Solid Projectile Research Articles

Compound cavity formation and splash crown suppression by water entry through proximally adjacent polystyrene beads

We move forward the important topic of water entry by documenting splash dynamics arising from the impact of hydrophilic spheres with buoyant millimetric microplastics, mimicked in our study by polystyrene beads. Collision with small, buoyant beads is yet another means to manipulate splash dynamics. In this experimental study, we investigate the fluid–structure interactions between beads and hydrophilic spheres for Froude numbers in the range of 20−100. Generally, hydrophilic spheres entering a liquid bath below the critical velocity of 8 m/s produce minimal fluid displacement and no cavity formation. The presence of proximally adjacent beads atop the fluid with respect to impacting spheres promote flow separation and compound cavities for sufficiently large Froude numbers, while suppressing the growth of splash crowns. Compound cavities consist of a shallow, quasi-static first cavity that seals near the water line, and a second, deeper cavity produced in the wake of descending spheres. A vertically protruding Worthington jet follows cavity collapse. The resulting splash metrics differ from those of hydrophobic spheres with respect to the properties of impacted beads. We find impactors traversing a deep liquid pool layered with beads experience drag reduction when compared to entry into a clean pool due to the drag-reducing benefits of flow separation while not offering a high inertial penalty. Our study unravels the physics behind the widely encountered interaction of solid projectiles impacting passively floating particles, and our results translate to the entry dynamics of water-diving creatures and projectiles into water bodies polluted by floating millimetric microplastics.

Crater depth prediction in granular collisions: A uniaxial compression model.

Impact crater experiments in granular media traditionally involve loosely packed sand targets. However, this study investigates granular impact craters on both loosely and more tightly packed sand targets. We report experiments that consistently adhere to power-law scaling laws for diameter as a function of impacting energy, similar to those reported by other groups for their experiments utilizing both solid and granular projectiles. In contrast, we observe significant deviations in the depth versus energy power law predicted by previous models. To address this discrepancy, we introduce a physical model of uniaxial compression that explains how depth saturates in granular collisions. Furthermore, we present an energy balance alongside this model that describes the energy transfer mechanisms acting during crater formation. We found a better way to transfer vertical momentum to horizontal degrees of freedom as the impact surface compacts, resulting in shallow craters on compacted sandbox targets. Our results reveal depth-to-diameter aspect ratios from approximately 0.051 to 0.094, allowing us to interpret the shallowness of planetary craters at the light of the uniaxial compression mechanism proposed in this work.

Ballistic performance of additive manufacturing 316l stainless steel projectiles based on topology optimization method

Material and structure made by additive manufacturing (AM) have received much attention lately due to their flexibility and ability to customize complex structures. This study first implements multiple objective topology optimization simulations based on a projectile perforation model, and a new topologic projectile is obtained. Then two types of 316L stainless steel projectiles (the solid and the topology) are printed in a selective laser melt (SLM) machine to evaluate the penetration performance of the projectiles by the ballistic test. The experiment results show that the dimensionless specific kinetic energy value of topologic projectiles is higher than that of solid projectiles, indicating the better penetration ability of the topologic projectiles. Finally, microscopic studies (scanning electron microscope and X-ray micro-CT) are performed on the remaining projectiles to investigate the failure mechanism of the internal structure of the topologic projectiles. An explicit dynamics simulation was also performed, and the failure locations of the residual topologic projectiles were in good agreement with the experimental results, which can better guide the design of new projectiles combining AM and topology optimization in the future.

