AUTOMATED COMPACT PARAMETRIC REPRESENTATION OF IMPACT CRATERS

We have developed a procedure for the measurement and automated parametric representation of 3-D impact craters. The 3-D stucture of the crater is measured using a lowcoherence optical interference technique and the parametric representation achieved in a two-step procedure. Sobel edge detection and morphological operations are used to define rigorously the impact region. The parameter set is defined with respect to a coordinate system whose origin and orientation are uniquely determined from the data. Subjective, arbitrary choices for this position and angle are thus eliminated and the coefficients of the Zernike polynomials are calculated by direct integration over this region to produce a parameter set. This set allows the representation of the three-dimensional geometry by a small set of numbers. Results are presented for impact craters generated at DERA Fort Halstead by the penetration of shaped charge jet particles. It is shown that the agreement between the crater profile generated by the Zernike parameter set and the real crater is very good.     

Penetration of HSLA-100 steel with tungsten carbide spheres at striking velocities between 0.8 and 2.5 km/s

A 51 mm thick plate of high-strength low-alloy (HSLA-100) steel was impacted by 6.4 mm diameter tungsten carbide spheres traveling at velocities ranging from 0.8–2.5 km/s. The width and depth of the crater for each impact event are provided in tabulated form and graphed as a function of velocity. The impacts were simulated using an explicit Lagrangian finite element model. A residual stress map over a cross-section through the crater was also measured by the Contour Method for the 2.2 km/s impact. The predominant feature of the stress map was a peak compressive stress of 1100 MPa, which is 1.6 times the yield strength, centered approximately one crater diameter below the crater floor. Residual stresses in the as-received HSLA-100 plate were also measured and were used to evaluate the effect of initial stresses on the model prediction. Good agreement is shown between the numerical simulation of the impact event and the experimental data.     

Jet-induced 2-D crater formation with horizontal symmetry breaking

We investigate the formation of a crater in a 2-D bed of granular material by a jet of impinging gas, motivated by the problem of a retrograde rocket landing on a planetary surface. The crater is characterized in terms of depth and shape as it evolves, as well as by the horizontal position of the bottom of the crater. The crater tends to grow logarithmically in time, a result which is common in related experiments. We also observe a horizontal symmetry breaking at certain well-defined conditions which, as we will demonstrate, could be of considerable practical concern for lunar or planetary landers. We present data on the evolution of these asymmetric states and attempt to give insights into the mechanism behind the symmetry-breaking bifurcation.     

结构缺陷对电子束诱发纯铝表面熔坑的影响

用Nadezhda-2强流脉冲电子束轰击不同预变形量的纯铝表面,对铝表面的结构缺陷和轰击后产生的表面熔坑进行了系统的表征,研究了表面熔坑与结构缺陷之间的关系.结果表明,结构缺陷对表面熔坑的形成有重要的影响,熔坑的数量随着结构缺陷数量的增加而增加,并且有沿晶界和高位错密度区择优形成的趋向,结构缺陷是表面熔坑形成的萌芽.另外,强流脉冲电子束还能在晶界和滑移带上诱发出大量由于空位聚集而产生的孔洞.     

云浮硫铁矿爆破漏斗试验研究

根据利文斯顿爆破漏斗理论,在云浮硫铁矿采场进行了单孔系列爆破漏斗试验,绘出爆破漏斗体积与药包埋深关系曲线图。采用MATLAB软件对试验数据进行三次项回归,计算出爆破漏斗的最佳埋深、临界埋深和最佳炸药单耗等。同时进行了宽孔距多孔同段爆破漏斗试验,确定了药包的炮孔排距和炸药单耗的参数范围。     

Crater formation in a plastic target under hypervelocity impact

The analytical models of crater formation in a semi-infinite target under hypervelocity impact have been analyzed. It has been shown by numerical calculations that to approximately calculate the crater size for a narrow range of impact velocities, the model of an ideal plastic body can be used. A new analytical model of crater formation in a plastic target, which describes the experimental data in a wide range of impact conditions, is suggested. The crater formes are analyzed.     

