Effect of alignment on the penetration of segmented rods

The purpose of this investigation is to study the effect of alignment on the performance of segmented penetrators. The Eulerian wave propagation code CTH is used for this purpose. A series of calculations using four L/D = 1 tungsten alloy segment trains at varying degrees of misalignment is performed, impacting a single finite-thickness oblique armor steel plate. Obliquity angles of 30° and 60° were considered. This study was performed primarily to investigate the effects of obliquity and is a continuation of a previous study [1] where semi-infinite armor steel plates were examined. It is shown that the obliquity of the plate can have a significant influence on the performance of the segment train. When misalignment is minimal, the performance of the segment train is not adversely affected, particularly if the misalignment positions the train in an orientation aligned with the plate normal. However, for large misalignments, degradation to the performance of the segment train is significant at all orientations.     

Reverse impact experiments against tungsten rods and results for aluminum penetration between 1.5 and 4.2 km/s

Reverse impact experiments against 0.76 mm diameter L/D = 20 tungsten rods have been conducted with a 38 mm diameter launch tube, two-stage light-gas gun using four 450 kV flash X-rays to measure penetration rates. Techniques for projectile construction, sample placement, alignment, and radiography are described. Data for penetration rate, consumption velocity, and total penetration were obtained for 28 mm diameter 6061-T651 aluminum cylinders at impact velocities between 1.5 and 4.2 km/s. It was found that penetration velocity was a linear function of impact velocity over this velocity range. Above 2 km/s impact velocity, penetration was completely hydrodynamic. There was substantial secondary penetration, and the total penetration increased linearly with impact velocity over the range 1.5 to 2.5 km/s.     

Hypervelocity penetration of tungsten alloy rods into ceramic tiles: experiments and 2-D simulations

A series of terminal ballistics experiments and 2-D simulations, with small scale tungsten alloy penetrators, was performed in order to quantify the ballistic efficiency of confined ceramic tiles. The data includes both depth of penetration (DOP), into thick steel backing and X-ray shadowgraphs during the penetration process. Impact velocities ranged between 1.25 to 3.0 km/s. The size of the tiles varied in order to check their performance as a function of thickness and lateral dimensions. We found that the differential ballistic efficiency of alumina tiles is practically independent on impact velocity and tile thickness, within the ranges of velocity and thicknesses, investigated here. A detailed simulation study, using the Eulerian processor of the PISCES 2-D ELK code, was performed in order to better understand the interaction between long-rods and ceramic tiles, and particularly, to adjust a proper failure criterion to the tiles. We found that a simple version of the Johnson-Holmquist model, with a single parameter, is fairly adequate to account for most of the data. These include: lateral confinement, tile thickness and impact velocity influence on the penetration depth. We used the code to further investigate the influence of lateral dimensions on tile performance.     

Monolithic and segmented projectile penetration experiments in the 2 to 4 kilometers per second impact velocity regime

A series of experiments was performed to evaluate the performance of projectiles impacting targets at velocities two to three times larger than conventional ordnance velocities. The results were positive, where low L/D ratio projectiles exceeded the theoretical hydrodynamic limit of penetration for the given projectile-target combination. High L/D ratio projectiles did not appreciably exceed the limit.

A second set of experiments was devised to test the hypothesis that a segmented projectile, - consisting of a series of low L/D projectiles, assembled in a long rod configuration, - could penetrate deeper into the target than a monolithic projectile of equivalent mass. The results were again positive, with a gain of about 10% shown in some cases. The balance of the experiments was devoted to developing a set of design rules and to exploring variations in the configuration and materials.     

High velocity impact of segmented rods with an aluminum carrier tube

In this paper, we apply the method of ballistic test to investigate the history and mechanism of the tungsten alloy segmented rod with aluminium carrier tube and corresponding continuous rod penetrating into semi-infinite steel target at velocities from 1.8 to 2.0 km / s. The length to diameter ratio of the segmented rod is 1 (L/D = 1), the ratios of length of spacing between segments to diameter (s/d) are 0.5, 1.0 and 2.0 respectively. The results show that the power of penetration of the segmented rod with carrier tube is obviously higher then that of the corresponding continuous rod with carrier tube. Raising of the impact velocity, suitably increasing of the length of spacing between segments and filling the spacing with non-metallic material, etc. all can increase the penetrating power of the segmented rod. When impact velocity is 2.0 km / s, s / D=2.0, the penetrating power of the segmented rod is 10% higher than that of the corresponding continuous rod, if the spacing is filled with glass steel (non-metallic material), the power will be 20% higher. In this paper, we present a simplified model of based on hydrodynamics and penetrating mechanics. This model can properly describe the whole penetrating process of segmented rod penetrating into semi-infinite target. The shape of the crater and depth of penetration, etc. calculated are in good agreement with the results obtained by experiments.     

