Hardened Steel Projectiles Research Articles

Perforation of Aluminum Plates with Ogive-Nose Projectiles Using Axisymmetric Smoothed Particle Hydrodynamics

The 2D axisymmetric smoothed particle hydrodynamics (ASPH) has been adopted to study the perforation of AA5083-H116 aluminum plates with ogive-nose hard steel projectiles. The deceleration history curves of the projectile by the ASPH were presented for three impact conditions. Impact vs. residual velocity curves were constructed and the ballistic limit velocity was determined. The computational residual velocities and the ballistic limit velocities from the ASPH agree well with available experimental data. The study shows that the ASPH is able to emulate the perforation of aluminum plates as observed in the experimental investigations of high velocity impact.

Impact of thick PMMA plates by long projectiles at low velocities. Part II: Effect of confinement

A hybrid experimental–numerical investigation of the penetration process in unconfined and confined thick polymethylmethacrylate (PMMA) plates was carried out. The confinement was applied by insertion of the polymeric plate into a conical steel ring. The response of such plates to the impact of long hard steel projectiles having an ogive-head shape in the range of velocities of 165<V0<260 (m/s), was investigated experimentally. The results show that unconfined targets were perforated and broken due to combined effect of penetration and cracking. By contrast, the confined targets were not perforated and could withstand repeated impacts due to suppression of the brittle damage mechanism by the confinement. The tests were modeled using 3D explicit finite element analyses. A good agreement regarding the trajectory of the projectile and the depths of penetration was obtained. The numerical results show that the confinement introduces a negative triaxiality and even some plasticity within the confined plates prior to impact. The increase of plastic failure strain of the PMMA at negative triaxiality reduces the ductile damage during penetration, while the hydrostatic pressure reduces significantly the brittle fracture mechanism. The resisting force to the penetration depends on the failure strain–triaxality relationship, and does not necessarily increase with higher confinement levels.

Impact of thick PMMA plates by long projectiles at low velocities. Part I: Effect of head’s shape

A hybrid experimental–numerical investigation of the penetration process in thick polymethylmethacrylate (PMMA) plates was carried out. The response of such plates to the impact of long hard steel projectiles having either blunt, hemispherical or ogive-head shapes was investigated experimentally in the range of velocities of 100 (m/s)<V0<250 (m/s). The penetration process can be divided into 3 stages: entrance, propagation and backwards bouncing. The last two stages are associated with brittle fracture of the plates. The tests were modeled using 3D explicit finite element analyses. The numerical results provide insight regarding the variations of field variables such as stresses, velocities, resisting forces and energies. A good agreement regarding the trajectory of the projectile and the depths of penetration is obtained. The enhanced backwards bouncing phenomenon is explained, and it is shown that the average deceleration during the penetration process is constant. The resisting force to the penetration is higher for blunt projectiles. It is 10% lower for the hemispherical head and 50% lower for ogive-headed projectiles.

Impact response of aluminum corrugated core sandwich panels

The mechanisms of projectile penetration of extruded 6061T6 aluminum alloy sandwich panels with empty and alumina filled, triangular corrugated cores have been experimentally investigated using zero obliquity, 12.7 mm diameter hard steel projectiles whose diameter was about a half that of the core's unit cell width. We find that low momentum impacts are laterally deflected by interactions with the inclined webs of the empty core. Complete penetration occurred by shear-off within the impacted front face sheet, followed by stretching, bending and tensile fracture of the core webs and finally shear-off within the back face sheet. This combination of mechanisms was less effective at dissipating the projectiles kinetic energy than the shear-off (plugging) mechanism of penetration of the equivalent solid aluminum panel. Inserting ballistic grade alumina prisms in the triangular cross section spaces of the corrugated core significantly increased the panel's ballistic resistance compared to the empty panel. The presence of the hard ceramic led to severe plastic deformation and fragmentation of the projectile and comminution and macroscopic fracture of the ceramic. The Al/Al2O3 hybrid panel ballistic limit was reached when pairs of parallel cracks formed in the rear face sheet at core web-face sheet nodes. The separation distance between these cracks was dependent upon the location of the impact with respect to that of the web-face sheet nodes. Nodal impacts resulted in pairs of fractures that were separated by one cell width and a critical velocity below that of the equivalent solid plate. Impacts mid-way between pairs of nodes resulted in back face sheet crack pairs separated by twice the cell width, and a critical velocity higher than the equivalent solid plate. Using X-ray tomography we show this resulted from the formation of oval (not circular) cross section fracture conoids in the ceramics. The conoid angle was about 60° in the extrusion direction but only 30° in the transverse direction. This observation may have interesting consequences for a panel's resistance to a second, close proximity impact.

