三角形波致LY12铝层裂的平板冲击实验研究

采用基于冲击波衰减动力学的实验技术,使平板碰撞层裂实验的LY12铝样品中,产生三角形冲击脉冲.采用VISAR技术记录样品自由表面历史,获得样品中发生层裂的信号.在经过高应力三角形波实验的软回收样品上,发现了两个层裂面.就样品中呈现三角形脉冲的平板冲击实验而言,样品中可以存在一些高拉伸应力区域并发生多次层裂.将该文提出的基于空穴聚集的层裂模型用于数值模拟这些特定条件的平板冲击试验,并将计算的样品自由面速度历史及样品中的损伤分布与实测的VISAR数据及软回收的样品层裂面等作了比较.研究表明,人为粘性、状态方程、本构方程以及层裂模型对于数值模拟三角形冲击脉冲引起的层裂有严重影响.     

Determination and interpretation of statistics of spatially resolved waveforms in spalled tantalum from 7 to 13 GPa

A suite of impact experiments was conducted to assess spatial variability in the dynamic properties of tantalum, on length scales of tens of microns to a few millimeters. Two different sample types were used: tantalum processed to yield a uniform refined grain structure (grain size ∼20 μm) with a strong axisymmetric {1 1 1} crystallographic texture, and tantalum processed to yield an equiaxial structure with grain size ∼42 μm. Impact experiments were conducted loading the samples to stress levels from 6 to 12 GPa, which are well above the Hugoniot Elastic Limit (HEL), then pulling the sample into sufficient tension to produce spall. These stress levels were specifically chosen to investigate the spall behavior of tantalum at levels ranging from the incipient spall stage to significantly above the spall strength, focusing on microstructural phenomena. A recently developed spatially resolved velocity interferometer known as the line-imaging VISAR allowed the point-to-point variability of the spall strength to be determined. Specifically, we have been able to determine in real time the nucleation and growth of void defect structures that lead to the eventual spallation or delaminating of the plate. Experiments indicate that the nucleation and growth process is time-dependent and heterogeneous since a time-dependent distribution of defects is measured. This strongly suggests that the spall strength of the material is not a single-valued function. When fitted to Weibull failure statistics, the results indicate a similar mean value and variability for the spall strength of both types of tantalum. The spatial dependence of the material distension of the spalled tantalum is also deduced, in the approximation of uniaxial strain.     

拉伸应变率对20钢层裂特性的影响

采用轻气炮加载技术和激光速度干涉(VISAR)测速技术相结合,对不同拉伸应变率条件下20钢的层裂特性进行了实验研究。通过改变飞片和样品的几何尺寸来调整拉伸应变率的大小,研究了拉伸应变率对20钢层裂强度的影响。实验的拉伸应变率的变化范围为104~106 s-1,最大拉伸应变率接近激光加载所能产生的拉伸应变率,相比激光加载,薄飞片技术容易保证一维应变条件。实验结果显示20钢的层裂特性明显依赖着拉伸应变率的大小,106 s-1条件下层裂强度比104 s-1时提高近70%。基于对数值计算结果的分析,讨论了影响层裂强度的主要外载荷因素。     

On probabilistic aspects in the dynamic degradation of ductile materials

Dynamic loadings produce high stress waves leading to the spallation of ductile materials such as aluminum, copper, magnesium or tantalum. The main mechanism used herein to explain the change of the number of cavities with the stress rate is nucleation inhibition, as induced by the growth of already nucleated cavities. The dependence of the spall strength and critical time with the loading rate is investigated in the framework of a probabilistic model. The present approach, which explains previous experimental findings on the strain-rate dependence of the spall strength, is applied to analyze experimental data on tantalum.     

