Blunt Projectiles Research Articles

Response of hot isostatically pressed Ti–6Al–4V targets to normal impact by conical and blunt projectiles

First experimental results are reported on ballistic performance of hot isostatically pressed (HIPed) texture-free targets using rapidly solidified powders of Ti–6Al–4V alloy (PREP and ELI-PREP). Plastic deformation by ball milling of these powders was performed to modify the microstructure of the materials. HIPed samples of 40 mm diameter and a thickness of 10–30 mm were shrinkfit into holes in larger diameter steel plates and struck by 50 caliber 60° cylindro-conical projectiles of hardness R c 60 with contact produced either by the conical or blunt surface. The mass of the projectile was about 31 g and the initial velocities ranged from 300 to 450 m/s for flat-ended and from 900 to 950 m/s for the conical impacts. Comparative behavior of HIPed samples and standard samples made from commercially available Ti–6Al–4V alloy MIL-T-9047G (bar, forged, annealed)—baseline material after impact with the same geometry was investigated. As a rule the final velocity of the plug for HIPed alloy was smaller (or no penetration was observed) than that of the baseline material for impact with the conical projectile. Slightly sloped cylindrical shear plugs were characteristic for both materials upon impact by flat projectiles and their velocities were used to evaluate the shear resistance of HIPed material to plugging. Comparative features of fracture in both cases are presented. The texture-free HIPed materials from powders demonstrated better ballistic performance than the baseline material and could be successfully used for ballistic applications. This is the first step in the development of high-gradient composite materials using a Ti–6Al–4V matrix from powder.

Perforation of 12 mm thick steel plates by 20 mm diameter projectiles with flat, hemispherical and conical noses : Part I: Experimental study

Projectiles with three different nose shapes (blunt, hemispherical and conical) have been used in gas gun experiments to penetrate 12 mm thick Weldox 460 E steel plates. Based on the experimental results, the residual velocity curves of the target material were constructed and compared. It was found that the nose shape of the projectile significantly affected both the energy absorption mechanism and the failure mode of the target during penetration. The ballistic limit velocities were about equal and close to 300 m/s for hemispherical and conical projectiles, while it was considerably lower for blunt projectiles. Blunt projectiles caused failure by plugging, which is dominated by shear banding, while hemispherical and conical projectiles penetrated the target mainly by pushing the material in front of the projectile aside. Also, the residual velocity curves were influenced by nose shape, partly due to the differences in projectile deformation at impact. The experimental study, given in this part of the paper forms the basis for explicit finite element analysis using the commercial code LS-DYNA presented in Part II of the paper.

Modeling Low-Velocity Impact Damage of Composite Sandwich Panels

Analytical solutions for the low-velocity impact damage of composite sandwich panels are found by considering systems of discrete masses, springs and constant-force dashpots. Equivalent spring and dashpot forces for the sandwich panel are derived from the static load-indentation response and adjusted with dynamic material properties of the facesheet and core. Predictions of the maximum impact force are about 15% higher than experimental values, which is the same order of accuracy that is obtained from the static load-indentation analysis. Damage initiation due to tensile failure by hemispherical-nose shape projectiles and shear failure by blunt projectiles are examined. Predicted tensile and shear failure loads are about 20% higher than experimental results.

Reaction mechanism requirements in shock-induced combustion simulations

A quasi-steady-state, shock-induced combustion problem due to the Mach 6.46 flight of a blunt projectile in a stoichiometric hydrogen and air mixture is examined as a case study. Three regions are identified where different elements of the chemical kinetics play key roles. The nose region is where the mixture is subjected to a near normal shock, the front separation region is where the shock and reaction front diverge, and the separated front region in the far field is where there is a oblique shock and a density front. These regions are at widely disparate temperatures and pressures. The near field is found to depend strongly on rate of heat release, while the front separation point depends only on equilibrium, and the far-field results depend strongly on a very accurate determination of the second explosion limit. Results also suggest that factors which impact induction time, such as high-pressure effects in the reaction mechanisms, can play an important role.

