     

Phase transition and dynamics of iron under ramp wave compression

The ramp wave compression experiments of iron with different thicknesses were performed on the magnetically driven ramp loading device CQ-4. Numerical simulations of this process were done with Hayes multi-phase equation of state (H-MEOS) and dynamic equations of phase transition. The calculated results of H-MEOS are in good agreement with those of shock phase transition, but are different from those under ramp wave compression. The reason for this is that the bulk modulus of the material in the Hayes model and the wave velocity are considered constant. Shock compression is a jump from the initial state to the final state, and the sound speed is related to the slope of the Rayleigh line. However, ramp compression is a continuous process, and the bulk modulus is no longer a constant but a function of pressure and temperature. Based on Murnaghan equation of state, the first-order correction of the bulk modulus on pressure in the Hayes model was carried out. The numerical results of the corrected H-MEOS agree well with those of pure iron in both ramp and shock compression phase transition experiments. The calculated results show that the relaxation time of iron is about 30 ns and the phase transition pressure is about 13 GPa. There are obvious differences between the isentropic and adiabatic process in terms of pressure–specific volume and temperature–pressure. The fluctuation of the sound speed after 13 GPa is caused by the phase transition.     

气相爆轰波冲击气固界面的透反射特性

为了研究气相爆轰波冲击气固界面过程中透射波和反射波的相关特性,建立爆轰波冲击气固界面的一维理论模型,对不同初始压力条件下爆轰波到达气固界面后的界面两侧的压力和界面速度变化进行分析。利用时空守恒元求解元方法对气相爆轰波冲击气固界面过程进行数值模拟,分析气体部分反射波的压力分布和速度变化规律及透射入固体中应力波的波形和波速特征,并搭建气相爆轰波冲击活塞实验装置进行进一步验证。结果表明:气体爆轰波到达气固界面后,在固体中透射指数形式的弹性波,并在界面处向气体区反射一道激波。爆轰波后的稀疏波与反射激波相交,削弱反射激波,最终形成稳定激波回传。气固界面在稀疏波和反射稀疏波的作用下,压力和速度逐渐下降,最终也形成稳定状态。在不同混气初始压力情况下,爆轰波冲击过程中产生的最高压力和爆压的比值基本保持不变。理论模型对特征点相关物理量的计算值和实验数据符合的较好。     

激光冲击下CoCrFeMnNi高熵合金微观塑性变形的分子动力学模拟

激光冲击强化技术可以有效地提高材料的疲劳寿命, 被广泛应用于航空航天领域. CoCrFeMnNi高熵合金作为一种经典的高熵合金体系, 研究其激光冲击强化后的微观组织变化以及冲击动态响应对该材料未来在航空航天领域中的应用具有重要意义. 采用分子动力学方法, 对CoCrFeMnNi高熵合金进行了冲击模拟, 发现冲击时弹、塑性双波分离现象以及微结构演化具有明显的取向相关性. 沿[100]方向进行冲击时未出现双波分离结构, 并且塑性变形过程中会产生中间相; 而沿[110]与[111]方向冲击时产生了双波分离结构, 并且受冲击区域存在大量的层错以及无序结构, 高位错密度是产生无序结构的重要原因. 双波分离现象与可开动滑移系个数有关, 而沿不同取向冲击时的Hugoniot弹性极限和发生塑性变形的临界冲击速度与其可开动滑移系的Schmid因子大小有关. 此外, 冲击诱导了梯度位错结构的产生, 位错密度沿冲击深度先增加后减小, 在沿原子密排方向冲击时产生了更高的位错密度. 冲击之后在模型两侧存在残余压应力, 芯部为残余拉应力, 残余应力的大小具有明显的取向相关性. 最后, 与具有相同尺寸及取向的纯Ni进行对比, 发现CoCrFeMnNi高熵合金在冲击过程中由于晶格畸变效应产生了较纯Ni更多的无序结构.      

基于分子动力学模拟的单晶硅冲击压缩相变研究

晶体硅具有复杂的相变机制,在相图研究中受到广泛关注,其在动载荷下的变形机制是当前研究热点。为揭示晶体硅在强动加载下的变形和相变行为特征,基于分子动力学方法,采用平板冲击加载方式,模拟研究了单晶硅在初始环境温度为300 K时分别沿[001]、[110]和[111]晶向的不同强度下的冲击压缩行为,冲击粒子速度为0.3～3.2 km/s。研究发现,随着冲击粒子速度的增加,单晶硅剪切应力在逐渐增加后由于结构相变发生急剧下降,相变阈值和相变机制均呈现各向异性。其中,沿[001]晶向冲击压缩下观察到多种固-固相变以及固-液相变,并观察到与最新文献的实验高度一致的固-液共存现象。研究结果可为动加载下晶体硅的相变研究提供纳米尺度的结果支撑。     

An Experimental Technique for Spalling of Concrete

The spalling strength of concrete is measured by examining the strain wave profiles in a polymer buffer bar behind the slender concrete bar specimen placed between a large diameter (Φ100 mm) Hopkinson bar and the buffer bar. The experimental results indicate that the spalling strength is related to not only the compressive strength of concrete but also the impact velocities (the loading rates). The rate effect of spalling strength mainly results from the different cracking paths in concrete under different impact velocities. However when the input compressive stress to specimen exceeds the threshold required to trigger the compressive damage, the spalling strength decreases due to the evolution and cumulation of compressive damage in concretes. The repeated impact loading experiments indicate that damage plays an important role in the spallation process of concrete. The high speed video of the spalling fracture process shows that multiple spalling fractures may occur in the scab and damage accumulation resulting from stress wave propagation in scab is the main reason for the producing of multiple spallations.     

