Failure mechanisms in impacting penetrators

Experiments are described on (i) the simple Taylor test where a flat-ended projectile mushrooms against a semi-infinite rigid target, (ii) penetration of conically tipped cylindrical projectiles into semi-infinite targets, (iii) the use of modified nose geometries to direct the pattern of shear failure and (iv) the impact of balls on thin plates. Macro- and micro-structural investigation of fractured penetrators illustrates mechanisms of ductile fracture, intergranular and transgranular brittle fractures, and adiabatic shear. Stress analysis highlights those regions subjected to prolonged hydrostatic tensile stresses during impact indicating fracture by spalling, and those regions where persistent velocity discontinuities or planes of maximum shear strain rate indicate adiabatic shear failure.     

Hypervelocity launch dynamics

The launch dynamics from a generic hypervelocity cannon are considered over the velocity range from 1500 through 3500 m/s. Both fin and flare stabilized projectiles are examined for the influence of in-bore dynamics, muzzle blast, sabot discard, and free flight aerodynamics upon their trajectory. Computations are performed using codes developed and validated for conventional cannon with ordnance muzzle velocities (approximately 1700 m/s). While the extension into the hypervelocity regime is not supported by experimental data, this initial study is intended to aid in defining potential problem areas.     

Stability of penetrators in dense fluids

The differential equation of yawing motion of a gyroscopically unstable, gigid body penetrator in a dense fluid is examined for application to flash radiographie measurements of a bullet penetrating gelatin. A functional form for the relationship of overturning moment to instantaneous yaw is derived. Using this expression for the overturning moment, a greatly simplified equation of gyroscopically unstable yawing motion is fitted to the bullet's yaw history. By pooling the data for the eight firings, a very good estimation of the overturning moment, valid for yaws up to 140°, is obtained.     

A modified jacketed cold storage design

The development of an energy efficient and economical modified jacketed vegetable storage concept and design is presented. Most fruits and vegetables require a storage environment near 0°C and 100%rh, which is difficult to achieve with conventional refrigeration systems. In a jacketed storage system, refrigerated air is circulated through air spaces surrounding the storage. High relative humidity levels can be achieved through the use of the large heat transfer surface of walls, ceiling and floor maintained at uniform temperature, and by preventing transmission heat gains into the storage space. While the jacket concept was initially introduced as a full jacket (covering walls, ceiling and floor), this paper discusses an alternative jacketed design which uses only the ceiling as a heat transfer surface. Based on construction simplicity, it is felt that this jacket design can be applied to conversion of conventional storage facilities and to new storage facilities. Acceptable relative humidity levels are attainable by the jacketed ceiling design with an extended heat transfer surface. It is expected that this concept will find favourable applications for storing fruits and vegetables wherever high humidity, medium temperature cold rooms are required.     

Hypervelocity penetration of concrete

Experiments have been conducted with 6.25 mm diameter tungsten rods striking concrete at 2.2 km/s. Three concretes were used—one was 2.35 g/cm³ and the other two were 2.27 g/cm³. The erosion rates were measured to be T/ΔL = 2.4–3.1 depending on the density of the concrete. This is greater than the hydrodynamic value, which shows that the strength of the penetrator is affecting the penetration. The cratering efficiency was computed (which included surface spall) and was found to be commensurate with the strength of the concrete, 28–34 MPa. CTH calculations were conducted using the brittle fracture kinetics (BFK) and Holmquist–Johnson–Cook (HJC) material models for concrete. Density in the calculations was 2.25 g/cm³. It was not possible to match erosion rates at 2.2 km/s, which were too high in the calculations. Also, computed crater volumes were much too small, mainly due to spall in the experiments that was not shown in the computations. Another significant inaccuracy of the calculations was the damage extent, which became unrealistically widespread as time increased in the BFK model.     

A note on the deformation of conical penetrators

Conical penetrators have been shown to give enhanced penetration over flat-ended projectiles up to a certain transition velocity. This transition velocity was found to be associated with a separation of the conical nose from the penetrator body which caused the penetrator to revert to the behaviour of a flat-ended penetrator at higher velocities. An explanation of the marked deformation resistance of the conical noses of these penetrators is presented.     

Hydrocode and microstructural analysis of explosively formed penetrators

A comparison of residual microstructural features in tantalum, iron, and copper explosively-formed projectiles (EFP's) utilizing optical and transmission electron microscopy (TEM) was undertaken and these observations correlated with corresponding microhardness maps measured on the recovered EFP half sections. These microstructural comparisons and residual microhardness maps were then used to validate AUTODYN-2D hydrocode simulations of residual yield stress profiles and the geometrical parameters measured from the recovered EFP's. Both Johnson-Cook and Zerilli-Armstrong constitutive relations were applied and compared in these simulations which also examined the initiation parameters on EFP evolution. Calculated plastic strain and temperature contour plots were also correlated with observed microstructures and the microhardness maps.     

