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Guest Editorial

J. Appl. Mech. 2013;80(3):030301-030301-1. doi:10.1115/1.4023491.

This issue of the Journal of Applied Mechanics contains 25 scientific papers presented at the 27th International Symposium on Ballistics (ISB), organized by the Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut (EMI, www.en.emi.fraunhofer.de), Freiburg i.B., Germany, on behalf of the International Ballistics Society (IBS, www.ballistics.org) and in collaboration with the National Defense Industrial Association (NDIA, www.ndia.org), USA.

Topics: Ballistics
Commentary by Dr. Valentin Fuster

Interior Ballistics

J. Appl. Mech. 2013;80(3):031401-031401-5. doi:10.1115/1.4023311.

Charge temperature is one of the main tactical and technical indices for a large caliber self-propelled gun, and is a critical firing datum affecting the projectile muzzle velocity and gun firing accuracy. In this paper, on the basis of analyzing the heat transfer character of propellant, an unsteady-state heat conduction model describing the variation of the charge temperature is built and the finite-difference implicit schemes are theoretically deduced using the volume equilibrium method for numerical simulation. Comparing simulation curves with experiment results indicates that the physical models used reflect the real-time change process of the charge temperature with the environment temperature. A noncontact automatic online charge temperature measurement unit is developed. This unit can accurately measure the temperature field and real-time average temperature of the charge for all charge zones placed in a combat vehicle and simultaneously transfer the temperature information to the gunner task terminal computer through a controller area network (CAN) bus interface for trajectory calculation and firing data correction, ensuring the weapon system meets the requirements of digitalization and informatization.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031402-031402-5. doi:10.1115/1.4023312.

The determination of pressure profiles behind a projectile has been a subject of investigations for more than 70 years. For lumped parameter models it was especially important to determine the pressure on the projectile base, pressure on the chamber base, and pressure for the propellant burning law. In the paper two analytical methods and one numerical method are considered. The analytical methods of proportionate expansion and two-phase mixture are studied. Pressure profiles are also computed numerically by TWO Phase Interior Ballistics (TWOPIB) code, which is based on the model of two-phase flow of solid propellant and its products of combustion, treated as separate phases with appropriate conservation laws and interactions between phases. Through comparison with experimental results on the real weapon system TWOPIB code showed great advantages over analytical methods.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031403-031403-6. doi:10.1115/1.4023313.

In conventional models for two-phase reactive flow of interior ballistic, the dynamic collision phenomenon of particles is neglected or empirically simplified. However, the particle collision between particles may play an important role in dilute two-phase flow because the distribution of particles is extremely nonuniform. The collision force may be one of the key factors to influence the particle movement. This paper presents the CFD-DEM approach for simulation of interior ballistic two-phase flow considering the dynamic collision process. The gas phase is treated as a Eulerian continuum and described by a computational fluid dynamic method (CFD). The solid phase is modeled by discrete element method (DEM) using a soft sphere approach for the particle collision dynamic. The model takes into account grain combustion, particle-particle collisions, particle-wall collisions, interphase drag and heat transfer between gas and solid phases. The continuous gas phase equations are discretized in finite volume form and solved by the AUSM+-up scheme with the higher order accurate reconstruction method. Translational and rotational motions of discrete particles are solved by explicit time integrations. The direct mapping contact detection algorithm is used. The multigrid method is applied in the void fraction calculation, the contact detection procedure, and CFD solving procedure. Several verification tests demonstrate the accuracy and reliability of this approach. The simulation of an experimental igniter device in open air shows good agreement between the model and experimental measurements. This paper has implications for improving the ability to capture the complex physics phenomena of two-phase flow during the interior ballistic cycle and to predict dynamic collision phenomena at the individual particle scale.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031404-031404-6. doi:10.1115/1.4023314.

The modular charge is developed to replace the conventional bagged charge systems for many advantages. In the interior ballistic cycle of a modular charge system, the nonsimultaneous ignition of propellant in different cases results in an increasing pressure wave, and can cause a launch safety problem of the gun. Because the charge structure is complicated, it is hard to simulate the interior ballistics process for the modular charge system currently. To simulate the interior ballistic of the modular charge system more accurately, an improved interior ballistic one-dimensional two phase flow model for modular charge system is established. The improvement of this model lies in that it takes account of the discontinuity of the propelling charge bed, the block of the cartridge wall to the flame spreading in propelling charge bed, effects of modular cartridge movement to the interior ballistic performance, the nonsimultaneous breakup of the modular charge cartridges, the ignition of the propelling charge in different cartridges, and flame spreading through the cap of the core tubes. Simulation for a full charge and three lower charge cases with different charge position were carried out based on the model. The simulation results proved that the model is reliable, and can be used to study the effects of cartridge mechanical properties, charge position, different charge zones on interior ballistic performance of modular charges.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031405-031405-7. doi:10.1115/1.4023315.

