无氧铜层裂的实验与数值研究

利用一级气体炮对国产的无氧铜（OFHC）进行了平板撞击致层裂实验,3 mm厚的OFHC飞片撞击6 mm厚的OFHC靶,采用锰铜应力计记录试样/有机玻璃（PMMA）界面附近的应力历史,获得试样发生层裂的信息。软回收试样,观测回收试样的层裂片。采用基于空穴聚集的层裂模型,数值模拟这些平板撞击致层裂实验。数值模拟的应力剖面以及试样层裂片厚度与实验结果基本一致。此外,对于国外相关的OFHC层裂实验,采用基于空穴聚集的层裂模型也作了相应的数值模拟,并进行了比较。     

三角波强加载下延性金属多次层裂破坏问题

基于应力瞬时断裂判据和Tuler-Butcher损伤累积判据，分析了没有升压的简单三角波强加载下延性金属的多次层裂破坏问题。分析结果显示:层裂片的厚度随着冲击波宽度与强度的比值的增大而增大，材料的破坏深度小于冲击波宽度的一半；在强冲击加载下，材料的破坏深度约为冲击波宽度的一半。     

液体炸药滑移内爆加载下钢管的变形与层裂破坏研究

用液体炸药滑移内爆加载方式,研究了不同炸药厚度和外壳条件下20钢圆管的内聚变形和破坏特征。实验中,钢管发生了较大径向应变,应变值大致随炸药厚度的增加而线性增大。在炸药厚度不低于3mm时钢管变形均匀,轴对称较好;药厚小于3mm时其变形不再是基本轴对称收缩,管子表面出现扭转褶皱。各种装药厚度下,均未观察到预期的层裂破坏。对未层裂的原因进行了初步分析。     

IGBT模块封装中大面积基板连接的应力翘曲分析

模拟绝缘栅双极型晶体管(IGBT)模块中用烧结银对直接覆铜(DBC)基板与铜底板进行大面积连接的过程,发现大面积基板连接会产生很大的翘曲与严重的残余应力．增加铜底板厚度使其刚度增大,能有效改善基板连接的翘曲问题．基板连接的最大残余应力产生于DBC上铜层与陶瓷层连接的角点处,随DBC铜层厚度的增加而增加,随陶瓷层厚度的增加而减小,与焊层厚度关系不大．其主要原因是各层材料厚度变化对结构整体热膨胀系数的改变程度不同,而DBC铜层与陶瓷层之间热失配程度最严重．结构厚度改变对连接层应力大小影响不大,但增加底板厚度会恶化连接层的应力分布,不利于模块长期可靠性．采用DBC下沉法和反向预翘曲法可以在控制连接层残余应力的条件下有效降低IGBT模块因烧结引起的翘曲,不会降低模块的可靠性．     

钢圆管在内爆加载下的层裂特性研究

利用完整回收技术,获得了20钢圆管在不同固体装药厚度、装药类型以及外壳条件的滑移内爆加载下形变和层裂的特征和规律,得到了钢圆管层裂的相对临界装药厚度等初步结果,分析了装药类型、装药厚度、外壳对钢圆管层裂的影响。     

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.     

纯钒在冲击加载下的动态拉伸断裂和弹性波衰减特性

利用平板撞击和激光干涉测速技术,实验研究了国产热等静压纯钒在压力5.2~9.0 GPa、拉伸应变率0.47×105~1.19×105 s-1冲击加载下的层裂特性。结果表明:国产热等静压纯钒具有较强的抗动态拉伸断裂能力,其层裂强度在4.0~5.3 GPa范围,明显高于相似加载条件下文献给出的熔炼钒结果,这主要与热等静压加工工艺下纯钒杂质含量更低、内缺陷更少有关;同时,纯钒层裂强度对冲击压力和拉伸应变率均比较敏感。此外,对弹塑性加载速度剖面的分析发现:在6 mm样品厚度范围,纯钒的弹性波幅值随样品厚度增大而减小,雨贡纽弹性极限随样品厚度的衰减规律较好地满足指数关系σ_HEL=3.246（h_s/h₀）-0.386,h₀为单位长度。     

无钴合金钢的层裂断裂及数值模拟研究

利用一级轻气炮作为加载手段,对无钴合金钢的层裂特性进行了研究,获得了Hugoniot弹性极限、层裂强度、层裂片厚度以及塑性应变率等动力学参数。对回收的样品进行了断口分析和金相分析,从宏微观角度分析了无钴合金在不同应变率下的断裂机制。利用LS-DYNA有限元程序对层裂现象进行了数值模拟,计算结果与实验结果基本吻合。     

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...     

