Critical Impact Velocity Research Articles

Theoretical analysis of projectile-target interface defeat and transition to penetration by long rods due to oblique impacts of ceramic targets

Integrated with related investigations, the present manuscript conducts systematical theoretical analysis on the interface defeat (the phenomenon where an impacting eroding projectile flows radially outward along the surface of target without any significant penetration) of long rods under the oblique impacts onto ceramic targets, and related comparison with the normal impact is also carried out. By taking the influence of obliquity angle into account, the corresponding formulae for the velocity decay and the mass erosion of long rods with time are deduced out, and the kinetic energy loss is further discussed. Moreover, the transition from interface defeat to penetration is analyzed in detail. The variation characteristic of critical impact velocity range with the increase of obliquity angle is discussed, and the critical transition time is deduced considering the damage and failure characteristics of the ceramic target. Furthermore, the corresponding theoretical expressions are formulated. Finally, some suggestions for optimizing the design of ceramic target are given based on related analysis.

Water Penetration through a Superhydrophobic Mesh During a Drop Impact.

When a water drop impacts a mesh having submillimeter pores, a part of the drop penetrates through the mesh if the impact velocity is sufficiently large. Here we show that different surface wettability, i.e., hydrophobicity and superhydrophobicity, leads to different water penetration dynamics on a mesh during drop impact. We show, despite the water repellence of a superhydrophobic surface, that water can penetrate a superhydrophobic mesh more easily (i.e., at a lower impact velocity) over a hydrophobic mesh via a penetration mechanism unique to a superhydrophobic mesh. On a superhydrophobic mesh, the water penetration can occur during the drop recoil stage, which appears at a lower impact velocity than the critical impact velocity for water penetration right upon impact. We propose that this unique water penetration on a superhydrophobic mesh can be attributed to the combination of the hydrodynamic focusing and the momentum transfer from the water drop when it is about to bounce off the surface, at which point the water drop retrieves most of its kinetic energy due to the negligible friction on superhydrophobic surfaces.

Dynamic Crushing of Cellular Materials with Temperature Gradient

Considering the applications of cellular materials in the sacrificial claddings for impact/blast alleviation, this study presents a method for the design of cellular foam claddings with temperature gradient under high initial velocity impacts. A one-dimensional model for the compaction of cellular foam claddings with temperature gradient is developed for the striker-rod impact scenario based on the R-P-P-L idealization. The predictions of the proposed model are compared with FE simulations with the realistic R-PLH material model based on the actual experimentally derived stress-strain curves. The predictions of the dependency of critical length, critical impact velocity and impact force of the cellular foam rod with temperature gradient on the temperature distribution and the relation of critical impact velocity of an aluminum foam rod with a given length to the temperature contrast at its two ends are compared well with the numerical simulations results.

Scaling invariance of spherical projectile fragmentation upon high-velocity impact on a thin continuous shield

The problem of aluminum projectile fragmentation upon high-velocity impact on a thin aluminum shield is considered. A distinctive feature of this description is that the fragmentation has been numerically simulated using the complete system of equations of deformed solid mechanics by a method of smoothed particle hydrodynamics in three-dimensional setting. The transition from damage to fragmentation is analyzed and scaling relations are derived in terms of the impact velocity (V), ratio of shield thickness to projectile diameter (h/D), and ultimate strength (σ p ) in the criterion of projectile and shield fracture. Analysis shows that the critical impact velocity V c (separating the damage and fragmentation regions) is a power function of σ p and h/D. In the supercritical region (V > V c ), the weight-average fragment mass asymptotically tends to a power function of the impact velocity with exponent independent of h/D and σ p . Mean cumulative fragment mass distributions at the critical point are scale-invariant with respect to parameters h/D and σ p . Average masses of the largest fragments are also scale-invariant at V > V c , but only with respect to variable parameter σ p .