In order to eliminate residual solvents in ammunition and to reduce the emissions of volatile organic compounds to the atmosphere, the U.S. Army ARDEC has teamed with TNO in developing a new process for the production of solventless propellant for tank ammunition. To reduce the costs of solventless propellants production, shear roll mill and continuous extrusion processing was investigated. As described in this paper JA-2 a double base propellant cannot be processed without solvent by the extrusion process. An alternative JA-2 equivalent propellant was defined. The aim of this work is to demonstrate the manufacturing of this propellant by solventless continuous twin screw extrusion processing while maintaining gun performance characteristics of conventional JA-2 propellant. This is elucidated by explicitly researching the relationship between interior ballistic properties of the gun propellant and utilizing a continuous manufacturing process. Processing conditions were established, and the propellant was manufactured accordingly. The extruded propellant has the desired properties, which resulted in a comparable gun performance as the conventional JA-2 propellant.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031406-031406-11. doi:10.1115/1.4023316.

A simulator is designed to explore the interactive mechanism of a plasma jet with the liquid medium in the bulk-loaded liquid electrothermal chemical launching process. The properties of the plasma jet expanding in the liquid and the mixing characteristics of the plasma jet with liquid in the cylindrical chamber are studied using a high speed camera system. According to the experimental results, a two-dimensional axisymmetric unsteady compressible flow model has been proposed. The transient characteristics of the jet in flow field have been simulated. The results indicate that, during the expansion of the plasma jet in the liquid medium, there is relatively strong turbulent mixing. The interface between the two phases is not smooth and fluctuates with time stochastically. The higher the discharge voltage is, the stronger the Helmholtz instability effect will be. The Taylor cavity forming during the jet expansion can be divided into three regions: the main flow region, the compression region, and the backflow vortex region. In the main flow region the temperature and velocity of the plasma jet are relatively high and both decrease along the axial and radial direction. The pressure near the Taylor cavity head is high. The high pressure region grows gradually while the pressure value decreases. The calculated axial expansion displacement of the Taylor cavity coincides well with the measured one from experiment.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031407-031407-8. doi:10.1115/1.4023317.

In a gun system, polydisperse phenomenon may occur due to the local combustion by an igniter system during the firing process. The Eulerian–Eulerian approach still lacks the capability of describing particle mixing under given conditions. A detailed insight of the interior ballistics must be predicted for the better safety and the lower cost at the development stage. The multiphase particle in cell (MP-PIC) model based on the Eulerian–Lagrangian approach, known to be more efficient than the conventional Eulerian–Lagrangian approach, has been initially applied for the simulation of the interior ballistics. A good efficiency with the MP-PIC model has been obtained in terms of the computational cost. The axisymmetric numerical code with the MP-PIC model has been developed for two-dimensional analysis of the interior ballistics. As part of the verification process for the code, several test computations have been performed: sod shock tube, free piston motion problem, and virtual gun calculated by IBHVG2 code. The code has become reliable with well-agreed results with the comparison data. Additionally, a numerical model for the orifices to describe the vent holes of the igniter on the coarse grid has been developed with the lumped parameter method used in the IBHVG2. Based on the model, the pressure behavior in the gun chamber according to the igniter length has been investigated. The computational results have shown that the negative differential pressure occurs clearly when the igniter is sufficiently short.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031408-031408-11. doi:10.1115/1.4023318.

