Time dependence of the spall strength under nanosecond loading

The problem of spall fracture is considered using an incubation time criterion. Some experimental fracture effects are discussed. The spall strength is found to depend substantially on the pulse parameters, in particular, the rate of decrease of the load. The strain-rate and time dependences of spall strength are shown to be considered as calculated characteristics rather than as functions of a material.     

冲击加载下LY12铝合金的动态屈服强度和层裂强度与温度的相关性

采用可测量任意反射表面的速度干涉仪对LY12铝合金在不同初始温度条件下的动态屈服与层裂行为进行了实验研究，温度范围从室温到接近熔化温度.实验结果显示：LY12铝合金的动态屈服强度随着温度升高而快速下降，当初始温度为847K (比熔化温度低86K) 时，其屈服强度仅为室温下的15％，层裂强度也随着温度升高而减小，在296—847K的实验温度范围内，层裂强度损失80%.通过实验结果与模型估算值的比较，发现Zerilli-Armstrong (ZA) 模型可以对LY12铝合金的动态屈服强度与温度的相关性进行较好 关键词： 温度相关性 LY12铝合金 动态屈服强度 层裂强度     

动态拉伸加载下高纯铝破坏的临界行为

以高纯铝(99.999%)作为延性金属的模拟材料，在一级气体炮上开展了一维应变平面冲击波加载实验.通过对不完全层裂的回收样品进行细观微损伤统计分析，讨论了高纯铝动态破坏时发生的临界行为.文中定义拉伸应力和拉伸作用时间的乘积为拉伸(作用)冲量，统计发现：随着拉伸冲量的增加，样品中的损伤呈现明显的临界行为特征：当拉伸冲量较小时，损伤以线性方式缓慢增长，当拉伸冲量足够大，且超过一定的临界阈值以后，损伤以幂指数形式快速增长.初步实验结果表明，高纯铝的拉伸冲量临界阈值约为0.34GPa·μs，对应的损伤临界值约为0.12.     

Interrelation between the threshold characteristics of erosion and spall fracture

Threshold diagrams of erosion and spall fracture are constructed based on the concept of incubation time of the fracture. It is shown that in the case of a defectless material, the incubation time can be estimated from the spallation or erosion experimental data. The temperature dependence of the threshold velocities of microparticle impact is considered. The effect of increasing the dynamic yield stress upon an increase in the surface temperature of the target material is obtained for small-size microparticles. The relationship with an analogous effect in the spallation experiments is discussed.     

Measuring the angle of optical rotation with subnanosecond time resolution

A method is presented for measuring the angle of optical rotation in the course of a single pulse of radiation with time resolution better than 0.5 nsec. The method is based on the combination of two data channels on a single photodetector, separated in time by an optical delay line. The feasibility of the method is illustrated by studying the dynamics of the rotation of the plane of polarization of radiation from a pulsed dye laser with an optically active object placed in the resonator.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 34–39, September, 1990.     

Temperature dependence of spall strength and the effect of anomalous melting temperatures in shock-wave loading

The effects of temperature anomalies in materials subjected to the action of shock waves are studied. The spall failure of aluminum single crystals and polycrystals at various temperatures was experimentally studied in [1]. An analysis of the experimental data for polycrystalline aluminum shows that the breaking strength only weakly changes with temperature when it increases from room temperature to 90% of the melting temperature and, then, drops sharply to zero with a further increase in the temperature. For aluminum single crystals, the effect of anomalously high temperatures was experimentally detected; i.e., their strength remained high in the state where melting was expected during tension. The criterion of incubation time of failure is used to obtain an analytical expression for the temperature dependence of the spall strength of the materials. A new melting criterion, which relates the instant of a phase transition to the melting incubation period, is introduced. This criterion allows one to naturally explain the effect of anomalously high melting temperatures detected during the pulsed action.     

On some principal features of data processing of spall fracture tests

A method for processing the results of dynamic spall fracture tests, based on the exact solution of the wave equation, and its commonly used simplified version based on the assumed unique relation between the free surface velocity drop and the ultimate medium fracture stress, are analyzed. Using the considered exact solutions of the wave technique, tensile stress pulses during spalling are determined. The obtained stress levels at the fracture point are compared with the spall strength calculated by the velocity drop technique. The cases of agreement and disagreement of the results obtained using both techniques are shown. By the example of differently shaped loading pulses, possible scenarios of sample fracture are presented, in particular, the probability of the fracture delay effect is shown, which can be lost in the simplified processing method.     