Deformation and perforation of cylindrical shells struck normally by blunt projectiles

An approximate theory is proposed in this paper to predict the deformation and perforation of metallic cylindrical shells struck normally by blunt projectiles at velocities up to 229 m s −1. On the basis of the experimental observations of quasi-static load–displacement characteristics, the problem of a cylindrical shell impacted normally by a blunt projectile can be tackled through the solution of an equivalent clamped circular plate struck transversely by the same projectile. It is shown that the approximate theoretical predictions are in good agreement with the experimental results for cylindrical shells in terms of the maximum permanent transverse displacements and the critical impact velocities (ballistic limits) when material strain rate sensitivity is taken into account.

Simulation of shock-induced combustion past blunt projectiles using shock-fitting technique

Two-dimensional axisymmetric, reacting viscous flow over blunt projectiles is computed to study shockinduced combustion at Mach 5.11 and 6.46 in hydrogen-air mixture. A finite difference, shock-fitting method is used to solve the complete set of Navier- Stokes and species conservation equations. In this approach, the bow shock represents a boundary of the computational domain and is treated as a discontinuity across which Rankine-Hugoniot conditions are applied. All interior details of the flow such as compression waves, reaction front, and the wall boundary layer are captured automatically in the solution. Since the shock-fitting approach reduces the amount of artificial dissipation, all intricate details of the flow are captured much more clearly than has been possible with the shock-capturing approach. This has allowed an improved understanding of the physics of shock-induced combustion over blunt projectiles and the numerical results can now be explained more readily with a one-dimensional wave-interacti on model.

Penetration of a rigid projectile into an elastic-plastic target of finite thickness

This paper considers the problem of non-steady penetration of a rigid projectile into an elastic-plastic target of finite thickness. A specific blunt projectile shape in the form of an ovoid of Rankine is used because it corresponds to a reasonably simple velocity field which exactly satisfies the continuity equation and the condition of impenetrability of the projectile. The target region is subdivided into an elastic region ahead of the projectile where the strains are assumed to be small, and a rigid-plastic region near the projectile where the strains can be arbitrarily large. Using the above mentioned velocity field, the momentum equation is solved exactly in both the elastic and the rigid-plastic regions to find expressions for the pressure and stress fields. The effects of the free front and rear surfaces of the target (which is presumed not to be too thin) and the separation of the target material from the projectile are modeled approximately, and the force applied to the projectile is calculated analytically. An equation for projectile motion is obtained which is solved numerically. Also, a useful simple analytical solution for the depth of penetration or the residual velocity is developed by making additional engineering approximations. Moreover, the solution procedure presented in this paper permits a straight forward approximate generalization to accommodate a projectile with arbitrary shaped tip. Theoretical predictions are compared with numerous experimental data on normal penetration in metal targets, and the agreement of the theory with experiments is good even though no empirical parameters are used. Also, simulations for conical and hemispherical tip shapes indicate that the exact shape of the projectile tip does not significantly influence the prediction of integral quantities like penetration depth and residual velocity.

Yawing impact on thin plates by blunt projectiles

Two experimental investigations and a corresponding analytical study were conducted to examine the phenomena attendant to the impact of blunt-nosed, hard-steel strikers on stationary thin plates of aluminum and steel at moderate angles of yaw and zero obliquity. The variation of ballistic limit with yaw angle or the terminal velocity and final trajectory angle in perforation tests were ascertained. Post-mortem examination of the plates indicated that damage and failure occurred by bulging, lateral indentation, and side and front petaling. A theoretical model based on a membrane representation was developed that analyzed the impact by dividing the process into five stages. This model underpredicted the ballistic limit by up to 14%, with better correlation found at higher yaw angles. Excellent agreement was observed between the experimental and analytical final velocities when the data points were corrected to reflect the difference between the experimental values of the ballistic limit and that predicted by the model. Fair agreement was found between the experimental and the analytical values of the trajectory angle.