Observation of Vertically Ejected Plumes Generated by the Impact of Hollow Projectiles at Various Velocities

Recently, impact experiments in space have been conducted in planetary exploration using hollow or internally structured projectiles. In laboratory experiments using hollow projectiles to investigate the differences in crater and ejecta from the case of solid projectiles, a plume perpendicular to the target surface has been observed, which has not been seen in conventional cratering experiments using solid projectiles. In this study, we conducted crater-formation experiments using hollow resin projectiles to understand the mechanism through which vertical plumes form in the case of hollow projectiles. We examined the generation of a vertical plume as a function of the impact velocity, v imp. We found that (i) no vertical plume occurs at v imp < 200 m s−1, (ii) the cases with or without a vertical plume are mixed at 200 < v imp < 350 m s−1, (iii) no vertical plume occurs at 350 < v imp < 800 m s−1, and (iv) a vertical plume occurs at 2 < v imp < 3 km s−1. We qualitatively discussed the generation mechanism of the vertical plume using the results of recovered projectiles. Depending on v imp, an empty hole in which there is no projectile materials can be opened along the central axis, resulting in the generation of a vertical plume.

Roles of packing fraction, microscopic friction, and projectile spin in cratering by impact.

From small seeds falling from trees to asteroids colliding with planets and moons, the impact of projectiles onto granular targets occurs in nature at different scales. In this paper, we investigate open questions in the mechanics of granular cratering, in particular, the forces acting on the projectile and the roles of granular packing, grain-grain friction, and projectile spin. For that, we carried out discrete element method computations of the impact of solid projectiles on a cohesionless granular medium, where we varied the projectile and grain properties (diameter, density, friction, and packing fraction) for different available energies (within relatively small values). We found that a denser region forms below the projectile, pushing it back and causing its rebound by the end of its motion, and that solid friction affects considerably the crater morphology. Besides, we show that the penetration length increases with the initial spin of the projectile, and that differences in initial packing fractions can engender the diversity of scaling laws found in the literature. Finally, we propose an ad hoc scaling that collapsed our data for the penetration length and can perhaps unify existing correlations. Our results provide new insights into the formation of craters in granular matter.

High-speed penetration of ogive-nose projectiles into thick concrete targets: Tests and a projectile nose evolution model

The majority of the projectiles used in the hypersonic penetration study are solid flat-nosed cylindrical projectiles with a diameter of less than 20 mm. This study aims to fill the gap in the experimental and analytical study of the evolution of the nose shape of larger hollow projectiles under hypersonic penetration. In the hypersonic penetration test, eight ogive-nose AerMet100 steel projectiles with a diameter of 40 mm were launched to hit concrete targets with impact velocities that ranged from 1351 to 1877 m/s. Severe erosion of the projectiles was observed during high-speed penetration of heterogeneous targets, and apparent localized mushrooming occurred in the front nose of recovered projectiles. By examining the damage to projectiles, a linear relationship was found between the relative length reduction rate and the initial kinetic energy of projectiles in different penetration tests. Furthermore, microscopic analysis revealed the forming mechanism of the localized mushrooming phenomenon for eroding penetration, i.e., material spall erosion abrasion mechanism, material flow and redistribution abrasion mechanism and localized radial upsetting deformation mechanism. Finally, a model of high-speed penetration that included erosion was established on the basis of a model of the evolution of the projectile nose that considers radial upsetting; the model was validated by test data from the literature and the present study. Depending upon the impact velocity, v0, the projectile nose may behave as undistorted, radially distorted or hemispherical. Due to the effects of abrasion of the projectile and enhancement of radial upsetting on the duration and amplitude of the secondary rising segment in the pulse shape of projectile deceleration, the predicted DOP had an upper limit.