More on the secondary penetration of long rods

The secondary penetration of long rods, impacting semi-infinite metallic targets, has been investigated since the early 60's, both experimentally and analytically. Several models have been proposed for the extra penetration which is achieved by these rods at the later stages of the process. However, the models are of limited applicability since they cover only limited regimes of the relevant parameters. In order to further understand the phenomenon of secondary penetration, we performed a large number of numerical simulations using the PISCES 2 DELK code. These simulations dealt with the relevant parameters in large ranges of variability, such as: the rod impact velocity, its aspect ratio (L/D), as well as the densities and strengths of rod and target material. We show that the semi-empirical formulations do not account for the whole range of these parameters. Our simulations show that the strength of the rod has a major influence on the values of the secondary penetrations. In addition, these values are strongly dependent on L/D and target strength.     

Penetration mechanics and performance of segmented rods against metal targets

Terminal ballistic experiments confirm theoretical predictions that a segmented rod will penetrate a semi-infinite metal target deeper than a continuous rod of the same material and having equal mass, diameter and velocity. For copper segmented rods impacting aluminum targets and tantalum segmented rods impacting 4340 (BHN 300) steel, penetration depths of at least 50 percent greater than that for a corresponding continuous rod are measured at impact velocities of 4 to 5 km/s. Spacing between segments of only about 2.5 segment diameters or more are required to achieve these results. Reducing the Li/D of the segments to less than 1 improves the penetration efficiency of a segmented rod. For segmented rods with segment L_i/D < 1, experiments suggest that penetration may increase with impact velocity rate greater than V^2/3.     

High velocity penetration of homogeneous, segmented and telescopic projectiles into alumina targets

Segmented and telescopic projectiles are designed to make efficient use of the higher impact velocities achievable with new acceleration techniques. This concept has been found to work against steel armour. In this study, we compare the penetration capability into an alumina target for these unconventional projectiles with that of a homogeneous projectile. The influence of segment separation distance and core-to-tube diameter ratio were simulated for the impact velocities 2.5, 3.0 and 3.5 km/s. The simulated final penetrations are compared to test results for one type of each of the homogeneous, segmented and telescopic projectiles at 2.5 and 3.0 km/s. Both simulations and tests show that the unconventional projectiles have better penetration capability than a homogeneous projectile with the same initial geometry.     

Modeling and simulation of progressive penetration of multilayered ballistic fabric shielding

 In this paper a simulation technique is developed to estimate the number of ballistic fabric sheets needed to stop an incoming projectile. Such sheets are free of any fortification by a resin. The computational approach is designed so that it can be easily implemented by a wide audience of researchers in the field, without resorting to more involved finite element formulations. This is achieved by taking advantage of the intrinsic characteristics of such fabric structures. Since the deformations of the fabric are (a) finite, (b) nonlinearly inelastic due to progressive fiber degradation and (c) dynamically coupled to the projectile, the system is highly nonlinear. A temporally adaptive, iterative scheme is developed to solve the system. Theoretical issues pertaining to convergence of the algorithm are investigated. Large-scale 3-D numerical examples are then given to illustrate the approach in determining the number of sheets needed to stop a projectile. Received 28 February 2002 / Accepted 2 April 2002     

Modeling the high strain rate behavior of titanium undergoing ballistic impact and penetration

Titanium is an important candidate in the search for lighter weight armors. Increasingly, it is being considered as a replacement for steel components. It is also an important component in the application of ceramics to armor systems, especially in armor modules that are capable of defeating kinetic energy penetrators while sustaining little or no penetration of the ceramic element. The best alloy available today for ballistic applications is Ti-6Al-4V, an aerospace grade titanium alloy. The principal deterrent to widespread use of this alloy as an armor material is cost, and a significant portion of the cost is in processing. Consequently, the U.S. Army Research Laboratory undertook a program to study a particular lower cost processing technique [1].

The objectives of this work are to characterize the low-cost titanium alloy by generating constants for the Johnson-Cook (JC) and Zerilli-Armstrong (ZA) strength models, and to use and compare these two models in simulations of ballistic experiments. High strain rate strength data for the low-cost titanium alloy are used to generate parameters for the two models. The approach to fitting the JC parameters follows one previously used successfully to model 2-in thick rolled homogeneous armor (RHA) [2]. The approach to fitting the ZA parameters is based on a method described by Gray et al. [3]. The resulting model parameters are used in the shock physics code CTH [4] to model a Ti-6Al-4V penetrator penetrating a Ti-6Al-4V semi-infinite block at impact velocities up to 2,000 m/s. Similar experiments are performed, and the predictions of the two models are compared to each other and to the experimental results.     