Trajectory instability and convergence of the curvilinear motion of a hard projectile in deep penetration

This paper studies the trajectory instability and convergence of the curvilinear motion of a hard projectile in deep penetration, based on rigid body dynamics and the differential area force law. It is observed that both sudden (instable) and gradual (curvilinear) changes of the projectile trajectory may happen under certain conditions depending on projectile geometry, target property, impact velocity and other striking parameters. A criterion is introduced to determine the occurrence of trajectory instability. Two characteristic velocities are identified, i.e. (a) the critical impact velocity for the occurrence of trajectory instability, and (b) the characteristic convergent velocity, at which the projectile trajectory turns to a straight line (linear motion) from the curvilinear motion, which is supported by experimental evidence. It is shown that both the critical impact velocity and the characteristic convergent velocity depend linearly on the relative location of projectile's centre of mass. However, the characteristic convergent velocity is independent of the non-axisymmetrical disturbance and impact velocity.

Impact of Viscous Droplets on Superamphiphobic Surfaces.

The impact of a liquid droplet on a solid surface is one of the most common phenomena in nature and frequently encountered in numerous technological processes. Despite the significant progress on understanding the droplet impact phenomenon over the past century, the impact dynamics, especially the coupling effects between the properties of a liquid and surface wettability on the impact process, is still poorly understood. In this work, we experimentally investigated the impact of viscous droplets on superamphiphobic surfaces, with the viscosity of liquids ranging from 0.89 to 150 mPa s. We showed that an increase in liquid viscosity will slow down the impact process and cause bouncing droplets to rebound lower and fewer times. The critical impact velocity, above which droplets can rebound from the superamphiphobic surface, was found to linearly increase with the liquid viscosity. We also showed that the maximum spreading factor increases with Weber number or Reynolds number but decreases with the liquid viscosity. Scaling analyses based on energy conservation were carried out to explain these findings, and they were found to be in good agreement with our experimental results.

Formation of few-layer graphene flake structures from graphite particles during thin film coating using dry spray deposition method

This work demonstrates the formation of few-layer graphene flake structures directly from graphite particles. A nano-particle deposition system, which is a dry spray deposition method generally used for deposition of metals and ceramics, has been introduced to deposit graphite particles at room temperature. In this study, graphite powder was deposited on a Cu substrate without using binders and the deposited thin film was characterized. The deposited thin film contained few-layer graphene flake structures, as observed using field effect scanning electron microscopy and confirmed using X-ray diffraction, Raman spectroscopy and field emission transmission electron microscopy. We also suggested the critical impact velocity for deposition of few-layer graphene flake structured thin film and a mechanism for forming few-layer graphene flake structures during thin film preparation. The suggested mechanism was interlayer separation of micron-sized graphite particles and fragmentation of particles into small pieces during deposition due to the impact of graphite particles on the substrate.

Numerical Study of Impact Behaviors of Angular Particles on Metallic Surface Using Smoothed Particle Hydrodynamics

ABSTRACTModeling and studying the impact behaviors of angular particles is critical in understanding the mechanisms of erosive wear on solid surfaces. This article focuses on effective mesh-free model based on the smoothed particle hydrodynamics (SPH) method to simulate impacts of angular particles on metallic surfaces. The predicted results are compared with the available experimental data, and good agreement has been achieved. Our simulations under different incident conditions successfully reproduce the general impact behaviors of angular particles, including rotating behavior and rebound behavior, which enables detailed examinations of erosion mechanisms. We find that the rotating behaviors are mainly determined by initial orientation and impact angle, whereas impact velocity has little effect. For backward impact involving a prying-off action, there generally exsits a critical impact velocity below which the cutting process would never be finished, which may result in a rebound angle greater than 90°. Further, multiple and overlapping impacts are simulated to reveal the effect of a pre-created crater on the subsequent impact. The results demonstrate the ability of the present model to handle the extremely deformed surface by overlapping impacts. The proposed SPH model and the present study could be useful in the study of erosive wear on the surface of metal devices that carry granular substances.