In this study, the effect of the initial temperature of a 120-mm mortar system on its interior ballistics was investigated using four different experiments: temperature-conditioned closed bomb firings for determining the temperature sensitivity of the ignition cartridge's M48 double-base propellant and instrumented firings of temperature-conditioned flash tubes, ignition cartridges, and an instrumented mortar simulator (IMS). The results of these experiments reveal that, for initial temperatures of –12 °C and greater, the mortar system and its subcomponents exhibited regular initial-temperature-dependent behavior, with increasing initial temperature causing monotonically increasing propellant burning rates and monotonically decreasing ignition delays, which produce monotonically increasing system pressures and pressure differentials in the flashtube, ignition cartridge, and IMS. However, some anomalous behavior was discovered for temperatures around –46 °C. At this initial temperature, the closed bomb firings indicated that brittle fracture of the M48 propellant granules used in the ignition cartridge occurs. This phenomenon explains the occurrence in the IMS firings of dramatically increased variation in pressure-time behavior and projectile muzzle velocity for charge 4 firings as compared to higher temperatures, as well as the occurrence of maximum tube pressures for –46 °C firings, being greater than those for 21 °C firings for charge 0. However, one of the –47 °C closed bomb firings does not exhibit evidence of grain fracture and yet produces a higher propellant burning rate than the –12 °C firings, suggesting that a fundamental change in reaction kinetics or flame structure is occurring at extremely low temperatures. This supposition is bolstered by evidence of a liquid layer existing on the surface of M48 propellant granules ejected from the ignition cartridge during the –46 °C firings—a phenomenon that does not occur at the higher initial temperatures and is not theorized to occur in double-base solid propellant combustion. Based on the flash tube experiments alone, the flash tube was determined to have a weak effect on the initial-temperature-dependent behavior of the mortar system; however, IMS testing with two different flash tube configurations revealed significant differences in longitudinal pressure wave amplitude and projectile muzzle velocity in charge 4 firings between the two configurations at –46 °C, suggesting that the uniformity of combustion product discharge from the flash tube could significantly affect the performance of the mortar at low temperatures.

Commentary by Dr. Valentin Fuster

Launch Dynamics

J. Appl. Mech. 2013;80(3):031501-031501-9. doi:10.1115/1.4023335.

The dynamics or “ringing” of a projectile structure at gun-muzzle exit has been observed to cause a large number of electronics failures in projectiles as well as possible safety concerns with respect to components impacting one another or structural components of the projectile coming apart. Current numerical tools allow accurate calculation of the muzzle exit event given that the engineer understands the forces acting on the projectile. Dynamic response of a structure is well understood by persons working in the field; however, engineers who do not regularly deal with dynamic analyses generally have difficulty interpreting results from both analyses and tests. This paper details the mathematics associated with this event so that the engineer confronted with a dynamics related issue can have a reference for understanding and interpretation. The results of a simple model show that accelerometer data should be used with caution and the support of a finite element analysis of the projectile structure with the proper pressure decay is usually necessary. Recommendations for use of measured acceleration data for modeling and simulation are provided.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031502-031502-7. doi:10.1115/1.4023336.

Smart projectiles use electronic components such as circuit boards with integrated circuits to control guidance and fusing operations. During gun-launch, the electronics are subjected to 3-dimensional g-forces as high as 15,000 G. The U.S. Army uses finite element analysis to simulate electronics with high-g, dynamic loads. Electronics are difficult to model due to the large variation in size, from large circuit boards, to very small solder joints and solder pads. This means that to accurately model such small features would require very large models that are computationally expensive to analyze; often beyond the capability of resources available. Therefore, small features such as solder joints are often not included in the finite element models to make the models computationally tractable. The question is: what is the effect on model accuracy without these small features in the model? The purpose of this paper is to evaluate the effect that solder joints and solder pads have on the accuracy of the structural analysis of electronic components mounted on circuit boards during gun shot. Finite element models of simplified circuit boards, chips, and potting were created to do the evaluation. Modal analysis and dynamic structural analysis using typical gun loads were done. Both potting at high temperature (soft) and potting at low temperature (stiff) were used in the dynamic analysis. In the modal analysis there was no potting. All of these models were run with and without solder. In all cases, the results differed between the models with solder and those without. In the models with potting, there was a difference in magnitude and stress distribution between the models with and without solder. This indicates that there is a significant reduction in accuracy when solder is not included in the model.