Hypervelocity impact of mixtures

The study of the behavior of mixtures during shock loading entails several special problems which are both physical and computational in nature. On the physical level, many mixtures of interest such as engineering composites and water-saturated geological materials have consitituents which are both soft and porus. Thus hypervelocity impact produces enormous heating. The distribution of this heating between the constituents of the mixture must be understood before accurate predictions of Hugoniot states and release paths can be achieved. On the computational level, numerical solutions within a wavecode framework require simultaneous solutions of an equation of state and conservation of energy equation for each constituent of the mixture. At present, to achieve these solutions a numerical subcycling scheme is required.

In this paper these problems are discussed in detail. A formulation of the theory of mixtures will be presented which is both complex enough to encompass the essential physics of many problems and simple enough to be incorporated into many wave propagation codes. Results of calculations will be compared to impact and release experiments on a mixtures of water and powdered calcite.     

Hypervelocity penetration of ceramics

The penetration resistance of alumina was found to decrease with velocity for armor-piercing bullets. However, it was relatively independent of velocity for rods and fragment-simulating projectiles. These results are explained in terms of compressive yielding caused by high velocity pointed projectiles.     

Hypervelocity impact penetration mechanics

Inert dense metal penetrators having a mass and geometry capable of missile delivery offer significant potential for countering underground facilities at depths of tens of meters in hard rock. The proliferation of such facilities among countries whose support for terrorism and potential possession of Weapons of Mass Destruction (WMD) constitutes threats to world peace and U.S. Security. The Defense Threat Reduction Agency (DTRA), in cooperation with the U.S. Army Corps of Engineers, the Department of Energy National Laboratories and private sector R&D firms have pursued an aggressive research effort to explore the attributes of high velocity impact penetrators for countering such facilities. The penetration of crustal rocks with metal rods (such as tungsten or steel alloys) at high velocities involves complex wave propagation phenomena within the rod and inelastic response of both the penetrator and target material. In this paper we examine the sensitivity of penetration depth (for a fixed tungsten alloy mass impacting a limestone target) to impactor velocity, strength and geometry. Analyses are based upon a matrix of first principle finite difference calculations using the Sandia CTH (release 7.1) Shock Physics Code. Results indicate that impact velocity, penetrator yield strength and target yield strength strongly influence the penetration depth. Maximum penetration depth is achieved by a delicate trade off between penetrator kinetic energy and penetrator inelastic deformation (erosion). Numerical analyses for the parameter variations exercised in this study (impact velocities 1–3.5 km/s and penetrator yield strengths of 1–4 GPa) produced penetration depths of a tungsten alloy rod (length 200 cm, diameter 20 cm) which varied from 5.1 m to 28 m in a homogeneous limestone target.     

The crater formation due to segmented rod penetrators

The efficiency of segmented rods penetrating into semi-infinite steel targets was investigated numerically by hydrocode simulations with impact velocities varying between 2000 and 5000 m/s.

In a second phase segmented elements were integrated in experimental projectiles and these projectiles were accelerated by means of a powder gun to verify the launchability of such projectiles and to confirm the results of the numerical simulations.

As predicted by the numerical simulations, we observe an increase of the penetration depth in the order of 10% with a 4 segments spaced projectile, in the case of an impact velocity of 2100 m/s.     

Hypervelocity impact damage to composites

This paper presents the results of hypervelocity impact tests conducted on graphite/PEEK laminates. Both flat plate and circular cylinders were tested using aluminum spheres of varying size, travelling at velocities from 2–7 km/s. The experiments were conducted at several facilities using light gas guns. Normal and oblique angle impacts were investigated to determine the effect of impact angle, particle energy and laminate configuration on the material damage and ejecta plumes. Correlations were established between an energy parameter and the impact crater size, spallation damage and debris cone angle. Secondary damage resulting from the debris plume on adjacent composite structures was studied using high-speed photography and witness plates. It was observed that for hypervelocity impacts, the debris plume particles have sufficient energy to penetrate adjacent structures and cause major structural damage as well.     