Measuring hypervelocity impact velocity from micrometeoroid crater geometry

Guided by half-space computer simulations showing hypervelocity impact crater formation for an iron particle impacting an aluminum target and characteristic crater geometry changes with impact velocity over the range 8–40 km s⁻¹, we examined normal surface crater views and cross-sectional views through craters (>0.5 mm diameter) from samples retrieved from the NASA LDEF satellite and examined in the scanning electron microscope (SEM). While geometrical features suggested in the computer simulations were indeed observed for micrometeoroid craters in 6061-T6 aluminum targets and 303 stainless steel targets, there was no consistent estimate for impact velocities in any of the experimental samples, and velocity estimates based on measuring ratios of ejecta width/crater diameter and ejecta height/crater depth as well as ejecta height/crater diameter varied from 8 to 42 km s⁻¹; over the same range simulated. These results point to the need to create reference data from actual hypervelocity impact experiments in the laboratory, and systematic observation of residual crater geometries in the SEM. These experiments also demonstrate the uncertainty in assuming a fixed impact velocity for all impact craters in space materials as well as an apparent futility in attempting to correlate impacting particle velocity with post-mortem characteristics of a given crater.     

Microparticle impacts at ultra-high velocities: Their relation to macroparticle impacts

In recent years the Hypervelocity Microparticle Impact (HMI) project at Los Alamos has utilized electrostatically accelerated iron spheres of microscopic dimensions to generate ultra-high velocity impact experiments to about 100 km/sec, about an order of magnitude beyond the data range for precisely controlled impact tests with ordinary macroscopic projectiles. But the extreme smallness of the micro impact events brings into question whether the usual shock-hydrodynamic size scaling can be assumed. It is to this question of the validity of size scaling (and its refinement) that the present study is directed.

Impact experiments are compared in which two comparable impact events at a given velocity, a microscopic impact and a macroscopic impact, are essentially identical except that the projectile masses and crater volumes differ by nearly 12 orders of magnitude—linear dimensions and times differing by 4 orders of magnitude. Strain rates at corresponding points in the deforming crater increase 4 orders of magnitude with the size reduction. Departures from exact scaling, by a factor of 3.7 in crater volume, are observed for cooper targets—with the micro craters being smaller than scaling would predict. This is attributed to a factor 4.7 higher effective yield stress occurring in the micro cratering flow. This, in turn, is because the strain rate there is about 10⁸/sec as compared to a strain rate of only 10⁴/sec in the macro impact.

The measurement of impact craters for very small impact events leads to the determination of metal yield stresses at strain rates an order of magnitude greater than have been obtained by other methods. The determination of material strengths at these exceedingly high strain rates is of obvious fundamental importance. Results are compared to recent theoretical models by Follansbee, Kochs and Rollett.

Finally, the problem is addressed of predicting crater sizes in a target material with strain rate effects. First some basic results are recalled pertaining to the late stage equivalence of hypervelocity impacts. It is then seen, for a strain rate dependent material, that the curve of dimensionless crater volume versus impact velocity is replaced by a family of curves, each member of which is for one final crater size. The spacing of the curves is determined by the stress versus strain properties of the material.     

Some recent advances in the scaling of impact and explosion cratering

The geotechnical centrifuge is a valuable experimental tool for studying impact and explosion craters. Performing experiments at elevated gravity effectively allows one to simulate crater formation at much larger scales than those otherwise attainable in the lab. The utility of the centrifuge has been demonstrated by successful simulations of explosive field tests with charges equivalent to a few kilotons of TNT. Impact experiments conducted on the centrifuge have provided measurements of final crater size, growth of the transient crater, formation time and material motions. These observations have been analyzed within the framework of a recently-developed theory of source coupling, which relates many aspects of crater formation through a single exponent. For example, the scaling of crater growth and formation time are found to be consistent with that one would predict from the observations of final crater size. The results are summarized and applied to develop improved estimates for scaling laws appropriate to the impacts of large bolides on planetary surfaces.     

爆坑尺寸的计算与试验研究

为了研究爆坑尺寸与爆炸位置和爆炸当量的函数关系,归纳了国内外炸药近地面爆炸成坑效应研究情况,分析了爆炸形成的爆坑尺寸计算公式和适用条件,并将野外爆炸试验的数据与计算公式及相关文献给出的试验结果进行对比,分析了结果偏差所产生的原因。结果表明:爆坑尺寸计算公式需考虑爆炸高度的影响,还应结合一定次数的场地试验来得到不同场地上爆炸成坑尺寸与爆炸当量和爆炸高度的函数关系,并且考虑炸药种类;同时爆坑试验结果离散性大,爆坑尺寸计算公式应采用上下限的形式。     

Further validation of a general approximation for impact penetration depth considering hypervelocity gouging data

A first-order approximation of penetration depth is developed for use in engineering design. A survey of available penetration data is used to construct a one-dimensional approach for estimating the geometry of a crater resulting from high-energy impact. The results are generalized to allow approximations to be made using existing experimental data without the requirement for laboratory testing. This approach for penetration depth approximation is validated using the hypervelocity gouging data from the Holloman High Speed Test Track (HHSTT) and hypervelocity gouging impact tests conducted by the authors.     