晶粒尺寸对高纯铝板材层裂特性的影响

采用不同热处理工艺制备了3种晶粒尺寸（60、100、500 μm）的高纯铝板材,利用平板撞击实验研究了其层裂行为。通过改变飞片击靶速度,在靶板中实现初始层裂状态和完全层裂状态。基于自由面速度时程曲线和微损伤演化及断口显微形貌分析,讨论了晶粒尺寸对高纯铝板材层裂特性的影响规律。实验结果显示:（1）晶粒尺寸对高纯铝板材层裂特性的影响强烈依赖于冲击加载应力幅值,在低应力条件下,层裂强度与晶粒尺寸之间表现出反Hall-Petch关系,而在高应力条件下,晶粒尺寸对层裂强度几乎没有影响;（2）随着晶粒尺寸的增大,靶板损伤区微孔洞的尺寸和分布范围均增大,但数量显著减少,在微孔洞周围还发现比较严重的晶粒细化现象;（3）随着晶粒尺寸的增大,层裂微观机制从韧性沿晶断裂向准脆性沿晶断裂转变,且在断口上观察到少量随机分布的小圆球,归因于微孔洞长大和聚集过程中严重塑性变形引起的热效应。     

预应力对LY12铝层裂强度的影响

通过过盈配合的热装配方法和机械夹紧方法对LY12铝合金平面样品施加了径向预应变,采用平面飞片撞击方法获得了预应力对层裂特性的影响。结果表明:当飞片以500 m/s的速度撞击样品,预应变为0（无预应变）、25110-6、1 10810-6、1 88610-6时,层裂强度分别为1.130、0.935、0.755、0.643 GPa,初始预应力（变）明显降低了LY12铝层裂强度,证实了材料的层裂强度与初始应力状态密切相关,可为以后的非一维平面应变下的层裂研究提供参考。     

Mixed-mode failure of thin films using laser-generated shear waves

A new test method is developed for studying mixed-mode interfacial failure of thin films using laser generated stress waves. Guided by recent parametric studies of laser-induced tensile spallation, we successfully extend this technique to achieve mixed-mode loading conditions. By allowing an initial longitudinal wave to mode convert at an oblique surface, a high amplitude shear wave is generated in a fused silica substrate and propagated toward the thin-film surface. A shear wave is obtained with amplitude large enough to fail an Al film/fused silica interface and the corresponding shear stress calculated from high-speed interferometric displacement measurements. Examination of the interfaces failed under mixed-mode conditions reveals significant wrinkling and tearing of the film, in great contrast to blister patterns observed in similar Al films failed under tensile loading.     

炸药爆炸驱动下铜板的层裂现

在进行炸药驱动铜板实验中,用激光速度干涉仪(VISAR)记录铜板自由面速度,观察到了铜板层裂现象,并用非线性有限元方法对爆炸载荷下铜板层裂过程进行数值模拟分析,得到了与实验结果吻合的计算结果。     

Mixed-mode interfacial adhesive strength of a thin film on an anisotropic substrate

The mixed-mode interfacial adhesion strength between a gold (Au) thin film and an anisotropic passivated silicon (Si) substrate is measured using laser-induced stress wave loading. Test specimens are prepared by bonding a fused silica (FS) prism to the back side of a 〈1 0 0〉 Si substrate with a thin silicon nitride (Si_xN_y) passivation layer deposited on the top surface. A high-amplitude stress wave is developed by pulsed laser ablation of a sacrificial absorbing layer on one of the lateral surfaces of the FS prism. Due to the negative non-linear elastic properties of the FS, the compressive stress wave evolves into a decompression shock with fast fall time. Careful selection of the incident angle between the pulse and the FS/Si interface generates a mode-converted shear wave in refraction, subjecting the Si_xN_y/Au thin film interface to dynamic mixed-mode loading, sufficient to cause interfacial fracture. A detailed analysis of the anisotropic wave propagation combined with interferometric measurements of surface displacements enables calculation of the interfacial stresses developed under mixed-mode loading. The mixed-mode interfacial strength is compared to the interfacial strength measured under purely tensile loading.     

Spall strength of glass fiber reinforced polymer composites

In the present paper results of a series of plate impact experiments designed to study spall strength in glass–fiber reinforced polymer composites (GRP) are presented. Two GRP architectures are investigated—S2 glass woven roving in Cycom 4102 polyester resin matrix and a balanced 5-harness satin weave E-glass in a Ciba epoxy (LY564) matrix. The GRP specimens were shock loaded using an 82.5 mm bore single-stage gas-gun. A velocity interferometer was used to measure the particle velocity profile at the rear (free) surface of the target plate. The spall strength of the GRP was obtained as a function of the normal component of the impact stress and the applied shear-strain by subjecting the GRP specimens to normal shock compression and combined shock compression and shear loading, respectively. The spall strengths of the two GRP composites were observed to decrease with increasing levels of normal shock compression. Moreover, superposition of shear-strain on the normal shock compression was found to be highly detrimental to the spall strength. The E-glass reinforced GRP composite was found to have a much higher level of spall strength under both normal shock compression and combined compression and shear loading when compared to the S2-glass GRP composite. The maximum spall strength of the E-glass GRP composite was found to be 119.5 MPa, while the maximum spall strength for the S2 glass GRP composite was only 53.7 MPa. These relatively low spall strength levels of the S2-glass and the E-glass fiber reinforced composites have important implications to the design and development of GRP-based light-weight integral armor.     