Behind-armor debris from the impact of hypervelocity tungsten penetrators

Behind-armor debris is the main mechanism by which targets are destroyed by projectile impact. The behind-armor debris generated from the impact of tungsten heavy alloy (THA) penetrators with a length-to-diameter ratio (L/D) of 20 against 6061-T6 aluminum targets was characterized. Behind-armor debris characteristics described were the number of debris particles, their positions, and their size distribution. Experiments were performed against two nominal target thicknesses, 100 and 150 mm, and covered a velocity range from 1.7 to 2.6 km/s. Two methods of obtaining data were used—radiographs were taken of the behind-armor debris, and perforation patterns were generated on steel witness packs placed behind the aluminum target. Debris particles recovered from the witness packs were also studied. Results are discussed for the effect of changes in target thickness and impact velocity on behind-armor debris particle characteristics.     

Numerical analysis and modeling of jacketed rod penetration

A computational study to assess terminal ballistic performance issues of adding a steel sheath, or jacket, to a depleted uranium (DU) penetrator has been performed. The CTH hydrocode was used to model DU penetrators with steel sheaths of various thicknesses against semi-infinite rolled homogeneous armor (RHA), finite RHA, and oblique plate targets. Guided by the initial results, additional semi-infinite RHA simulations were performed to support the development of a generalized penetration model for jacketed rods. The model computes RHA penetration as a function of impact velocity and normalized jacket thickness (thickness over diameter) and compares very favorably with experimental DU and steel data. The model indicates that “bulk” density (areal density) can considerably underestimate jacketed rod penetration. In addition, some insight into the penetrator and target flow shape factors (k_p and k_t) is obtained.     

Experiments with jacketed rods of high fineness ratio

Cylinders of high fineness ratio can show severe integrity and stability problems during acceleration and free-flight phase. The paper describes a method to overcome these problems by adding an envelope to the slender cylinder thereby augmenting the stiffness under flexure. Theoretical considerations treat the pros and cons of jackets with different Young's moduli while looking at various parameters such as maximal deflection, total mass as well as muzzle and impact velocity.

Special emphasis is given to the terminal ballistic efficiency which has been tested using jacketed model penetrators made of tungsten heavy metal with carbon-fibre reinforced plastic (CFRP) and steel envelopes. Some experiments were carried out with cannon-launched CFRP-jacketed tungsten rods of aspect ratios from 45 to 60 being accelerated up to 2000 m/s. In other penetration tests L/D=25 and 40 jacketed penetrators were shot onto homogeneous semi-infinite RHA targets and also spaced targets at 60° incidence at velocities up to 2500 m/s by aid of a light-gas gun.

The experiments with jacketed model penetrators of 3 and 4 mm diameter at high impact velocities showed a good penetration power into homogeneous targets, whereas there is a loss of penetration efficiency into spaced targets of 20% and more. Furthermore it seems that the relative thickness of the jacket should not exceed a certain value in order not to risk a detrimental effect on the penetration performance.     

Hypervelocity macroparticle accelerator experiments at CEM-UT

Several railgun experiments designed to accelerate projectile masses of 2 to 5 g to velocities greater than 6 km/s were performed. Two parallel rail-type accelerators with 12.7 mm square bores were used for the experiments. One gun is 2 m long, has molybdenum rails and alumina ceramic insulators. The other is 1 m long, has molybdenum rails and granite insulators. The greatest velocity achieved to date during the experiments was 5.1 km/s. During the test program, the following ideas to enhance launcher performance were tested: stiff-gun structures to reduce plasma leakage and rail movement, refactory bore materials to reduce ablation and frictional losses, and prefilling the gun bore with gases which will eliminate precursor arcs. After three experiments utilizing the 2 m long launcher, with peak currents ranging from 660 to 780 kA, a gun barrel comprised of 96% pure alumina ceramic insulators and 99.9% pure molybdenum rails has survived with minimal damage and no degradation of seals     

Pressure-bar experiments determine forces on penetrators into geological targets

We developed a laboratory procedure to determine forces on projectiles penetrating geological targets. A gas gun accelerated foundry core targets (a simulated foft sandstone) to constant velocities, and these targets impacted conical-nosed, cylindrical-rod penetrators. Strain-time pulses on the cylinders and displacement-time profiles at the ends of the cylinders were measured for times corresponding to 2.3 nose lengths of penetration. These data were used as input to equations derived from a stress-wave analysis to determine forces on the projectile during penetration.     

Hypervelocity impact tests against metallic meshes

The purpose of this study is to investigate fragment clouds generated by hypervelocity impacts (HVIs) on metallic meshes. The HVIs tests have been conducted with a two-stage light gas gun facility at Kyushu Institute of Technology. Projectiles were launched at 3 km/s. Radiographs of the fragment cloud generated by the impact of the projectile on the mesh were taken with three flash X-rays. As a result, it was found that the velocity of the fragment cloud at the cloud center of gravity was up to 35% slower than the impact velocity of the projectile.