Multiple impact penetration of semi-infinite concrete

An experimental study was performed to gather multiple impact, projectile penetration data into concrete. A vertical firing range was developed that consisted of a 30-06 rifle barrel mounted vertically above a steel containment chamber. 0.41 m cubes of an Air Force G mix concrete were suspended in wet sand and positioned in the steel chamber. The concrete targets were subjected to repeated constant velocity impacts from 6.4 mm diameter steel projectiles with an ogive nose shape and a length to diameter ratio of 10. A laser sight was adapted to the rifle to ensure alignment, and a break screen system measured the projectile velocity. After each impact, the projectile penetration and crater formation parameters were recorded. The penetration and crater formation data were consistent with single impact penetration data from previous studies conducted at Sandia National Laboratories. In addition, an analytic/empirical study was conducted to develop a model that predicted the penetration depth of multiple impacts into concrete targets. Using the multiple impact penetration and crater formation data, a single impact penetration model, developed by Forrestal at Sandia National Laboratories, was extended to account for the degradation of the target strength with each subsequent impact. The degradation of the target was determined empirically and included in the model as a strength-modifying factor. The model requires geometry parameters of the ogive nose projectile, projectile velocity, the number of impacts, and target compressive strength to calculate the overall penetration depth of the projectile.     

A Simulation of the Shape of a Crater in an Organic-Glass Target under High-Velocity Impact

The results of measurements of craters caused by the interaction between impactors moving at velocities of 2–6 km/s and organic-glass targets are used to find the crater depth depending on the impact velocity and on the density ratio between the impactor and target, as well as on the target strength. A differential equation is derived for determining the crater shape, whose solution fits well the measurement results. The results of calculation of the dimensionless volume of the crater, its diameter, and the slope of its generatrix by the equation for the crater shape also fit well the measured values.     

Fractional fourier domain encrypted holographic memory by use of an anamorphic optical system

We propose and demonstrate a fractional Fourier domain encrypted holographic memory using an anamorphic optical system. The encryption is done by use of two statistically independent random-phase codes in the fractional Fourier domain. If the two random-phase codes are statistically independent white sequences, the encrypted data are stationary white noise. We exploit the capability of an optical system to process information in two dimensions by using two different sets of parameters along the two orthogonal axes to encode the data. The fractional Fourier transform parameters along with the random-phase codes constitute the key to the encrypted data. The knowledge of the key is essential to the successful decryption of data. The decoding of the encoded data is done by use of phase conjugation. We present a few experimental results.     

Random Approach to Peening Coverage in Ultrasonic Shot-Peening

The so-called ultrasonic shot-peening is a new process using calibrated peening hard balls motioned by walls vibrating at 20000 Hz. As in conventional shot-peening, a fully peened surface coverage is required tor the process to be efficient. However, it is intricate to determine the coverage ratio experimentally. A statistical approach, issued from the Random Sets Theory, was used to establish relationships between coverage, impact superficial density and impact crater area. The random modelisation was tested with different materials treated by ultrasonic shot-peening. Individual crater diameter measurements and local repartition analysis were made using quantitative image analysis to test the model. Calculations provide the mean number of impacts everywhere on the surface for any coverage ratio. This statistical approach can also be applied to any impact treatment involving an impact flow.     

Unusual Nano-Microcrystals of Natural Diamond

Described are unusual nano-microcrystals of natural diamond found in a meteorite crater of Ukraine and advised about the earlier unknown mechanism of diamond polyhedra growth—the formation by globules. It was revealed that diamond nano-microcrystals in a meteorite crater are very similar to globular crystals, and at the same time have octahedral faceting. The morphology and composition of diamond nano-microcrystals are studied by scanning electron microscopy and with an energy dispersive X-ray analyser. These tiny crystals are grown on the grains of impact apographitic diamond from the Bilylivka meteorite crater (Zapadnaya impact crater) on the Ukrainian Shield. Their surface morphology indicates that the nano-microdiamonds are grown, most probably, by a vapor deposition process immediately after the formation of the impact diamond–transformation of the graphite into diamond and lonsdaleite.     