An experimental study on the deformation and fracture modes of steel projectiles during impact

Previous investigations of the penetration and perforation of high-strength steel plates struck by hardened steel projectiles have shown that under certain test conditions the projectile may fracture or even fragment upon impact. Simulations without an accurate failure description for the projectile material will then predict perforation of the target instead of fragmentation of the projectile, and thus underestimate the ballistic limit velocity of the target plate. This paper presents an experimental investigation of the various deformation and fracture modes that may occur in steel projectiles during impact. This is studied by conducting Taylor bar impact tests using 20mm diameter, 80mm long, tool steel projectiles with three different hardness values (HRC 19, 40 and 52). A gas gun was used to fire the projectiles into a rigid wall at impact velocities ranging from 100 to 350m/s, and the deformation and fracture processes were captured by a high-speed video camera. From the tests, several different deformation and fracture modes were registered for each hardness value. To investigate the influence of material on the deformation and fracture modes, several series of tensile tests on smooth axisymmetric specimens were carried out to characterise the mechanical properties of the three materials. To gain a deeper understanding of the various processes causing fracture and fragmentation during impact, a metallurgical investigation was conducted. The fracture surfaces of the failed projectiles of different hardness were investigated, and the microstructure was studied for each hardness value.

Experimental and numerical study on fragmentation of steel projectiles

A previous experimental study on penetration and perforation of circular Weldox 460E target plates with varying thicknesses struck by blunt-nose projectiles revealed that fragmentation of the projectile occurred if the target thickness or impact velocity exceeded a certain value. Thus, numerical simulations that do not account for fragmentation during impact can underestimate the perforation resistance of protective structures. Previous numerical studies have focused primarily on the target plate behaviour. This study considers the behaviour of the projectile and its possible fragmentation during impact. Hardened steel projectiles were launched at varying velocities in a series of Taylor tests. The impact events were captured using a high-speed camera. Fractography of the fragmented projectiles showed that there are several fracture mechanisms present during the fragmentation process. Tensile tests of the projectile material revealed that the hardened material has considerable variations in yield stress and fracture stress and strain. In the finite element model, the stress-strain behaviour from tensile tests was used to model the projectile material with solid elements and the modified Johnson-Cook constitutive relation. Numerical simulations incorporating the variations in material properties are capable of reproducing the experimental fracture patterns, albeit the predicted fragmentation velocities are too low.

Quasi-brittle fracture during structural impact of AA7075-T651 aluminium plates

The stress–strain behaviour of the aluminium alloy 7075 in T651 temper is characterized by tension and compression tests. The material was delivered as rolled plates of thickness 20 mm. Quasi-static tension tests are carried out in three in-plane directions to characterize the plastic anisotropy of the material, while the quasi-static compression tests are done in the through-thickness direction. Dynamic tensile tests are performed in a split Hopkinson tension bar to evaluate the strain-rate sensitivity of the material. Notched tensile tests are conducted to study the influence of stress triaxiality on the ductility of the material. Based on the material tests, a thermoelastic–thermoviscoplastic constitutive model and a ductile fracture criterion are determined for AA7075-T651. Plate impact tests using 20 mm diameter, 197 g mass hardened steel projectiles with blunt and ogival nose shapes are carried out in a compressed gas-gun to reveal the alloy's resistance to ballistic impact, and both the ballistic limit velocities and the initial versus residual velocity curves are obtained. It is found that the alloy is rather brittle during impact, and severe fragmentation and delamination of the target in the impact zone are detected. All impact tests are analysed using the explicit solver of the non-linear finite element code LS-DYNA. Simulations are run with both axisymmetric and solid elements. The failure modes are seen to be reasonably well captured in the simulations, while some deviations occur between the numerical and experimental ballistic limit velocities. The latter is ascribed to the observed fragmentation and delamination of the target which are difficult to model accurately in the finite element simulations.