Validation of the use of firearms for euthanising stranded cetaceans

Efforts to euthanise stranded cetaceans remain highly variable in their outcomes, with few field tested operational procedures available. This study sought to validate the efficacy of using modern firearms technology to euthanise small (&lt;6m length) stranded cetaceans. Post-mortem evidence was gathered from the standardised shooting of cetacean cadavers (n = 10), representing six species, using .30 caliber (7.62mm) firearms and blunt solid copper-alloy non-deforming projectiles, in southwestern Australia. The six species studied were Risso’s dolphin, common dolphin, bottlenosed dolphin, pygmy sperm whale, Cuvier’s beaked whale, and humpback whale. Post-mortem data revealed that 100% of bullet wound tracts fully penetrated the skulls of shot animals, with associated indirect skull fracturing, secondary bone missiles and brain parenchyma laceration. The results suggest that appropriate firearms technology is fully capable of inducing instantaneous fatal pathology to the central nervous system of these species. In comparison to alternative methods for the euthanasia of stranded cetaceans, the use of firearms is associated with superior animal welfare outcomes, public safety levels and accessibility. This paper provides a template for the safe, humane and repeatable use of this technique to euthanise &lt;6m length stranded cetaceans.

Modular electromagnetic railgun accelerator for high velocity impact studies.

A modular electromagnetic railgun accelerator facility named "RAFTAR" (i.e., Railgun Accelerator Facility for Technology and Research) has been commissioned and its performance has been characterized for high velocity impact testing on materials in a single-shot mode. In the first tests, RAFTAR demonstrated an acceleration of more than 1000m/s for an 8g solid aluminum-7075 armature projectile. The current fed was 220kA, having a muzzle time of about 1.75ms. It is a single pulse breech-fed rectangular bore (14 × 13mm2) railgun, and its 1.15m long barrel assembly consists of two parallel copper bars with an inter-gap of 13mm that are encased within 50mm thick high strength reinforced fiberglass sheets (Garolite G10-FR4) and bolted from both the sides. RAFTAR is powered by two capacitor bank modules that have a maximum stored energy of 160 kJ each (containing eight 178 μF/15kV capacitors), two high power ignitron switches, and a pulse shaping inductor. To obtain consistent acceleration of the armature inside the barrel, reversal of driving current is prevented, and its pulse duration is stretched by tactical integration of the crowbar switch and bitter coil inductor in the circuit. Armature projectile velocity measurement in-bore and outside in free space was performed by the time-of-flight technique using indigenously made miniature B-dot sensors and a novel shorting-foil arrangement, respectively. The time resolved measurement of the in-bore armature evidenced a velocity-skin-effect in the high acceleration phase. There is good agreement between the experimentally measured and theoretically predicted efficiency, confirming the optimal choice of operating parameters. The conclusion summarizes important experimental findings and analyzes the underlying causes that limit the performance of railguns.

Numerical modelling study of a modified sandbag system for ballistic protection

Modern field fortification systems are very large and cause a logistical burden during setup, thus setting the requirement for their modification. A simulation study was conducted to study the correlation between the compaction of sand and the energy absorbed by it when impacted by a solid steel projectile at 850 m/s. Furthermore, the sandbag was modified by the addition of plates in the system to observe the change in projectile penetration and the energy absorption behaviour of the sand. The factors considered for this study were the plate thickness (15 mm, 25 mm), plate material (aluminium, concrete, plexiglass, polycarbonate, steel) and plate location (226 mm, 236 mm, 256 mm from the point of impact). It was observed that a layer of compressed sand is formed around the projectile, aiding the energy absorption and dissipation process during impact. Upon addition of the plate, it was observed that the modified system (plate and sand) absorbed maximum energy when the plate is placed closest to the point of maximum penetration without the plate. It was also noted that the addition of the plate enhanced energy absorption characteristics of the system compared to conventional sandbags because of increased compaction of sand. From the study, it was observed that the plexiglass and polycarbonate plates had the maximum energy absorption and maximum deformation. The steel plate had the least energy absorption and minimal deformation. Concrete and Aluminium had comparable areal density, energy absorption and lowest deformation, making them the preferred choice of plate material for a modified sandbag. The numerical studies were verified using a gas gun and a modified sandbag with Aluminium plates to show that the addition of a plate improves compaction behaviour of sand.