内爆和柱壳条件下无氧铜的层裂特性

利用任意反射面位移干涉系统(DISAR)激光测速技术,成功地获得了滑移内爆加载和柱壳结构条件下无氧铜的内表面（自由面）速度剖面,并对其层裂特性进行了初步分析。结果表明:(1)在固定炸药和改变无氧铜圆管壁厚条件下,层裂片厚度随着圆管壁厚h的减小而增加;以圆管壁厚h为参照进行归一化,则相对层裂片厚度(/h)随相对装药厚度(he/h)的增大而增加,这种规律与以往对20钢的研究结果一致,但圆管发生层裂的临界条件,却显示出明显的材料相关性。(2)初步来看,无氧铜的层裂强度对结构的依赖性不明显,而与加载脉冲的幅值和宽度相关。(3)受无氧铜粘性和Taylor波衰减的影响,无氧铜的层裂强度随管壁厚度的增加而略有降低;同时,材料分散性也对此有一定影响。     

Spallation of polycarbonate under laser-driven shocks

Laser driven shocks have been used to investigate dynamic failure (spallation) of polycarbonate under uniaxial tensile loading at very high strain rate, of the order of 10 s. First, uninstrumented recovery shots have been performed, post-test examination of the fracture damage has been carried out, and the influences of the experimental parameters (loading conditions and target thickness) have been analyzed. Then, an attempt to model the response of polycarbonate to plane shock loading has been made. On one hand, in-situ measurements have been performed in polycarbonate samples submitted to the plane detonation wave of a strong explosive, and the results have led to content with simple constitutive relations. On the other hand, piezoelectric measurements under laser shocks have provided a characterization of the loading pressure pulse, and comparisons of the measured and computed signals have confirmed the ability of the model to describe wave propagation in polycarbonate. Finally, the spallation experiments have been simulated. A spall strength has been estimated, on the basis of the experimental data, and the predictive capability of the model has been tested. Received: 18 February 1997 / Accepted 1 April 1997     

PROGRESSIVE FRACTURE MODELING OF THE FAILURE WAVE IN IMPACTED GLASS

The failure wave has been observed propagating in glass under impact loading since 1991. It is a continuous fracture zone which may be associated with the damage accumulation process during the propagation of shock waves. A progressive fracture model was proposed to describe the failure wave formation and propagation in shocked glass considering its heterogeneous meso-structures. The original and nucleated microcracks will expand along the pores and other defects with concomitant dilation when shock loading is below the Hugoniot Elastic Limit. The governing equation of the failure wave is characterized by inelastic bulk strain with material damage and fracture. And the inelastic bulk strain consists of dilatant strain from nucleation and expansion of microcracks and condensed strain from the collapse of the original pores. Numerical simulation of the free surface velocity was performed and found in good agreement with planar impact experiments on K9 glass at China Academy of Engineering Physics. And the longitudinal, lateral and shear stress histories upon the arrival of the failure wave were predicted, which present the diminished shear strength and lost spall strength in the failed layer.     