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。     

复合圆柱壳冲击压缩数值模拟及稳定性研究

针对复合圆柱壳在炸药爆轰作用下的动力学响应及在此过程中伴随的失稳问题,研究了其制造工艺中可能出现的缺陷以及圆柱壳中铜线螺旋角和直径对复合圆柱壳稳定性产生的影响。采用SPH-FEM耦合算法,建立了复合圆柱壳二维细节模型,并提出了一种基于圆柱壳内壁粒子速度历史的失稳判据,计算了在不同参数条件下复合圆柱壳的失稳时间及对应的压缩率,对影响复合圆柱壳稳定性的因素进行了评估。分析结果表明,在复合圆柱壳制备过程中存在的折返层缺陷和铜线直径对复合圆柱壳的稳定性有较大影响,而螺旋角度对其稳定性影响不大。     

Thermomechanical vibration analysis of a functionally graded shell with flowing fluid

This paper reports the results of an investigation into the vibration of functionally graded cylindrical shells with flowing fluid, embedded in an elastic medium, under mechanical and thermal loads. By considering rotary inertia, the first-order shear deformation theory (FSDT) and the fluid velocity potential, the dynamic equation of functionally graded cylindrical shells with flowing fluid is derived. Here, heat conduction equation along the thickness of the shell is applied to determine the temperature distribution and material properties are assumed to be graded distribution along the thickness direction according to a power-law in terms of the volume fractions of the constituents. The equations of eigenvalue problem are obtained by using a modal expansion method. In numerical examples, effects of material composition, thermal loading, static axial loading, flow velocity, medium stiffness and shell geometry parameters on the free vibration characteristics are described. The new features in this paper are helpful for the application and the design of functionally graded cylindrical shells containing fluid flow.     

Evaluation of the Material Properties of an OFHC Copper Film at High Strain Rates Using a Micro-Testing Machine

The material properties of an oxygen-free high thermal conductivity (OFHC) film with a thickness of 0.1 mm were evaluated at strain rates ranging from 10⁻³/s to 10³/s using a high-speed material micro-testing machine (HSMMTM). The high strain-rate material properties of thin films are important especially for an evaluation of the structural reliability of micro-formed parts and MEMS products. The high strain-rate material testing methods of thin films, however, have yet to be established to the point that the testing methods of larger specimens for electronics, auto-body, train, ship, and ocean structures are. For evaluation, a new type of HSMMTM was developed to conduct high-speed tensile tests of thin films. This machine is capable of testing at a sufficiently high tensile speed with an electromagnetic actuator, a novel gripping mechanism, and an accurate load measurement system. The OFHC copper film shows high strain-rate sensitivity in terms of the flow stress, fracture elongation, and strain hardening. These measures increase as the tensile strain rate increases. The rate-dependent material properties of an OFHC copper film are also compared with those of a bulk OFHC copper sheet with a thickness of 1 mm. The flow stress of an OFHC copper film is relatively low compared to that of a bulk OFHC copper sheet in the entire range of strain rates, while the fracture elongation of an OFHC copper film is much larger than that of a bulk OFHC copper sheet. A quantitative comparison would provide material data at high strain rates for the design and analysis of micro-appliances and different types of micro-equipment.     

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.     