Influence of plastic deformation in flexible pad laser shock forming – experimental and numerical analysis

Flexible Pad Laser Shock Forming (FPLSF) is a new microforming process using laser-induced shock pressure and a hyperelastic flexible pad to induce high strain-rate (~10⁵ s^?1) plastic deformation on metallic foils to produce 3D microcraters. This paper studies the effect of two significant process parameters of FPLSF, flexible pad material and its thickness, on the deformation characteristics of the metal foils using experiments and finite element analysis. A finite element model is developed to simulate the FPLSF process. The stress-strain distribution across the foil and the flexible pad at different process stages of FPLSF are studied using FE analysis. Flexible pad materials including silicone rubber, polyurethane rubber, and natural rubber with thicknesses ranging between 300 μm and 3000 μm have been investigated in detail. Experimental results highlight that both the hardness and thickness of the flexible pad significantly influence the deformed crater geometry, thickness distribution across the formed crater and surface hardness at the crater surfaces. The experimental results are correlated with the stress-strain distributions from finite element analysis to study the underlying behaviors.     

Existence of phase explosion during laser ablation and its effects on inductively coupled plasma-mass spectroscopy

A sudden increase in crater depth was observed during high irradiance (> 10(10) W/cm2) laser ablation of silicon, and it is attributed to the phenomenon of phase explosion. The threshold irradiance for phase explosion showed a dependence on two laser parameters: laser beam spot size and wavelength. For a larger beam size and longer incident wavelength, a higher laser irradiance was required to generate phase explosion. The rapid increase of crater depth above the phase explosion threshold irradiance correlated with a significant increase in the ICPMS signal intensity. The ratio of crater volume to ICPMS intensity, which represents entrainment efficiency, remained the lowest at laser irradiances slightly above the phase explosion threshold. However, this ratio increased at irradiances well above the threshold (> 10(11) W/cm2). Chemical analysis using laser ablation at irradiance above 10(11) W/cm2 provides increased sensitivity via improved entrainment and transport efficiency and increased ablation rate.     

Investigation of processes leading to damage growth in optical materials for large-aperture lasers

Damage growth in optical materials used in large-aperture laser systems is an issue of great importance to determine component lifetime and therefore cost of operation. Small size damage sites tend to grow when exposed to subsequent high-power laser irradiation at 355 nm. An understanding of the photophysical processes associated with damage growth is important to devise mitigation techniques. We examine the role of laser-modified material and cracks formed in the crater of damage pits in the damage growth process using fused-silica and deuterated KDP samples. Experimental results indicate that both of the above-mentioned features can initiate plasma formation at fluences as low as 2 J/cm2. The intensity of the recorded plasma emission remains low for fluences up to approximately 5 J/cm2 but rapidly increases thereafter, accompanied by an increase of the size of the damage crater.     

An analytical model of hole size in finite plates for both normal and oblique hypervelocity impact for all target thicknesses up to the ballistic limit

This paper presents, for the first time, a single comprehensive analytical model for the hole size produced by hypervelocity impact into finite plates. This model is based on experimental data for 2017 aluminum spheres impacting 2014, 2024 and 6061 aluminum plates.

The significance of this model is that it spans the entire range of target thickness from very thin to very thick, which makes it possible to determine when the impact conditions are those of thin target behavior (where the hole size increases with increasing target thickness and debris formation and damage is important) and when the impact conditions are those of thick target behavior (where the hole size decreased with increasing target thickness and the debris formation is significantly decreased). The model makes it clear that the target thickness that divides the thin target regime from the thick target regime is a function of velocity. This means that an impact configuration which exhibits thick target behavior at common experimental velocities could actually exhibit thin target behavior at velocities in the tens of kilometers per second such as that of meteroid impacts. This hole size model also includes the effects of oblique impact and computes both the major and the minor diameters of the hole.

This paper also raises, for the first time, the possibility that the commonly accepted models for crater diameter (and by implication those for penetration depth as well), which are taken to be a power function of velocity, might be wrong. Only a linear dependence on velocity for the crater diameter is consistent with the linear velocity dependence of this and all other accepted models of hole diameter in finite plates. If this is correct, it would raise questions about the validity of using any target damage computer models, that are based on the old crater modeling equations, to extrapolate to higher velocities.