延性金属层裂自由面速度曲线特征多尺度模拟研究

以延性金属钽为研究对象,对钽在平板撞击下的层裂行为进行了多尺度下的数值模拟研究,从微观视角对自由面速度曲线上的典型特征进行了新的解读。在宏观尺度,对比分析了光滑粒子流体动力学法（smootfied particle hydrodynamics, SPH）与Lagrange网格法以及几种本构模型的模拟结果及其适用性。通过与实验数据的对比表明,Steinberg-Cochran-Guinan本构模型在层裂模拟中与实验数据吻合较好,通过改变加载条件获得了不同应变率下的自由面速度曲线,分析了不同应变率下的自由面速度曲线中的典型特征。在微观尺度,采用分子动力学方法获得层裂区域内损伤演化情况,揭示了宏观尺度自由面速度曲线典型特征所蕴含的物理内涵。分析表明,层裂表现为材料内部微孔洞形核、长大和聚集的损伤演化过程,自由面速度曲线上的典型特征与层裂区域的损伤演化过程存在密切关联。Pullback信号是层裂区域内微孔洞形核的宏观表征;自由面速度曲线的下降幅值在一定程度上反映了微孔洞的形核条件,由此计算得到的层裂强度实际上是微孔洞的形核强度。此外,Pullback信号后的速度回跳速率反映了微损伤演化的速率。     

改进SPH方法在陶瓷材料层裂数值模拟中的应用

为了探讨铝飞片撞击陶瓷材料时的层裂现象,采用改进SPH方法模拟应力波在陶瓷材料中的传播。结果表明,当离散粒子分布不均匀时,数值模拟计算的自由面速度时程曲线与实测曲线吻合良好。对比CSPM方法,改进SPH方法的精度更高。提出适用于数值模拟的陶瓷材料损伤演化方程,对脉冲载荷下陶瓷/钢层合板层裂的破坏过程进行数值模拟,结果表明,由于陶瓷的波阻抗高于钢的,且抗压强度远高于抗拉强度,因此拉应力引起的层裂破坏是主要的。即使在材料内部传播的只是弹性压缩波,当弹性波到达材料界面时,由界面反射引起的卸载波也能导致陶瓷发生层裂破坏。     

激光选区熔化增材制造GP1不锈钢动态拉伸力学响应与层裂破坏

采用选择性激光熔化增材制造技术，制备了GP1不锈钢单轴拉伸板条试样和层裂圆片试样，并对材料微观结构进行了表征。借助Zwick-HTM5020 高速拉伸试验机，并结合数字图像相关性全场应变测量技术，开展了增材制造GP1不锈钢材料的轴向拉伸力学性能实验研究，得到了不同应变率下材料的拉伸应力-应变曲线，结果显示:(1) GP1不锈钢流动应力具有比较显著的应变强化效应；(2)通过回收试样的电子背散射衍射表征，发现GP1不锈钢在拉伸变形过程中会发生奥氏体与马氏体之间的相变；(3) GP1不锈钢的屈服应力随着应变率呈幂指数增大，断裂应变在中低应变率下保持不变，但在高应变率下则显著减小。采用一级轻气炮实验装置和激光干涉粒子速度测量技术，开展了增材制造GP1不锈钢的层裂实验，发现GP1不锈钢的层裂强度随着飞片撞击速度增大而减小。单轴拉伸试样断口和层裂试样断口的显微分析结果表明:随着应变率增大，单轴拉伸断裂模式和断裂机理都发生了转变；层裂损伤易成核于激光熔池边界线的交汇处，断口韧窝形貌明显区别于单向拉伸断口。     

基于“钉床型”飞片的斜波加载技术及应用

为了实现斜波加载,设计了一种“钉床型”广义波阻抗梯度飞片,即在基座上密排叠加许多小圆锥,简称“钉床型”飞片。该飞片采用激光选区熔化金属增材制造技术进行制备。利用一级轻气炮加载装置和全光纤激光位移干涉测试系统,开展不同工况下“钉床型”飞片高速击靶压缩实验和层裂实验,重点讨论小圆锥高度和撞击速度对斜波压缩加载波形的影响规律,以及斜波加载对不锈钢靶板层裂特性的影响。实验结果显示:（1）“钉床型”飞片对靶板产生的压缩是逐步的,从自由面速度剖面上观察到压缩波上升前沿时间被显著延长,形成了斜波波阵面,明显不同于冲击压缩的陡峭波阵面;（2）在飞片击靶速度近似恒定条件下,斜波波阵面的上升沿时间、平台速度峰值都明显依赖于“钉床型”飞片上的小圆锥高度,随着小圆锥高度增大,上升沿时间呈线性增大,而平台速度峰值呈线性减小;（3）在“钉床型”飞片的几何尺寸保持不变的条件下,斜波波阵面的上升沿时间随着飞片击靶速度的增大而线性减小,平台速度峰值则线性增大;（4）与冲击加载相比,“钉床型”飞片产生的斜波加载不会对材料的层裂强度产生明显影响,但对材料内部损伤演化速率有一定的影响。     