Heat transfer characteristics of rotating triangular thermosyphon

An experimental investigation is carried out to study heat transfer characteristics of a rotating triangular thermosyphon, using R-134a refrigerant as the working fluid. The tested thermosyphon is an equilateral triangular tube made from copper material of 11?mm triangular length, 2?mm thickness, and a total length of 1,500?mm. The length of the evaporator section is 600?mm, adiabatic section is 300?mm, and condenser section is 600?mm. The effects of the rotational speed, filling ratio, and the evaporator heat flux on each of the evaporator heat transfer coefficient, h_e, condenser heat transfer coefficient, h_c, and the overall effective thermal conductance, C_t are studied. Experiments are performed with a vertical position of thermosyphon within heat flux ranges from 11 to 23?W/m² for the three selected filling ratios of 10, 30 and 50?% of the evaporator section volume. The results indicated that the maximum values of the tested heat transfer parameters of the rotational equilateral triangular thermosyphon are obtained at the filling ratio of 30?%. Also, it is found that the heat transfer coefficient of the condensation is increased with increasing the rotational speed. The tested heat transfer parameters of the thermosyphon are correlated as a function of the evaporator heat flux and angular velocity.     

On fracture of a thin spherical shell under blast loading

The nature of fracture under impulsive loading is examined. It is shown as a consequence to Drucker's stability hypothesis that, under impulsive loading, it is strain rate and not total strain that plays the dominant role in determining fracture criteria. Rinehart and Pearson's criteria for rupture of cylindrical shells, which are consistent with the above, are extended to spherical shells. Experimental program on polystyreneplastic shells of different sizes is reported. Dominant fracture pattern observed was brittle. Also, critical velocity of straining (V _cr) was found to be directly proportional to the cube of the diameter of the shell. On the basis of dimensional considerations, it is shown that the shell-wall thickness has no effect upon the number of fragments. The effect of material properties on fracture is also examined.     

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.     

Fracture behavior of explosively loaded spherical molded steel shells

Experimental and numerical works are made to study the fracture of explosively loaded spherical molded steel shells. The first series of experiments included three sawdust recovery shots to save fragments for examination. In this series, detonation was initiated from the center of the sphere. Results of the experiments show that two types of fractures are observed in spherical shells: radial and shear as in cylindrical shells. Spall fracture is also observed in spherical shells. To assist understanding of the experimental results, a computer simulation of expanding shells is performed to provide information on the stress, strain, strain rate and position of each element of the shell wall as a function of time after detonation. For t=7.5 μs after detonation, triaxial non-uniform strain prevails in the middle of the thickness of the wall. The peak of the stress equals to 6.5 GPa and exceeds the spall strength of carbon steel. In the second series of experiments, spall fracture is suppressed by displacing the point of detonation initiation from the center to periphery of HE charge.     

Core temperature and density profile measurements in inertial confinement fusion implosions

We have measured time-integrated and time-gated electron temperature (T_e) and density (N_e) spatial profiles from indirect-drive implosions. In our experiments, we used a multiple-pinhole two-dimensional imaging spectrometer to obtain multispectral X-ray images of the imploded core. Quantitative comparisons between quasi-monochromatic images in different energy bands allowed T_e and N_e spatial profiles to be determined using two independent and validated techniques: a multi-objective search and reconstruction analysis, and an analytical analysis. We then compared the results to a simple one-dimensional (1D) mix-free hydrodynamics simulation in order to evaluate the ability of such a model to predict our experiments. Our data show spatial T_e profiles that are qualitatively consistent with the predictions of our 1D simulations, but we observe central cores that are 10–25% cooler and emit X-rays as late as 200 ps after peak compression. We infer time-gated spatial N_e profiles that are consistent with our 1D simulations near the times of peak compression, but we find significant disagreement between time-integrated data and 1D simulation predictions at large radii. Careful analysis of the time-gated and time-integrated T_e and N_e spatial profiles, together with streaked X-ray emission spectra from core and shell dopants, suggests mixing of shell material into the core is an important process that our 1D hydrodynamics simulations fail to capture, and comparison between image data and a simple analytical model suggests that 2–5 μm of the initial inner shell thickness mixes into the core during the time period of significant X-ray emission. This mix width is consistent with the predictions of a growth-factor analysis that treats instability growth seeded by capsule surface roughness, and points to the need to consider time-dependent mixing effects when interpreting T_e and N_e spatial profiles derived from multispectral X-ray image data, particularly at large radii where mixing effects will be most significant.