Commentary by Dr. Valentin Fuster

Exterior Ballistics

J. Appl. Mech. 2013;80(3):031601-031601-7. doi:10.1115/1.4023337.

The supersonic aerodynamics of a high spin armor-piercing fin-stabilized discarding sabot projectile (APFSDS) is numerically and experimentally investigated. Classically, using a 120 mm smooth bore gun, the value of the nondimensional steady spin rate of the inflight APFSDS projectile is about 0.01. In our case, a medium caliber APFSDS projectile is fired using a rifled barrel gun. The initial nondimensional spin rate, which is, with the Mach and the Reynolds numbers, the third similitude parameter governing the projectile aerodynamics, is significantly greater and about 0.09. Complex aerodynamics and flight dynamics were found and are detailed in the paper. In particular, the side force and moment evolutions with spin rate and angle of attack reflect a highly nonlinear behavior of the Magnus effect. The numerical predictions are mainly based on Reynolds-averaged Navier–Stokes (RANS) and URANS (Unsteady RANS) equations. Flight tests have been performed in an aeroballistics corridor. The exterior ballistics of the projectile was investigated using a yaw cards method, the experimental results are analyzed using a 6DOF flight dynamics model. The comparison between computational fluid dynamics (CFD) computations and flight tests results is satisfactory. CFD computations show, for the first time at our knowledge, that a roll-pitch coupling may appear.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031602-031602-6. doi:10.1115/1.4023338.

The revolving barrel gun is the principal component of the close-in weapons system (CIWS) that provides important terminal defense against anti-ship cruise missiles that have penetrated fleet defenses. The muzzle flow field of the revolving barrel firing is extraordinarily complex. The 3D computational model was formulated to illustrate the details of the flow field produced by the revolving barrel gun firing. The algorithm of a second order monotone upstream-centered schemes (MUSCL) approach with the advection upstream splitting method (AUSM) solver was used to simulate the high pressure muzzle flow field. The interior ballistic process was coupled with the simulation. The predicted muzzle velocity and maximum bore pressure were in good agreement with those measured in gun firing. Moreover, the muzzle flow field was obtained during the revolving barrel firing and was subsequently analyzed. The maximum lateral velocity of the first and second projectile fired was about 1.6 and 3.8 m/s.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031603-031603-9. doi:10.1115/1.4023434.

The Magnus effect on a generic 6.37 diameter long tangential-ogive-cylinder type projectile was studied by means of 3D Reynolds-averaged Navier–Stokes (RANS) simulations and wind tunnel measurements. The nominal Mach number was 3 and the Reynolds number, based on the model length, was 1.09 × 107. The simulations provided a profound insight into the flow structure and revealed a shift of the cross-flow separation lines as a consequence of the spin. This was shown to be the primary source of the Magnus side force for the higher angles of attack in the investigated range. The nonlinear dependence of the Magnus side force on the angle of attack was analyzed and reached a maximum value between 10 and 15 deg before decreasing again. The occurrence of secondary vortices in this range of angles of attack is presented as an explanation for a locally negative Magnus side force portion acting on the model.

Commentary by Dr. Valentin Fuster

Explosion Mechanics

J. Appl. Mech. 2013;80(3):031701-031701-13. doi:10.1115/1.4023339.

A nickel/aluminum (NiAl) reactive powder system has been investigated to determine its mechanical properties under quasi-static and high rate compression to understand its deformation behavior. A shock recovery system has been used to define shock reaction thresholds under a triaxial loading system. Two nickel/aluminum (NiAl) shaped charge liners have been fired into loose kiln dried sand to determine whether the jet material reacts during the formation process. A simple press tool was developed to press the liners from a powder mixture of nickel and aluminum powder and a simple conical design was used for the liner. The shaped charge jet particles were recovered successfully in the sand and subjected to a detailed microstructural analysis. This included X-ray diffraction (XRD) and optical and electron microscopy on selected particles. The analysis demonstrated that intermetallic NiAl was detected and all the aluminum was consumed in the particles examined. In addition, different phases of NiAl were detected as well as silicon oxide in the target material. There was also some evidence that the aluminum had melted along with evidence of a dendritic microstructure. This is the clearest evidence that the shaped charge jet material has reacted during the formation process. Simulations have been performed using the GRIM Eulerian hydrocode to compare with flash X-rays of the jet.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031702-031702-6. doi:10.1115/1.4023561.