Failure mechanism of single lap, adhesively bonded composite-titanium joints subjected to solid projectile impact

The failure mechanism of single lap, adhesively bonded composite-titanium joints subjected to solid projectile impact is addressed in this article. Three-dimensional finite element analysis was conducted in Abaqus/Explicit using two damage models. The first damage model accounts for debonding failure modes of adhesive–adherend interface using cohesive zone method. The second damage model accounts for in-plane failure modes of composite adherend using Hashin criterion and continuum damage mechanics. Mechanical tests were carried out by impacting the adhesively bonded joints with solid projectile using a one-stage gas gun. The recorded strain responses and damage patterns were found to be in good correlation with those of the finite element analysis. Results have shown that, for single lap, adhesively bonded composite-titanium joints under increasing impact velocities of solid projectile, the progression of failure modes is: local debonding in the adhesive–titanium adherend interface, matrix cracking in the composite adherend and global debonding in the adhesive–titanium adherend interface.

Modeling and Simulation Techniques Used in High Strain Rate Projectile Impact

A series of computational models and simulations were conducted for determining the dynamic responses of a solid metal projectile impacting a target under a prescribed high strain rate loading scenario in three-dimensional space. The focus of this study was placed on two different modeling techniques within finite element analysis available in the Abaqus software suite. The first analysis technique relied heavily on more traditional Lagrangian analysis methods utilizing a fixed mesh, while still taking advantage of the finite difference integration present under the explicit analysis approach. A symmetry reduced model using the Lagrangian coordinate system was also developed for comparison in physical and computational performance. The second analysis technique relied on a mixed model that still made use of some Lagrangian modeling, but included smoothed particle hydrodynamics techniques as well, which are mesh free. The inclusion of the smoothed particle hydrodynamics was intended to address some of the known issues in Lagrangian analysis under high displacement and deformation. A comparison of the models was first performed against experimental results as a validation of the models, then the models were compared against each other based on closeness to experimentation and computational performance.

Centroid migration on an impacted granular slope due to asymmetric ejecta deposition and landsliding

ABSTRACT For a fundamental understanding of terrain relaxation occurring on sloped surfaces of terrestrial bodies, we analyse the crater shape produced by an impact on an inclined granular (dry-sand) layer. Owing to asymmetric ejecta deposition followed by landsliding, the slope of the impacted inclined surface can be relaxed. Using the experimental results of a solid projectile impact on an inclined dry-sand layer, we measure the distance of centroid migration induced by asymmetric cratering. We find that the centroid migration distance xmig normalized to the crater minor-axis diameter Dcy can be expressed as a function of the initial inclination of the target tan θ, the effective friction coefficient μ, and two parameters K and c that characterize the asymmetric ejecta deposition and oblique impact effect: xmig/Dcy = K tan θ/[1 − (tan θ/μ)2] + c, where K = 0.6, μ = 0.8, and c = −0.1 to 0.3. This result is consistent with a previous study that considered the effect of asymmetric ejecta deposition. The obtained results provide fundamental information for analysing the degradation of sloped terrain on planetary surfaces, such as crater-shape degradation due to the accumulation of micro-impacts.

Close-in blast resistance of large-scale auxetic re-entrant honeycomb sandwich panels

The protection of critical infrastructure, including government buildings, airports, religious buildings, military buildings and military vehicles, which are at risk to blast loads, has become important due to increasing terrorist activities in recent years. Sacrificial cladding systems based on negative Poisson’s ratio core topologies have recently received more attention as a protective technology due to its excellent energy absorption capability. In this study, field blast tests were performed on metallic re-entrant honeycomb-cored sacrificial cladding systems as protective structures for steel plate structures. This study focused on the near-field blast loading conditions where liquid Nitromethane (NM) spherical charges were detonated in close proximity to the main structure. Two 6 mm thick mild steel plates and two steel plates protected with re-entrant honeycomb-cored sacrificial cladding systems were among the specimens tested. The proposed auxetic cladding system was fabricated from aluminium sheets using a novel in-house built folding machine. Numerical simulations were conducted utilising LS-DYNA software and the Blast Impact Impulse Model (BIIM). The results obtained from the numerical simulations are in good agreement with the experimental results. It was found that the deformation pattern of the sacrificial auxetic cladding system varies with the intensity of the blast loading, and there is a limit at which the proposed protective system ceases to effectively absorb the applied blast loading. The variation of negative Poisson’s ratio of the system with blast loading was studied. It was found that the auxetic cladding system could become a solid projectile leading to damage amplification for very close-range blast loads due to rapid densification of the auxetic core. The proposed cladding systems with narrow re-entrant angles performed well under blast loads due to relatively low stiffness of the panels. Finally, the optimisation study was performed for the protective system. Overall, the experimental and numerical results assure that auxetic-based cladding systems are suitable for applications requiring blast protection such as armoured vehicles and critical physical infrastructure but need to be carefully designed for the given blast threat to prevent overloading of the protected structures.