The influence of plate hardness on the ballistic penetration of thick steel plates

This investigation describes and analyses the experimental results pertinent to the penetration of steel plates of varying hardness in the range HV295–HV520 and of thickness 20 and 80 mm by ogive-shaped 20-mm-diameter projectiles over the velocity range 300–800 m s⁻¹. All the tests were carried out at normal impact angle, i.e. zero obliquity. The experimental results presented include the variation of depth of penetration, crater volume, lip height, bulge height and diameter, plugged length and diameter and specific energy absorption capacity with impact velocity for tests on each plate of a given hardness and thickness. Selected data and observations relating to the plastic zone size and shape surrounding the penetrating projectile, incidence and extent of adiabatic shear band (ASB) formation and plate spalling have also been presented. These experimental data have been interpreted in terms of the appropriate penetration mechanisms like ductile hole formation, bulging followed by star cracking, ASB-induced shear plugging, etc., and also by making use of the fact that the projectile undergoes substantial deformation when penetrating the harder plates (HV450 and HV520). It is also demonstrated that the resistance to penetration and hence the mechanism of penetration is very much dependent on whether the penetration occurs under plane strain or plane stress conditions. For example, ASB-induced plugging occurs only under plane stress conditions while projectile deformation is dominant only under plane strain conditions even in harder plates.     

Re-examination of the evidence for a failure wave in SiC penetration experiments

Previous work suggested the possibility that the effects of a failure wave, evidenced through a change in the slope of the penetration velocity vs. impact velocity (u–v_p) curve resulting from an increase in target penetration resistance, could be observed in penetration experiments of SiC. However, the previous work had to combine two different sets of experimental data, one using long tungsten rods and the other copper shaped-charge jets. A new set of experiments was conducted to address the uncertainties associated with combining the two disparate data sets. Analysis of the new experiments showed no evidence of a distinct change in the slope of the u–v_p response of SiC, up to an impact velocity of 6.2 km/s. We re-examine the original data and analysis in light of the new experiments to understand the origins of the original misinterpretation.     

Influence of scale on the penetration of tungsten rods into steel-backed alumina targets

As ballistic tests are often performed in reduced geometrical scale, the scaling laws are important for the interpretation of the results. In this study, we tested the validity of replica scaling, by which we mean that all geometrical dimensions are scaled uniformly, while the materials and the impact velocity are kept the same. Long tungsten projectiles with length-to-diameter ratio 15 were fired against unconfined alumina targets with steel backing. The tests were carried out with impact velocities 1500 m s⁻¹ and 2500 m s⁻¹, and in three different scales with projectile lengths 30, 75 and 150 mm (diameters 2, 5 and 10 mm). The alumina targets were photographed by means of a high-speed camera, and the tungsten projectiles were photographed inside the alumina targets by means of flash radiography. Also, the residual penetrations in the steel backings were measured. The Johnson-Holmquist model for ceramic materials was implemented into the AUTODYN code, which was used for simulation of the experiments. The agreement between results of experiment and simulation was fair, and over the tested interval of scales replica scaling was found to be valid with reasonable accuracy.     

A computational study of the influence of thermal softening on ballistic penetration in metals

A two-dimensional axisymmetric computational study of the penetration of a tungsten heavy alloy (WHA) rod into a 6061-T6 aluminum target has been performed using a Lagrangian formulation. Adaptive remeshing has been used to alleviate the problem of excessive distortion of elements which occurs during large deformation studies (such as ballistic penetration). Strain hardening, strain-rate hardening and thermal softening in both the penetrator and target materials are taken into full consideration. The computed depth of penetration (DOP), residual penetrator length and maximum crater diameter match very well the experimental results reported by Yadav and Ravichandran (Int. J. Impact Eng., Submitted for publication) for an impact velocity of 1100 m/s. Computer simulations reveal that in the absence of failure mechanisms (such as shear banding), introduction of thermal softening in the penetrator material decreases its depth of penetration in a metal target, when compared to a penetrator material which does not soften thermally. These results are in contrast to the recent work of Rosenberg and Dekel (Int. J. Impact Eng. 21 (1998) 283–296) and a plausible explanation for this discrepancy is presented.     