This paper discusses the development of a fiber optic probe that can obtain temperature measurements from the interior of explosive fireballs, which are generated when unreacted detonation products react with oxygen in the surrounding air. Signatures of the thermochemical environment and chemical species involved can often be deduced from their light emissions, but the limited optical depth of fireballs means that remote sensing techniques can only sample emissions from the outer shell. By developing a protected fiber optic probe that can be placed adjacent to an exploding charge, giving it the ability to become enveloped by the fireball, the thermal radiation from the interior of the fireball can be sampled. Measurement from five shots using Detasheet-C explosives were carried out and could be obtained over the course of about 20 ms. Blackbody-type radiation with temperatures in the 1600 K to 1900 K range were observed, peaking at about 1850 K after 12 ms. The magnitude and time behavior of the temperature was not significantly different when taken at different locations within the fireball, indicating that temperature is fairly uniform throughout. The lack of specific spectral emission lines implies that in the interior of the fireball any combustion that occurred was probably primarily with carbonaceous soot, though differences in optical depth at different locations in the fireball indicate that it was much more fuel-rich closer to the center.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031703-031703-6. doi:10.1115/1.4023340.

Production of shear-formed liners for rotating shaped charges is of interest since it causes the jet self-spinning effect, which drastically affects the penetration process. A novel four-part methodology for calculating the performance parameters of such charges is suggested. First, the liner strained state is related to the feed rate and mandrel angular velocity during the shear-forming process. Second, based on the polycrystal plasticity theory, a methodology for determining the liner's plastic anisotropy parameters depending on its strained state is realized. Third, a general dependence of the jet angular velocity on the liner plastic anisotropy parameters is obtained. The fourth part presents the methodology of shaped charge performance calculation with respect to spinning effects. The results of calculations performed according to the suggested methodology are in good agreement with experimental data. Calculations also show that the penetration depth increases 9% to 15% compared to a spinning shaped charge with a drawn liner (i.e., without the self-spinning effect) when the self-spinning jet rotates oppositely to the shaped charge. When they rotate in the same direction, penetration depth decreases critically (more than 50%).

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031704-031704-9. doi:10.1115/1.4023341.

The purpose of this paper is to verify the applicability of innovative technologies for manufacturing controlled fragmentation warheads, with particular attention paid to guided ammunition. Several studies were conducted by the authors during the warhead development of DART and Vulcano family munitions. The lethality of the guided munitions can be considerably increased with controlled fragmentation warheads. This increase can compensate a lower payload of the guided munitions. After introducing the concept of warhead and its natural fragmentation, the paper describes both the elements of fracture mechanics related to the fragmentation and the state of the art of controlled fragmentation. A preliminary evaluation of controlled fragmentation technologies is illustrated along with the numerical models developed for predicting the natural and controlled fragmentations. The most promising technologies are presented in detail and the features of the warheads used for the experiments are defined. A description of the entire experimental phase is provided, including results of arena tests, data analysis and revision of numerical models. The applicability of some innovative technologies for controlled fragmentation warheads is fully demonstrated. Two technologies in particular, the laser microdrilling and the double casing solution, provide a high increase of the reference warhead lethality.

Topics: Lasers , Warheads , Melting
Commentary by Dr. Valentin Fuster

Terminal Ballistics and Impact Physics

J. Appl. Mech. 2013;80(3):031801-031801-11. doi:10.1115/1.4023342.

The influence of pitch (vertical yaw) angle on the penetration reduction of rod projectiles into oblique targets has been investigated for tungsten sinter alloy rods with a blunt nose and L/D = 20. Semi-infinite RHA targets with an obliquity of 30 deg, 45 deg, and 60 deg were impacted at 1650 m/s. The pitch angles were varied between ±90 deg. The strong asymmetric behavior of the target crater is dependent on whether the pitch is positive or negative relative to the obliquity of the target. The experiments provide a good overview of the penetration characteristics of long rods for the whole pitch angle range. The penetration data are described by empirical relations that show good agreement with the experiments.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031802-031802-6. doi:10.1115/1.4023343.

In this study, two different scale projectile high velocity penetration experiments with concrete targets that had an average compressive strength of 35 MPa were conducted in order to find the velocity limits and nose erosion properties. We conducted the penetration experiments for the small-scale (48 mm diameter, 195 mm long, 2 kg) and the large-scale (144 mm diameter, 680 mm long, 50 kg) ogive-nose projectiles with the hard steel 4340 whose dynamic compression strength is 2.2 GPa. A 100-mm-diameter powder gun was used to launch the five tests of the 2 kg projectiles with striking velocities between 1100 m/s and 1600 m/s and a 320-mm-diameter Davis gun was used to launch the two tests of the 50 kg projectiles with striking velocities 1100 m/s and 1300 m/s. The experimental results showed that the nose material was missing, indicating an apparent eroding process when the striking velocity exceeded 1400 m/s, where the rigid body penetration made a transition into the elastic-plastic hydrodynamics regime and penetration depth begin to decrease when the striking velocity exceeds 1400 m/s. Furthermore, nose changes and mass loss due to nose erosion did not significantly affect the penetrating ability before rigid body penetration made a transition into the hydrodynamic regimes. In addition, nose erosion was analyzed with SEM surface microstructures, and the SEM image showed that the mass loss of projectiles was due to the shear cracks preceded by adiabatic shear bands.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031803-031803-7. doi:10.1115/1.4023344.