A fully implicit time accurate method for hypersonic combustion: application to shock-induced combustion instability

A new fully implicit, time accurate algorithm suitable for chemically reacting, viscous flows in the transonic-to-hypersonic regime is described. The method is based on a class of Total Variation Diminishing (TVD) schemes and uses successive Gauss-Seidel relaxation sweeps. The inversion of large matrices is avoided by partitioning the system into reacting and nonreacting parts, but a fully coupled interaction is still maintained. As a result, the matrices that have to be inverted are of the same size as those obtained with the commonly used point implicit methods. In this paper we illustrate applicability of the new algorithm to hypervelocity unsteady combustion applications. We present a series of numerical simulations of the periodic combustion instabilities observed in ballistic-range experiments of blunt projectiles flying at subdetonative speeds through hydrogenair mixtures. The computed frequencies of oscillation are in excellent agreement with experimental data.

A study of fragmentation in the ballistic impact of ceramics

Experiments in which both confined and unconfined ceramic targets are perforated by pointed and blunt projectiles are described, and a correlation is established between increased degree of fragmentation and reduced ceramic toughness. Front confinement of the ceramic results in greater overall fragmentation, however fewer, very fine fragments are produced for confined targets compared to unconfined targets. By attributing the fine fragments principally to crushing ahead of the impacting projectile, and coarse fragmentation to the interaction of stress relief waves, the effects of confinement can be qualitatively explained in terms of a simple model for loading and stress relief during perforation. The use of blunt projectiles increases the degree of fragmentation in those cases where the ceramic strength itself is insufficient to fracture the tip on impact. Measurements of fractured ceramic surface area and calculations of fracture work demonstrate that very little of the projectile kinetic energy is consumed in creating new ceramic fracture surface, and it is shown that a high proportion of the projectile impact kinetic energy is redistributed to residual kinetic energy of ejected ceramic debris. For cases where the projectile does not deform during penetration it is possible to derive a value for the average pressure resisting the penetrator. The ballistic efficiency of the ceramic increases with hardness for the lower strength ceramics, however, for the hard ceramics, where the principal influence of the ceramic is to destroy the projectile nose and create an inefficient penetrator, it is found in these tests that the residual penetration depths are similar, and ballistic efficiency is then unrelated to ceramic strength.

Ballistic evaluation of ceramics: Influence of test conditions

Summary This work presents data on the penetration of high quality 99.5% alumina tiles backed with “semi-infinite” metal plates by both sharp and blunt tungsten alloy and hard steel projectiles. The three metallic backing materials used are an aluminium alloy 5083, rolled homogeneous steel armour and high hardness steel armour. Merit ratings which quantify the performance of the ceramic against the impacting projectile, are a function of the projectile type and of the backing material. High values of merit rating are achieved for pointed projectiles, in cases where the projectile is hard relative to the reference backing material. With such conditions deformation of the penetrator is minimal when fired into the reference material alone, but use of the hard ceramic tile results in the destruction of the penetrator tip converting the residual penetrator into one of low efficiency. Low merit ratings are obtained for pointed projectiles fired against hard backing targets (which destroys the tip) and for blunt penetrators. Two mechanisms are involved in the defeat of pointed projectiles by a hard ceramic: destruction of the nose configuration, which converts the penetrator into one of low efficiency, and penetrator erosion. For blunt projectiles, on the other hand, the interaction with the ceramic is dominated by penetrator erosion.

Response of simulated propellant and explosives to projectile impact—I. Material behavior and penetration studies

Summary As the first part of a broad investigation of the response of two propellant and explosive simulants to projectile impact, this paper first delineates their measured mechanical behavior under static and dynamic loading. Subsequently, a detailed description is given of the damage patterns generated upon penetration and perforation of homogeneous disks or composites where these disks were constrained axially by metallic plates or completely confined by the addition of a circumferential steel band. The two substances, labeled Propergol A and B, are composed of particle-embedded polymers and were found to be highly viscoelastic. Properties were determined from quasi-static, creep and relaxation and Hopkinson bar tests, as well as from acoustic wave experiments. Ballistic limits were determined both for pure Propergol and combinations of this material with adjacent metallic plates. On the basis of areal density, monolithic Propergol exhibits a substantially lower perforation resistance than steel, aluminum, or Propergol disks either prefaced or sandwiched by metallic plates. Damage patterns observed in the vicinity of the ballistic limit include radial and conical cracking, cylindro-conical (or mushroom) plugging for blunt projectiles, specimen cleavage due to slow crack propagation in constrained samples upon removal of the constraint, and debonding and rebonding in the interior around the projectile trajectory. In this velocity regime, fragmentation is minimal; at higher speeds, plug formation is inhibited and the phenomenon is fragmentation dominated. This feature and phenomenological or numerical analyses of the processes are discussed in subsequent papers.