Contact Layer as a Propelling Advantage in Throwing

Adding a soft, springy material to a solid projectile leads to a threefold increase in kinetic energy when the projectile is launched upward.

Floquet theory for the electronic stopping of projectiles in solids

A theoretical framework for the study of electronic stopping of particle projectiles in crystalline solids is proposed. It neither relies on perturbative or linear-response approximations nor on an ideal metal host. Instead, it exploits the discrete translational invariance in a space-time diagonal for a constant velocity projectile following a trajectory with crystalline periodicity. By means of Floquet theory, (stroboscopically) stationary solutions are characterized, and previous perturbative and jellium models are recovered. The threshold-velocity effect in insulators is analyzed based on quasienergy conservation.Received 16 September 2019Revised 30 June 2020Accepted 2 July 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.033151Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBand gapIrradiation effectsScattering theoryPhysical SystemsFloquet systemsCondensed Matter, Materials & Applied Physics

Impact Study on Composite Targets by a Nylon Projectile

In the present paper, impact study of nylon projectile on composite targets using a water-jet disruptor was addressed with damage mechanism. The three different composite targets such as teak, ply and pine woods were used in the experiment. The strength of wood and the damage under impact load is very crucial. The failure under load is very intricate as it is made up of natural composite. The composite target damage mechanism depends on characterisation and impact velocity of projectile. Simultaneously stresses and strains are generated which makes it responsible to cause the damage. Disruptor using water-jet plays a very crucial role in neutralising Improvised Explosive Devices (IEDs) with the minimum risk. These devices are designed to generate a high velocity water-jet using an electrically initiated gas generator cartridge. This helps in separating the critical components of IED’s and avoiding them from operating as anticipated. The damage caused by a waterjet disruptor is not quantified in assessing the performance. However, at present no technique was available to evaluate the performance. The terminal ballistics tests at the target end are found appropriate for solid projectiles, such as penetration of bullet into target plates or impact of fragments. These terminal effects are the good indicators considering the destruction. The available alternative measurement techniques are cost effective that forms a basis for future common test protocols. The authors suggested that craters made by a nylon projectile using the disruptor provide a quantitative measure. This should be considered as a potential for upcoming standard test for evaluating the performance. The impact caused by a nylon projectile is dependent on projectile kinetic energy and the dissemination of stress wave in the surrounding zone. The energy dissipated by a nylon projectile gets absorbed by the targets through a certain mechanisms. Ballistic velocities for various targets are also worked out. Pinewood has less ballistic velocity limit as compared to other two targets. Study and methods for the disruptor performance on various composite targets (plywood, teak wood and pinewood with 15 mm thickness) at 0.5 and 1m stand-off distances are evaluated. This compares with the respective common target tests. From the experiments, it is concluded that a nylon projectile has potential to penetrate through composite targets.