Thermal–mechanical–chemical responses of polymer-bonded explosives using a mesoscopic reactive model under impact loading

A mesoscopic framework is developed to quantify the thermal–mechanical–chemical responses of polymer-bonded explosive (PBX) samples under impact loading. A mesoscopic reactive model is developed for the cyclotetramethylenetetranitramine (HMX) crystal, which incorporates nonlinear elasticity, crystal plasticity, and temperature-dependent chemical reaction. The proposed model was implemented in the finite element code ABAQUS by the user subroutine VUMAT. A series of three-dimensional mesoscale models were constructed and calculated under low-strength impact loading scenarios from 100m/s to 600m/s where only the first wave transit is studied. Crystal anisotropy and microstructural heterogeneity are responsible for the nonuniform stress field and fluctuations of the stress wave front. At a critical impact velocity (≥300m/s), a chemical reaction is triggered because the temperature contributed by the volumetric and plastic works is sufficiently high. Physical quantities, including stress, temperature, and extent of reaction, are homogenized from those across the microstructure at the mesoscale to compare with macroscale measurements, which will advance the continuum-level models. The framework presented in this study has important implications in understanding hot spot ignition processes and improving predictive capabilities in energetic materials.

Influence of impact conditions on plasma generation during hypervelocity impact by aluminum projectile

For describing hypervelocity impact (relative low-speed as related to space debris and much lower than travelling speed of meteoroids) phenomenon associated with plasma generation, a self-developed 3D code was advanced to numerically simulate projectiles impacting on a rigid wall. The numerical results were combined with a new ionization model which was developed in an early study to calculate the ionized materials during the impact. The calculated results of ionization were compared with the empirical formulas concluded by experiments in references and a good agreement was obtained. Then based on the reliable 3D numerical code, a series of impacts with different projectile configurations were simulated to investigate the influence of impact conditions on hypervelocity impact generated plasma. It was found that the form of empirical formula needed to be modified. A new empirical formula with a critical impact velocity was advanced to describe the velocity dependence of plasma generation and the parameters of the modified formula were ensured by the comparison between the numerical predictions and the empirical formulas. For different projectile configurations, the changes of plasma charges with time are different but the integrals of charges on time almost stayed in the same level.

Effect of wetting difference across junction on dynamics of drops impacting on the junction of dual‐textured surfaces

The effect of wetting difference across the junction of smooth and groove‐textured portions of a dual‐textured surface on the various stages in the dynamics of liquid drops impacting on the junction is reported. Two dual‐textured surfaces with substantially different wetting difference across their junction, Δθe = 65° and 25°, made of intrinsically hydrophilic stainless steel and intrinsically hydrophobic poly‐di‐methyl‐siloxane (PDMS) are considered. The effect of wetting difference across the junction is dominantly seen on the net drop drift velocity and subsequent bulk drop movement, ξ. ξ on the PDMS dual‐textured surface is less than that on the stainless dual‐textured surface due to the lower wetting difference across the junction of the former. The variation of ξ with impact velocity shows a contrasting trend between the stainless steel and PDMS dual‐textured surfaces due to the difference in critical impact velocity corresponding to drop impregnation on their groove‐textured portions. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4109–4118, 2016

Penetration depth of torpedo anchor in cohesive soil by free fall

Developing deep sea technologies, many marine novel facilities have been introduced; and mooring systems, however, have become more expensive, complex, and hard cooperative in deep-water marine industry. The torpedo anchor is regarded as a modern technology benefits from easy installation, cost efficiency, and high level of anchor force. In this study, the penetration depth of a free falling torpedo anchor into cohesive soil has been laboratory investigated. 128 Sets of tests have been conducted with nine different torpedo anchors not only in shape, but also in size. Three anchor aspect ratios and three different types of muds were tested while mud rheological properties such as the yield stress and flow curves were also measured. Finally, a formula calculating the penetration depth of the anchor, regardless of soil separation has been proposed based on energy conservation principle and experimental measurements. Results indicate that the predicted penetration depth has a good conformity with the measured penetration depths in laboratory and field tests. Static undrained shear strength values should be reduced in formula when the impact velocity exceeds a critical value. Soil separation occurs depending on the anchor nose angle and surface roughness, and also soil properties. Nevertheless, the critical impact velocity resulting into soil separation and its degree require further study in the future.