Normal impact of blunt projectiles on moving targets: Analytical considerations

An analytical model for the normal impact of blunt, cylindrical and undeformable projectiles on thin metallic targets moving in a plane normal to the initial bullet trajectory has been developed. This analysis is intended to depict the principal phenomena that occur when such a projectile strikes the center of a circular plate mounted at the end of an arm rotating about the other end at constant angular velocity. The first step in the simulation constructions of the construction of a three-stage plugging process involving plastic wave propagation, joint projectile/target motion and tensile failure designed for impact on a stationary plate, using a membrane theory for the target response. This axisymmetric representation was then applied to moving targets by employment of impulse-momentum laws. Petaling failure was portrayed by an upper-bound plasticity approach, while projectile motion was described by rigid-body dynamics. The principal assumption inherent in this representation were guided by geometric considerations and observations from tests described in a companion paper. Model evaluation was performed for several cases of projectile and target speed combinations. Relevant solutions were compared with results from corresponding experiments. In general, good correlation were obtained; discrepancies could be attributed to the effect of features that had not been incorporated in the model.

Normal impact of blunt projectiles on moving targets: experimental study

An experimental investigation was performed to study impact on targets moving at right angles to the striker trajectory with a velocity of 40 m/s. Tests were executed with a compressed gas gun using 6.35 mm diameter, 38.1 mm long hard steel blunt-nosed cylindrical strikers in the velcity range 49–213 m/s and with a powder gun using a blunt 9.53 mm diameter cylindrical projectile with an aspect ratio of 2 up to velocities of 492 m/s. The post-impact projectile trajectory and attitute as well as the configuration of the craters produced in thin targets of cold-rolled steel and soft aluminum were observed. The impact process produced asymmetric plugging followed by either front or side petaling. The latter phenomenon was found to be controlled by the ultimate tensile strain of the target and the ratio of the projectile to target speed. The test parameters and resulting data were primarily selected to permit comparison with the predictions of an analytical model of the process described in a companion paper.

A structural model for thin plate perforation by normal impact of blunt projectiles

A model is developed which allows deformation of a plate by plug shearing, bending and membrane stretching during perforation by a projectile. The model uses equations for the impact bending of a beam with appropriate modifications to the mass distribution, moment of inertia, deformation and failure criteria to adapt it to the case of a thin plate. The method is applied to a range of impact problems on thin plates, relative to the projectile diameter, in the sub-ordinance velocity range.

Wounding mechanism of projectiles striking at more than 1.5 km/sec.

In 1976 Charters and Charters (2) described experiments intended to study effects of projectiles with a striking velocity greater than 1 km/sec. They postulated that the projectiles at higher velocity would cause shallow wounds with wide tissue destruction on the surface, especially when striking velocity exceeded the speed of sound in tissue (about 1.5 km/sec). We found no other studies reported dealing with projectiles in this velocity range, the conclusions and assumptions of Charters and Charters have been quoted by others and accepted as fact. We designed and performed experiments to test the hypothesis of Charters and Charters by comparing temporary cavity morphology and penetration in gelatin. We fired two types of blunt projectiles over a velocity range from 650 m/sec (2,137 ft/sec) to 2,016 m/sec (6,614 ft/sec). In these studies we found no evidence to indicate that shape and characteristics of the disruption in ordnance gelatin change significantly when missile striking velocity exceeds sonic speed in the target.

Impact response of a circular membrane

The major objective of this study is to investigate the transverse deformation of a finite-circular membrane after normal impact by a blunt projectile. A separation-of-variables technique is used to solve the governing two-dimensional-membrane equation. From observations obtained from a high-speed film of the impact, it is shown that the predicted displacements are in close agreement with experimentation. The theoretical formulation is also supperted by comparison with the experimental results obtained by Beynet and Plunkett, who studied the analogous problem of a thin plate subjected to ballistic impact.