Dynamics of oblique impact in a quasi two-dimensional granular medium

When a solid projectile impacts a granular target, it experiences a drag force and abruptly comes to rest as its momentum transfers to the grains. An empirical drag force law successfully describes the force experienced by the projectile, and the corresponding grain-scale mechanisms have been deciphered for normal impacts. However, there is little work exploring non-normal impacts. Accordingly, we extend studies to explore oblique impact, in which a significant horizontal component of the drag force is present. In our experiments, a projectile impacts a quasi-two-dimensional bed of bidisperse photoelastic grains. We use high-speed imaging to measure high-resolution position data of the projectile trajectory and simultaneously visualize particle-scale force propagation in the granular medium. When the impact angle becomes important, the spatial structure of the stress response reveals relatively weak force chain propagation in the horizontal direction. Based on these observations, we describe the decrease of the inertial drag force with impact angle.

Numerical Simulation on the Penetration Behavior of the Projectile during the Water Injection Stage of Water-Projectile-Assisted Injection Molding Process

Water-projectile-assisted injection molding (W-PAIM) is a novel molding process for plastic pipes with complicated shape. It utilizes high-pressure water as a power to push a solid projectile to penetrate through the melt to form a hollow space. In order to investigate the penetration behavior of the projectile during the water injection stage of W-PAIM process, numerical simulation of the water injection stage of a W-PAIM pipe with straight and curved segments was carried out. A turbulent flow for the driving water was considered in the motion equation, and the dynamic mesh technology was used to deal with the moving solid projectile. The simulation results, including RWT and the flow fields, were compared with those of water-assisted injection molding (WAIM) pipe with the same outer dimensions. It was found that the residual wall thickness (RWT) of the W-PAIM pipe is much thinner than that of the WAIM pipe. The projectile has a crucial influence on the RWT. The pressure fields of W-PAIM and WAIM are very similar in both straight and curved segments. The velocity field and strain rate field near the penetration front in W-PAIM are quite different from those in WAIM due to the drag flow caused by the projectile penetration.

Making a splash with fabrics in hydrophilic sphere entry

Splash dynamics associated with impacts of solid projectiles into aqueous pools are traditionally investigated with respect to impactor geometry, velocity, and surface roughness. Fluid surface alteration in some instances, may be more easily accomplished for the tuning of splashes. In this combined experimental and theoretical study, smooth, free-falling, hydrophilic steel spheres impact a quiescent liquid pool for Weber numbers in the range of 430−2700. Spheres strike fabrics resting atop the fluid surface which are either punctured or remain intact. As spheres strike fabrics, flow separation is tripped at low speeds which would otherwise not produce air-entraining cavities. Punctured fabrics suppress splash crowns normally seen for cavity-producing impacts while intact fabrics generate deeper cavities, higher Worthington jets, and pronounced splash crowns. Some fabrics, both punctured and intact reduce drag with respect to clean surface impacts by providing the drag-reducing benefits of flow separation while not offering a high inertial penalty. Such observations augur well for interfacial fluid–structure interactions where splashes warrant control.

A novel experimental setup for an oblique impact onto an inclined granular layer.

We develop an original apparatus of the granular impact experiment by which the incident angle of the solid projectile and the inclination angle of the target granular layer can be systematically varied. Whereas most of the natural cratering events occur on inclined surfaces with various incident angles, there have not been any experiments on oblique impacts on an inclined target surface. To perform systematic impact experiments, a novel experimental apparatus has to be developed. Therefore, we build an apparatus for impact experiments where both the incident angle and the inclination angle can be independently varied. The projectile-injection unit accelerates a plastic ball (6 mm in diameter) up to vi ≃ 100 m s-1 impact velocity. The barrel of the injection unit is made with a three-dimensional printer. The impact dynamics is captured by using high-speed cameras to directly measure the impact velocity and incident angle. The rebound dynamics of the projectile (restitution coefficient and rebound angle) is also measured. The final crater shapes are measured using a line-laser profiler mounted on the electric stages. By scanning the surface using this system, a three-dimensional crater shape (height map) can be constructed. From the measured result, we can define and measure the characteristic quantities of the crater. The analyzed result on the restitution dynamics is presented as an example of systematic experiments using the developed system.