Ballistic testing and theoretical analysis for perforation mechanism of the fan casing and fragmentation of the released blade

Ballistic tests with plate and stiffened targets impacted by cylindrical and blade-like projectiles were conducted using compressed gas gun to investigate the perforation-resistance performance of aero-engine fan casing during a blade-out event. It was observed that for the plate target impacted by the blade-like projectile, fragmentation on the projectile nose occurred and the targets were failed by petaling; but for the other combinations, the main failure mode of the targets was plugging and the projectiles were intact except in cases when the blade-like projectile hit the stiffeners. Due to the distinct failure mode, ballistic limit for the plate target impacted by the blade-like projectile was much higher. To analyze the perforation resistance of the target, the energy absorption modes were classified and described with analytical models. A new dishing energy calculating method was proposed for a plate normally impacted by an arbitrary cross-section projectile. The energy absorbed by each mode and the ballistic limits for the plate target were calculated using these analytical models. It was found that fragmentation of the projectile led to a larger amount of dishing energy. The formation conditions of the fragmentation on projectile nose were theoretically analyzed, and the influencing factors for the fragmentation such as the critical thickness of the target and the critical impact velocity of the projectile were derived with established formulas. The theoretical analysis agreed well with the test results. It is argued that if the target is thicker than the critical thickness and the initial velocity of the projectile is larger than the critical impact velocity, fragmentation on projectile nose will occur, failure mode of the target will change to petaling, and the perforation resistance of the target will be significantly enhanced.

Threshold for plasma phase transition of aluminum single crystal induced by hypervelocity impact

Molecular dynamics method is used to study the threshold for plasma phase transition of aluminum single crystal induced by hypervelocity impact. Two effective simulation methods, piston-driven method and multi-scale shock technique, are used to simulate the shock wave. The simulation results from the two methods agree well with the experimental data, indicating that the shock wave velocity is linearly dependent on the particle velocity. The atom is considered to be ionized if the increase of its internal energy is larger than the first ionization energy. The critical impact velocity for plasma phase transition is about 13.0 km/s, corresponding to the threshold of pressure and temperature which is about 220 GPa and 11.0 × 103 K on the shock Hugoniot, respectively.

Impact resistance of uniform and functionally graded auxetic double arrowhead honeycombs

Double arrowhead honeycombs (DAHs) are auxetic cellular materials with negative Poisson's ratio (NPR). The quasi-static and impact behaviors of both uniform and functionally graded DAHs are explored. Analytical expressions for the in-plane Young's modulus, Poisson's ratio and quasi-static yield stress of uniform DAHs are derived and are found to be consistent with finite element (FE) predictions, showing that the quasi-static yield stress of DAHs is dependent upon the geometrical parameters and NPR. Systematical simulations of uniform DAHs subject to different impact velocities reveal that there exists a critical impact velocity beyond which the dynamic plateau stress is insensitive to the effect of NPR. A semi-empirical expression of the plateau stress is obtained. For functionally graded DAHs, they are shown to only have improved energy absorption capacity under high velocity impacts. Moreover, the results of uniform DAHs are shown to be able to be adopted to interpret the simulated plateau stress of functionally graded DAHs under impact loading. The deformation mechanisms associated with the impact resistance of DAHs are also discussed.

On the transition from interface defeat to penetration in the impact of long rod onto ceramic targets

Based on the related research on the penetration of long rod into ceramic targets, the present manuscript summarizes qualitatively three deformation modes of the rod and target under different impact velocities: interface defeat, normal penetration and transition from interface defeat to penetration. Then the phenomenon of transition is investigated in detail. Firstly, integrated with the analysis on the mechanical property of ceramic material, the critical impact velocity range within which the transition phenomenon occurs, i.e., the highest impact velocity corresponding to interface defeat and the lowest impact velocity corresponding to normal penetration, is further identified. Subsequently, the critical transition time from interface defeat to penetration is discussed, in which the detailed damage and failure characteristics of ceramic material during the interface defeat process are taken into account. Finally an explicit half-analytical expression of the critical transition time is formulated.