Microstructural aspects of impact erosion in single crystals of LiF, NaCl, KCl and CaF2 by blunt projectiles

In an attempt to clarify the fundamental mechanism of material removal in erosion by blunt solid particles entrained in a fluid stream impinging on a solid surface, single crystals of CaF2 (relatively brittle), LiF (intermediate between brittle and ductile), and KCl and NaCl (relatively ductile) were eroded with 0.25 mm glass beads and 0.50 mm quartz sand grains. The velocity of the particles was varied between 2 and 120 m sec−1. Erosion damage was studied with optical and scanning electron microscopy. In the single-impact mode, the damage is highly dependent on the mechanical properties of the target; material spalled off or micromachined out in individual impacts, or as chips produced upon the intersection of fractures resulting from several neighbouring impacts. At normal impact, the predominant mechanism is intersecting fractures, but at impact angles away from the normal, micromachining occurs in NaCl and KCl and, in fact, becomes the major mechanism of material removal. In LiF, only a little micromachining occurs and in CaF2, none at all; hence in these materials spalling is the controlling mechanism for material loss in individual impacts.

A phenomenological penetration model of plates

An analysis has been developed to describe phenomenologically the plugging process occurring in thin plates or those of intermediate thickness when struck by blunt projectiles at normal incidence. The present model is concerned with an underformable striker and a target whose mechanical behaviour is characterized by a strain-rate independent rigid/plastic constitutive relation. Plastic wave theory is employed to construct a five-stage penetration process that consists sequentially of indentation, plug formation, separation and slipping, and of post-perforation deformation. The dynamic response of the target is included in terms of the action of a plastic hinge resulting from shear applied at the projectile periphery. The equations of motion are obtained for the small number of bodies defined by the system components, the wave fronts and the hinges. The problem was coded in Fortran IV and run on a PDP-11/60 computer. Experimental results for residual projectile velocity obtained by other investigators were found to be in excellent correspondence with the calculations of the model. The axial shear deformation mode does not predict the total target deflection and contributes only within a narrow zone beyond the projectile radius, while bending is a more dominant mechanism. The inclusion of shear was found to provide closer correspondence with experimental data near the ballistic limit.

Instabilities in reacting flows

This paper reviews nonsteady phenomena in reacting flows, arising through coupling between chemical kinetics and gas dynamics. Problems examined include piston-supported detonations, exothermic hypersonic flows about blunt and conical projectiles, transient development of instabilities in blunt-body flows and in propagating detonations, acoustic-kinetic interactions and vorticity generation. The interrelationships among the various problems are examined in terms of the governing instability mechanism. Unsolved but critical problem areas are also identified. In the theoretical studies of piston-supported shock waves in an exothermally reacting mixture, periodic instabilities have been found to develop to well-defined amplitudes and periods. These results agree well with those predicted on the basis of a wave-interaction model, which clearly demonstrates the physics involved in the coupling between gas dynamics and chemical kinetics. They also agree well with the experimental results observed in exothermic hypersonic flows about blunt projectiles in a ballistic range. Recent experiments on exothermic hypersonic flows about conical projectiles show the existence of two instability regimes in addition to steady overdriven and Chapman-Jouguet detonations. One of the two instability regimes shows flow features that resemble remarkably well with those observed around blunt projectiles, while the other regime clearly shows features peculiar to conical flows. In order to understand the coupling mechanisms in greater detail, experiments have been conducted in a ballistic range to study the initiation and the decay of the two instability regimes around blunt projectiles across compositional interface. Similar transient studies have been performed with propagating detonation waves that encounter either compositional or tube-area changes. Much deeper understanding has been achieved. Unlike sound propagation in gaseous media at chemical equilibrium, when only attenuation is observed, traveling acoustic waves can be amplified under some conditions through coupling between nonequilibrium chemical kinetics and acoustic fluctuations. In another theoretical study, generation of vorticity fluctuations has been predicted, as the consequence of sound-entropy interactions in the presence of chemical reactions. These studies have led to a better understanding of the role of chemical reactions in basic instability mechanisms.