The high velocity impact performance in hybrid woven carbon and S2 and E glass fabric laminates manufactured by resin transfer molding (RTM) was studied. Specimens with different thicknesses and glass-fiber content were tested against 5.5 mm spherical projectiles with impact velocities ranging from 300 to 700 m/s to obtain the ballistic limit. The resulting deformation and fracture micromechanisms were studied. Several impacts were performed on the same specimens to identify the multihit behavior of such laminates. The results of the fracture analysis, in conjunction with those of the impact tests, were used to describe the role played by glass-fiber hybridization on the fracture micromechanisms and on the overall laminate performance under high velocity impact.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031804-031804-9. doi:10.1115/1.4023345.

One observation from interface defeat experiments with thick ceramic targets is that confinement and prestress becomes less important if the test scale is reduced. A small unconfined target can show similar transition velocity as a large and heavily confined target. A possible explanation for this behavior is that the transition velocity depends on the formation and growth of macro cracks. Since the crack resistance increases with decreasing length scale, the extension of a crack in a small-scale target will need a stronger stress field, viz., a higher impact velocity, in order to propagate. An analytical model for the relation between projectile load, corresponding stress field, and the propagation of a cone-shaped crack under a state of interface defeat has been formulated. It is based on the assumption that the transition from interface defeat to penetration is controlled by the growth of the cone crack to a critical length. The model is compared to experimentally determined transition velocities for ceramic targets in different sizes, representing a linear scale factor of ten. The model shows that the projectile pressure at transition is proportional to one over the square root of the length scale. The experiments with small targets follow this relation as long as the projectile pressure at transition exceeds the bound of tensile failure of the ceramic. For larger targets, the transition will become independent of length scale and only depend on the tensile strength of the ceramic material. Both the experiments and the model indicate that scaling of interface defeat needs to be done with caution and that experimental data from one length scale needs to be examined carefully before extrapolating to another.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031805-031805-7. doi:10.1115/1.4023391.

Vital parts of nuclear facilities are commonly protected by reinforced concrete (rc-) structures. In order to assess the barrier effectiveness of these structures, different internal and external loads have to be considered. Among others, external missile impacts, for instance due to an airplane crash, are assumed to be relevant loading cases. In this context, impacts of nondeformable (“hard”), deformable (“soft”) as well as liquid filled (“wet”) missiles are considered. Major rc-target failure mechanisms are global bending, punching and perforation. This paper presents simulations with the computer program AUTODYN (ANSYS INC., 2010, ANSYS AUTODYN, Version 13.0 & Theory Manual) on intermediate- and large-scale impact experiments dealing with the aforementioned failure mechanisms. Missile velocities are in the range of 110 to 250 m/s. In particular, two different intermediate scale test series are considered. One test deals with predominant punching failure and perforation of a rc-slab (target) hit by a hard missile. Further, bending vibration of slabs impacted by soft missiles is analyzed, whereupon the influence of liquid infill on loading and target response is pointed out. Finally, a large scaled test with combined bending and punching failure of a rc-slab due to soft missile impact is considered. Results of numerical simulation and tests are compared. It is found, that the used concret material model developed by Riedel, Hiermaier, and Thoma (RHT) (Riedel, 2004, “Beton Unter Dynamischen Lasten: Meso-Und Makromechanische Modelle Und Ihre Parameter, Fraunhofer–Ernst-Mach-Institut, Freiburg/Br., ISBN 3-8167-6340-5) is suitable to reproduce the responses of rc-structures subjected to various kinds of impact conditions. Sensitivities of simulation results on modeling parameters are discussed.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031806-031806-10. doi:10.1115/1.4023349.