Influence of plastic deformation on fracture of an aluminum single crystal under shock-wave loading

The plastic deformation and the onset of fracture of single-crystal metals under shock-wave loading have been studied using aluminum as an example by the molecular dynamics method. The mechanisms of plastic deformation under compression in a shock wave and under tension in rarefaction waves have been investigated. The influence of the defect structure formed in the compression wave on the spall strength and the fracture mechanism has been analyzed. The dependence of the spall strength on the strain rate has been obtained.     

Study of breakdown in yttrium aluminum garnet at subnanosecond times of increasing voltage

Crystallographically oriented channels with bottlenecks in the regions of reflection of the pulses have been obtained in aluminum yttrium garnet during multipulsed nanosecond breakdown from the anode under voltages of 100–140 kV, and the propagation rate of the fronts of the phase transition in this voltage range has been determined. It has been shown that the character of the observed pattern of the sequential formation of separate chains of the complete breakdown structure indicates that the channel is formed only locally and immediately at the point of time of passing the breakdown front since the maximal field strength, the magnitude of which determines the diameter of the breakdown channel, is observed in the vicinity of the breakdown front.     

Explosion-type chain fracture reaction during the formation of a complete spall damage zone

Some aspects of the available experimental results on the spall fracture of steel and duralumin sample caused by the explosion of a sheet HE charge are discussed. In particular, the location of the damage zone is considered based on the condition that the density of deformation energy is larger than the density of energy of formation new surfaces upon continuity break. The localization of the complete damage zone is determined by the threshold values the induction time and tensile stress. No dynamic branch of longevity associated with spall fractures is observed at times shorter than 10⁻⁷ s.     

Development of a laser-based fluorescence microscope with subnanosecond time resolution

This report describes the development of a fluorescence microscope based on a standard inverted optical microscope which incorporates a pulsed picosecond dye laser excitation source and a detector consisting of a gated image intensifier coupled to a CCD camera. Fluorescence images have been obtained using gate durations of 0.5 ns from this apparatus, representing a reduction in gate duration of an order of magnitude compared with similar instruments reported by others recently. Subnanosecond gated fluorescence images of V79-4 Chinese hamster lung fibroblasts stained with a phthalocyanine photosensitizer used in photodynamic therapy are presented. The results of these measurements are discussed in terms of the intracellular distribution of the sensitizer. Other potential applications and limitations of this technique are also outlined.     

On the initial stage of a subnanosecond pulsed breakdown of high-pressure gas discharge gaps

The model of background electron multiplication is used for deriving the time dependence of ionization, which correctly describes experiments on initiation of a homogeneous high-voltage subnanosecond pulsed breakdown under a gas pressure on the order of ten atmospheres in a discharge gap ～1 mm.     

Formation of a volume discharge at a subnanosecond rise time of the voltage pulse

A study is made of the formation of a volume discharge in atmospheric-pressure air in a nonuniform electric field without additional preionization. It is shown that the spatial distribution of the plasma glow between a plane and a spherical (as well as a point) electrode at a subnanosecond rise time of the high-voltage pulse is volumetric in character. The change of the voltage polarity does not qualitatively affect the character of the glow. The propagation of a spherical ionization wave in nitrogen is calculated in the drift-diffusion approximation. The fact that the character of the discharge glow is essentially independent of the voltage polarity is explained by the multiplication of the background electrons in the dense working gas.     

冲击加载下铝的剪切模量

分别用Steinberg-Cochran-Guinan (SCG)模型、修正的SCG模型和有限应变理论对材料的剪切模量做了数值计算，并与一维平面应变加载下铝的实验结果进行了比较.结果表明，修正的SCG模型与实验结果较为符合.在10—80GPa的压力范围下，剪切模量随冲击压力的增加而逐渐增大，这是由于压力的影响占主要地位，发生了加工硬化.在80—125GPa的压力范围下，剪切模量随冲击压力的增大快速减小，这是因为温度的影响比较严重，发生了高温软化现象.剪切模量最终在冲击压力为125GPa处趋于零，这是由于在该压力点冲击熔化发生，剪切强度消失. 关键词： 剪切模量 Steinberg-Cochran-Guinan模型 有限应变理论 铝     

Chemical transformations in the zone of spall damageability

The results of experiments on studying the perlite–ferrite structure in steels under short-term negative pressures are described. It is shown that in the localized deformation bands formed in the zone of interference of unloading waves, where the tension stress is lower than the dynamic strength of the material, the cementite bands in perlite are crushed, their fragments are in part dissolved and enriched with carbon, and the cementite can pass into a steady spherical form on the boundary with ferrite. At relatively high shock-wave amplitudes, the perlite in its entirety acquires a spheroidal shape.