Multi-scale finite element analysis for tension and ballistic penetration damage characterizations of 2D triaxially braided composite

The multi-scale finite element model is presented to analyze tension and ballistic penetration damage characterizations of 2D triaxially braided composite (2DTBC). At the mesoscopic level, the damage of fiber tows is initiated with 3D Hashin criteria, and the damage initiation of pure matrix is predicted by the modified von Mises. The progressive degradation scheme and energy dissipation method are adopted to capture softening behaviors of tow and matrix. The macro-scale damage model is established by maximum-stress criteria and exponential damage evolution. To simulate interface debonding and inter-ply delamination, a triangle traction–separation law is adopted in each scale. Both scale damage models are verified with available experimental results. Based on numerical predictions, the stress–strain responses and damage developments of 2DTBC under axial and transverse tension loading are studied. For ballistic penetration loading, the meso-scale damage mechanisms of 2DTBC are predicted using 1/4 model, 1/2 model, 1-layer model, 2-layer model and 3-layer model. Then, effects of different model and impactor radius on damage modes are analyzed. Additionally, the macro-scale ballistic penetration behaviors of 2DTBC are simulated and compared with experiment. The prediction results for tension and penetration correlate well with experiment results. Both tension and penetration damage characterizations for tow, matrix within tow, pure matrix, interface and inter-ply delamination are revealed. A comparison of penetration damage between meso- and macro-scale presents a similar crack mechanism between two scales.     

Numerical and experimental investigations into ballistic performance of hybrid fabric panels

Strong, low density fibres have been favoured materials for ballistic protection, but the choice of fibres is limited for making body armour that is both protective and lightweight. In addition to developments of improved fibres, alternative approaches are required for creating more protective and lighter body armour. This paper reports on a study on hybrid fabric panels for ballistic protection. The Finite Element (FE) method was used to predict the response of different layers of fabric in a twelve-layer fabric model upon impact. It was found that the front layers of fabric are more likely to be broken in shear, and the rear layers of fabric tend to fail in tension. This suggested that using shear resistant materials for the front layer and tensile resistant materials for the rear layer may improve the ballistic performance of fabric panels. Two types of structure, ultra-high-molecular-weight polyethylene (UHMWPE) woven and unidirectional (UD) materials, were analyzed for their failure mode and response upon ballistic impact by using both FE and experimental methods. It was found that woven structures exhibit better shear resistance and UD structures gives better tensile resistance and wider transverse deflection upon ballistic impact. Two types of hybrid ballistic panels were designed from the fabrics. The experimental results showed that placing woven fabrics close to the impact face and UD material as the rear layers led to better ballistic performance than the panel constructed in the reverse sequence. It has also been found that the optimum ratio of woven to UD materials in the hybrid ballistic panel was 1:3. The improvement in ballistic protection of the hybrid fabric panels allows less material to be used, leading to lighter weight body armour.     

Three-wave approximation for the modal field inside high-index dielectric rods of hybrid plasmonic waveguides

Abstract

An approximate three-wave model is suggested for describing the modal field inside the high-index dielectric rod of a hybrid plasmonic waveguide. An evanescent wave, an uniform wave and a propagating wave are considered along the direction perpendicular to the metal surface. The superposition of these three waves forms the modal field inside the high-index rod. Through numerical tests, we find that this model is highly valid for a large range of waveguide sizes.     

Hydrocode simulations of air and water shocks for facility vulnerability assessments

Hydrocodes are widely used in the study of explosive systems but their use in routine facility vulnerability assessments has been limited due to the computational resources typically required. These requirements are due to the fact that the majority of hydrocodes have been developed primarily for the simulation of weapon-scale phenomena. It is not practical to use these same numerical frameworks on the large domains found in facility vulnerability studies. Here, a hydrocode formulated specifically for facility vulnerability assessments is reviewed. Techniques used to accurately represent the explosive source while maintaining computational efficiency are described. Submodels for addressing other issues found in typical terrorist attack scenarios are presented. In terrorist attack scenarios, loads produced by shocks play an important role in vulnerability. Due to the difference in the material properties of water and air and interface phenomena, there exists significant contrast in wave propagation phenomena in these two medium. These physical variations also require special attention be paid to the mathematical and numerical models used in the hydrocodes. Simulations for a variety of air and water shock scenarios are presented to validate the computational models used in the hydrocode and highlight the phenomenological issues.     

On one new modification of Alekseevskii-Tate model for nonstationary penetration of long rods into targets

New ballistics equations for long rods penetrating both elastic-plastic and brittle materials are proposed which generalize the well-known Alekseevskii-Tate equations. These equations are derived from the basis of the Lagrange-Cauchy integral for equations of nonstationary irrotational motion of ideal fluids, as well as the equations of the dynamics of expansion of a spherical cavity. Their solutions determine both the translational movement of the penetrator and the dynamics of cavity formation in the target. The structure of the equations is sufficiently simple for analysis and solution. The calculations demonstrate good qualitative agreement of the theoretical predictions with the experimental data.