A physical model of the structure and attenuation of shock waves in metals

In this paper, a physical model of the structure and attenuation of shock waves in metals is presented. In order to establish the constitutive equations of materials under high velocity deformation and to study the structure of transition zone of shock wave, two independent approaches are involved. Firstly, the specific internal energy is decomposed into the elastic compression energy and elastic deformation energy, and the later is represented by an expansion to third-order terms in elastic strain and entropy, including the coupling effect of heat and mechanical energy. Secondly, a plastic relaxation function describing the behaviour of plastic flow under high temperature and high pressure is suggested from the viewpoint of dislocation dynamics. In addition, a group of ordinary differential equations has been built to determine the thermo-mechanical state variables in the transition zone of a steady shock wave and the thickness of the high pressure shock wave, and an analytical solution of the equations can be found provided that the entropy change across the shock is assumed to be negligible and Hugoniot compression modulus is used instead of the isentropic compression modulus. A quite approximate method for solving the attenuation of shock wave front has been proposed for the flat-plate symmetric impact problem.     

成分和组织对衬板钢在腐蚀料浆环境下的冲击磨损性能与机理的影响

利用改造后的MLD-10型冲击磨损试验机研究了3种冶金矿山湿磨衬板钢在铁矿石酸性矿浆中的冲击腐蚀磨损行为;采用扫描电子显微镜观察了试样磨损表面形貌,用光学显微镜分析了垂直于试样磨损表面的亚表层金相组织.结果表明:低碳高合金钢的耐冲击腐蚀磨损性能优于高锰钢及中碳合金钢;低碳高合金钢的冲击腐蚀磨损机制主要为挤出硬化棱剥落、轻微的腐蚀磨损及浅层疲劳剥落,高锰钢的主要冲击腐蚀磨损机制为较深层的累积变形疲劳剥落和严重的腐蚀磨损,而中碳合金钢的主要冲击腐蚀磨损机制为深层脆性剥落和严重的腐蚀磨损.     

斜波压缩下锡的相变和层裂行为

获取不同热力学路径下锡的动态响应实验数据,是深入研究其相变和损伤物理过程的基础.利用小型磁驱装置CQ-4完成了金属锡的斜波加载实验,获取了锡含有相变和层裂损伤物理信息的实验数据.实验结果显示,在加载段锡依次经历了弹塑性转变和β-γ相变两种物理过程,屈服强度约0.194 GPa,相变压力随着锡厚度的增加从7.54 GPa减小到7.14 GPa.在卸载段出现了明显的层裂损伤,层裂强度约1.1 GPa,与相同加载压力下冲击实验结果有巨大差异,层裂片厚度约0.38 mm.结合由锡的多相Helmholtz自由能计算的多相状态方程、Hayes相变动力学方程和损伤度理论,对斜波压缩实验过程进行一维流体动力学数值模拟,计算结果可以很好描述锡的弹塑性转变、相变和层裂三个物理过程.     

脆性材料中应力波衰减规律与层裂实验设计的数值模拟

对混凝土、岩石类脆性材料的层裂实验进行了有限元模拟,研究了应力波在此类材料中传播的衰减规律,包括两类机制:弹性波因大尺寸试样的几何弥散产生的小幅度线性衰减、与应变率相关的黏塑性波因本构关系导致的指数衰减。在此基础上,提出了包含常数项的指数型应力波峰值拟合公式。建议采用可以忽略应力波衰减影响的细长形试样进行层裂实验。混凝土类脆性材料层裂破坏模拟结果显示,有限元模拟得到的层裂片厚度与一维应力波理论得到的结果非常吻合,验证了按一维应力波理论确定层裂强度的实验方法的有效性。通过对比3种不同入射波形下层裂片的形状和净拉应力波形,发现不对称的入射波形状更有利于实验获得平直的层裂断面和较准确的层裂强度。     

压剪复合平板冲击加载技术进展及其应用

自20世纪70年代末发明压剪炮以来, 压剪复合冲击加载实验技术和诊断技术有了长足进展, 应用也日益广泛.由于压剪联合加载波直接反映了材料的动态剪切特性, 对于认识材料的屈服、损伤演化、失效、相变、界面滑移等动态行为和机理, 构筑更全面的本构模型能够提供必要的附加信息.本文主要讨论气炮实验中压剪复合应力波的产生方式, 诊断技术, 以及在压剪复合塑性波和动高压本构模型、聚合物压剪冲击行为、剪切波跟踪法(SWT)和水泥基复合材料的损伤和失效、界面动摩擦行为、冲击相变、动态损伤和断裂等方面的研究与应用进展.      

Deformation rate effects on failure modes of open-cell Al foams and textile cellular materials

The compressive behavior of open-cell aluminum alloy foam and stainless steel woven textile core materials have been investigated at three different deformation rate regimes. Quasi-static compressive tests were performed using a miniature loading frame, intermediate rates were achieved using a stored energy Kolsky bar, and high strain rate tests were performed using a light gas gun.In agreement with previous studies on foam materials, the strain rate was not found to have a significant effect on the plateau stress of metallic foams. For all the tests, real time imaging of the specimen combined with digital image correlation analysis allowed the determination of local deformation fields and failure modes. For the Kolsky bar tests, the deformations in the foam specimen were found to be more distributed than for the quasi-static test, which is attributed to moderate inertia effects. The differences in failure mode are more dramatic for the gas gun experiments, where a full compaction shock wave is generated at the impact surface. The stresses in front and behind the shock wave front were determined by means of direct and reverse gas gun impact tests, i.e., stationary and launched specimen, respectively. A one-dimensional shock wave model based on an idealized foam behavior is employed to gain insight into the stress history measurements. We show that the predictions of the model are consistent with the experimental observations. Woven textile materials exhibited moderate dependence of strength on the deformation rate in comparison with open-cell foam materials.