Research results of working body of fertilizing machine

The fertilizer centrifugal spreaders not provide uniformity of application of the process material. The high cost of fertilizers and crop losses provoke to address the problem of uneven application of fertilizers. This problem is important not only for Ukraine but also for the whole world. It is the fact that the technologically to do exactly same granules is expensive. Therefore, manufacturers admit a range of 1-5mm diameter. More than 5 mm granules are fragile and the dose of mineral substances will be excessively large at its falling. Granules of less than 1 mm are greatly different in aerodynamic properties. Global manufacturers are constantly stay in search of promising solutions by centrifugal spreaders, because of the disc spreaders are most beneficial in terms of performance and quality indicators. The program and methodology are developed and results of experimental research of the centrifugal working body for application of mineral fertilizers are obtained. The rotary working body is proposed. The design features of this organ can improve the uniformity of dispersion of mineral fertilizers. The original methods of determining the critical impact velocity of the metal surface with the moisture granules and influence the action of wind on the final distribution are developed. Techniques of airflow rate influence research to determine the effect, both in laminar and turbulent conditions that are not provided by known techniques. The work will identify the optimal design parameters of centrifugal disc of fertilizer spreader.

Phase transition modeling of polytetrafluoroethylene during Taylor impact

The complex pressure and temperature dependent phase behavior of the semicrystalline polymer polytetrafluoroethylene (PTFE) has been investigated experimentally. One manifestation of this behavior has been observed as an anomalous abrupt ductile-to-brittle transition in the failure mode of PTFE rods in Taylor cylinder impact tests when impact velocity exceeds a narrow critical threshold. Earlier, hydrocode calculations and Hugoniot estimates have indicated that this critical velocity corresponds to the pressure in PTFE associated with the transition from a crystalline phase of helical structure to the high pressure crystalline phase (phase III) of a planar form. The present work represents PTFE as a material in a simplified phase structure with the transition between the modeled phases regulated by a kinetic description. The constitutive modeling describes the evolution of mechanical characteristics corresponding to the change of mechanical properties due to either an increase of crystallinity or the phase transition of a crystalline low-pressure component into phase III. The modeling results demonstrate that a change in the kinetics of the transition mechanism in PTFE when traversing the critical impact velocity can be used to explain the failure of the polymer in the Taylor cylinder impact tests.

A Numerical Investigation Into Cold Spray Bonding Processes

Specific mechanisms underlying the critical velocity in cold gas particle spray applications are still being discussed, mainly due to limited access to in situ experimental observation and the complexity of modeling the particle impact process. In this work, particle bonding in the cold spray (CS) process was investigated by the finite element (FE) method. An effective interfacial cohesive strength parameter was defined in the particle–substrate contact regions. Impact of four different metals was simulated, using a range of impact velocities and varying the effective cohesive strength values. Deformation patterns of the particle and the substrate were characterized. It was shown that the use of interfacial cohesive strength leads to a critical particle impact velocity that demarcates a boundary between rebounding and bonding type responses of the system. Such critical bonding velocities were predicted for different interfacial cohesive strength values, suggesting that the bonding strength in particle–substrate interfaces could span a range that depends on the surface conditions of the particle and the substrate. It was also predicted that the quality of the particle bonding could be increased if the impact velocity exceeds the critical velocity. A method to predict a lower bound for the interfacial bonding energy was also presented. It was shown that the interfacial bonding energy for the different materials considered would have to be at least on the order of 10–60 J/m2 for cohesion to take place. The general methodology presented in this work can be extended to investigate various materials and impact conditions.

A Simplified Approach for the Determination of Critical Velocity for Cold Spray Processes

A simple technique employing the first law of thermodynamics was used to predict the critical impact velocity for cold spray processes based on material properties of the particles and substrates. It has been shown that during its interaction with the substrate, a particle should reach around 70% of its melting temperature to obtain good mechanical bonding. To characterize the results in a general way, a non-dimensionalization of the relevant parameters was conducted and validated to determine the combination of cold spray process variables required for the particle to reach the critical impact velocity.