Single-yarn impact results have been reported by multiple authors in the past, providing insight on the fundamental physics involved in fabric impact. This insight allowed developing full fabric models that were able to reproduce properly wave propagation, deflection, and ballistic limits. This paper proposes a similar experimental methodology but for a specific composite material made of ultra-high molecular weight polyethylene. The presence of the polyurethane matrix in the composite is expected to slow down wave propagation. But the high-speed photographic tests reported in this paper indicate that wave propagation in strips and single-layer material is similar to that expected for dry fiber. An explanation is proposed for this unexpected result. This paper also reports the critical velocities (i.e., impact velocities that fail the fibers immediately) measured for the composite material and compares them to the velocities expected from the theory. The velocity is accurately predicted when taking into account wave interactions in front of the projectile. Finally, tests on multilayer composites are presented. In particular, a flash produced under the projectile during the first few microseconds was recorded with high-speed video cameras. A simplified study of the temperature increment upon impact indicates that the material may be reaching the autoignition point. This mechanism is speculated to be the origin of the flash systematically observed.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2013;80(3):031807-031807-11. doi:10.1115/1.4023573.

It has been demonstrated that significant weight reductions can be achieved, compared to conventional glass-based armor, when a transparent ceramic is used as the strike face on a glass-polymer laminate. Magnesium aluminate spinel (MgAl2O4) and AlON are promising candidate materials for application as a hard front layer in transparent armor. Comprehensive, systematic investigations of the fragmentation of ceramics have shown that the mode of fragmentation is one of the key parameters influencing the ballistic resistance of ceramics. In the study described here, the fragmentation of AlON and three types of spinel was analyzed: two types of fine grained spinel with nominal average grain sizes 0.6 μm and 1.6 μm and a bimodal grain-sized spinel with large grains of 250 μm size in a fine grain (5–20 μm) matrix were examined. The ceramic specimens of 6-mm thickness were glued to an aluminum backing and impacted with armor piercing (AP) projectiles of caliber 7.62 mm at two different velocities—850 m/s and 1100 m/s. The targets were integrated into a target box, which allowed for an almost complete recovery and analysis of the ceramic fragments. Different types of high-speed cameras were applied in order to visualize the different phases of fragment formation and ejection. A laser light-sheet illumination technique was applied in combination with high-speed cameras in order to determine size and speed of ejected ceramic fragments during projectile penetration. The application of the visualization techniques allowed for the analysis of the dynamics of the fragment formation and interaction with the projectile. A significant difference in the fragment size distributions of bimodal grain-sized spinel and AlON was observed.

Commentary by Dr. Valentin Fuster

Vulnerability

J. Appl. Mech. 2013;80(3):031901-031901-5. doi:10.1115/1.4023346.

Many police body armor systems are dual purpose, offering both ballistic and knife resistance by combining a flexible ballistic textile pack with a stiffer knife resistant layer. The two types of protection differ in materials and mechanisms such that each individual component may help or interfere with the function of the other. This paper investigates the effect on knife and ballistic penetration resistance when a single thin metal plate was placed at various different positions within an aramid textile armor pack. Two metallic layers were used: aluminum 7075 and commercial purity titanium; these had similar areal densities and were positioned in the front, middle, and back of a 20 layer pack of woven Kevlar® 49. An instrumented drop weight machine was used to deliver a repeatable knife blade impact at comparable energy levels to those specified in the UK Home Office test standards for knife resistance. Ballistic tests were used to determine the V50 ballistic limit velocity against typical 9 mm and 0.357 Magnum handgun threats. Against a stabbing threat, it was found that positioning the metal plate in the middle of the pack provided the greatest resistance to knife penetration by a factor of almost two, while a plate at the front of the pack provided less resistance and plates positioned at the rear of the pack provided the least resistance to penetration. Against the ballistic threat, the penetration resistance of the textile pack can be significantly improved when a metal plate is at the front of the pack, while for all other positions the effect is negligible. However, this effect is sensitive to both the ammunition type and the metal plate composition. When the metal plate is positioned at the rear of the pack there is a significant decrease in the back-face deformation of the armor pack although, again, this effect is only present for certain ammunition and metal combinations. The overall effect of combining soft and hard elements was that specific performance parameters could be substantially increased by the correct combination. There were no significant negative effects, however, in a number of cases, the combined systems performance was no greater than that of a single element type, despite the added weight.

Commentary by Dr. Valentin Fuster

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