An examination of dynamic fracture under biaxial-strain conditions

An experiment to investigate the influence of biaxial strain on the dynamic fracture of metals is outlined. A hollow cylindrical specimen is loaded on the inner diameter by a polyethylene-coated exploding wire which results in a circumferential spallation pattern. Prior to fracture, the spall surface undergoes biaxial deformation with a total hoop strain of the same order of magnitude as the total radial strain. This is in contrast to earlier studies in which fracture induced by stress waves was examined in experiments which are characterized by uniaxial-strain conditions. Comparing the results of the two configurations, it can be shown that the maximum principal-stress history required to cause fracture is the same in both cases, although the stresses normal to the fracture surface are quite different. High-speed photographs of the coated exploding wires and dynamic-flash X-rays of the specimens are included to demonstrate the axisymmetry of both the loading and the spall phenomenon.     

CoCrFeMnNi高熵合金冲击波响应与层裂强度的分子动力学研究

高熵合金未来有望应用于航空航天和深海探测等领域, 并且不可避免地会受到极端冲击载荷作用, 甚至会发生层裂. 本文采用分子动力学(MD)方法, 研究了CoCrFeMnNi单晶高熵合金冲击时的冲击波响应、层裂强度以及微观结构演化的取向相关性和冲击速度相关性. 模拟结果表明, 在沿[110]和[111]方向进行冲击时产生了弹塑性双波分离现象, 且随着冲击速度的增加呈现出先增强后减弱的变化趋势, 但在沿[100]方向冲击时未出现双波分离现象. 在冲击过程中, 大量无序结构产生且随冲击速度的增加而增加, 使得层裂强度随冲击速度的增加而减小. 此外, 层裂强度也具有取向相关性. 沿[100]方向冲击时产生了大量体心立方(BCC)中间相, 抑制了层错以及无序结构的产生, 使得[100]方向的层裂强度最高; 层裂初期微孔洞形核区域无序结构含量大小关系的转变, 使得[111]方向的层裂强度在冲击速度较低时(U_p≤0.9 km/s)大于[110]方向, 而在冲击速度较大时(U_p≥1.2 km/s)略小于[111]方向. 研究成果有望为 CoCrFeMnNi高熵合金在极端冲击条件下的应用提供理论支撑和数据积累.      

Experimental and theoretical/numerical investigations of thin films bonding strength

A laser spallation facility has been developed to measure the strength of planar interfaces between a substrate and a thin coating. This quantity is a central requirement in contemporary thin film and protective coatings technology and its successful measurement should improve the scientific/technological potential for the design of advanced composites, protective coatings of composites that operate in hostile environments, and in joining of dissimilar materials. The technique involves impinging a laser pulse of ultra short duration on the rear surface of the substrate, which is coated by a thin layer of energy absorbing metal such as Sn and Pb. The explosive evaporation of the metallic layer, confined between a fused quartz crystal and the substrate, induces a compressive shock wave, which propagates through the substrate toward the material interface. Upon reflection from the free surface of the coating, the pressure pulse is converted into a tensile wave which, under certain conditions, can lead to spallation at the interface. It is shown by mathematical simulation that atomic bond rupture is the mechanism of separation in this experiment. Since the interaction of laser energy with matter is a complicated, highly non-linear process, our investigations, at first, were based on measurement of the pressure pulse generated by the threshold flux level that leads to spallation, by using a micro-electronics device with a piezo-electric crystal, and on computation of the tensile stress experienced at the material interface, by numerical simulation of the induced stress wave propagation. Several substrate/coating (ceramic/ceramic and ceramic/metal) systems have been investigated such as, 1–15 μm SiC by CVD, 1–4 μm TiC and TiN by PVD coatings on sapphire substrates, as well as 1–2 μm Au, Sn and Ag coatings by sputtering on sapphire, fused quartz and glass substrates. For identically prepared specimens, the measured threshold energy levels are reproducible, thus leading to reproducible bond strength values, while the spall size, as expected, is dependent on the laser pulse energy level. Finally, the bond strength values obtained are in very good agreement with similar data derived by direct experimental techniques based on Laser-Doppler-Interferometry.