Foreign object damage behavior in a silicon nitride ceramic by spherical projectiles of steels and brass

Assessments of foreign object damage (FOD) of a commercial, gas-turbine grade, in situ toughened silicon nitride ceramic (AS800) were made using different projectile materials at ambient temperature. AS800 flexure target specimens rigidly supported were impacted at their centers in a velocity range from 100 to 450 m/s by spherical projectiles with a diameter of 1.59 mm. Three different projectile materials were used including hardened steel, annealed steel, and brass. Post-impact strength of each target specimen impacted was determined as a function of impact velocity to appraise the severity of local impact damage. For a given impact velocity, the extent of FOD was greatest for hardened steel projectiles, least for brass projectiles, and intermediate for annealed steel projectiles. The key material parameter affecting FOD the most was identified as the hardness (or yield stress) of projectile materials. Prediction of impact force as a function of impact velocity for each projectile material was made based on a quasi-static plastic model incorporated with the average ‘contact yield pressure’ determined from static indentation testing.

Perforation of AA5083-H116 aluminium plates with conical-nose steel projectiles – Calculations

The use of aluminium alloys in lightweight protective structures is increasing. Even so, the number of experimental and computational investigations that give detailed information on such problems is limited. In an earlier paper by some of the authors, perforation experiments were performed with 15–30 mm thick AA5083-H116 aluminium plates and 20 mm diameter, 98 mm long, HRC 53 conical-nose hardened steel projectiles. In all tests, initial and residual velocities of the projectile were measured and the ballistic limit velocity of each target plate was determined. In the present paper, an analytical perforation model based on the cylindrical cavity-expansion theory has been reformulated and used to calculate the ballistic perforation resistance of the aluminium plates. In addition, non-linear finite element simulations have been carried out. The target material was modeled with the Johnson–Cook constitutive relation using 2D axisymmetric elements with adaptive rezoning. To allow ductile hole growth, a pin-hole was introduced in the target. The analytical and numerical results have been compared to the experimental findings, and good agreement was in general obtained. A parametric study was also carried out to identify the importance of the different terms of the Johnson–Cook constitutive relation on the perforation resistance of the target. The results indicate that thermal softening cannot be neglected, so an alternative procedure for identification of the material constants in the power-law constitutive relation used in the cavity-expansion theory has been proposed.

Localised shear banding in Weldox steel plates impacted by projectiles

Results from metallurgical investigations of Weldox 460 E, 700 E and 900 E steel plates impacted by hardened steel projectiles are presented. The strength is considerable higher for Weldox 700 E and 900 E than for Weldox 460 E, while the opposite is true regarding ductility. Penetration tests on the different alloys using blunt projectiles have shown that the ballistic limit velocity decreases with increasing yield strength, while in similar tests with conical and ogival projectiles the ballistic limit velocity increases with increasing yield strength. In a preliminary metallurgical study, only deformed shear bands were observed in 12 mm thick Weldox 460 E plates, while intense white etched shear bands were obtained in both Weldox 700 E and 900 E targets. The latter suggests that a phase transformation has occurred in these materials during penetration. However, based on detailed SEM and TEM studies of the localised shear zones, it was found difficult to prove such behaviour. Therefore, the authors believe that even though white shear bands were observed, the material inside the localised zone may not have reached the temperature necessary for a phase transformation from tempered to untempered martensite. In contrast, based on the TEM study, acicular ferrite and martensite twins were observed in the shear band in a 20 mm thick Weldox 460 E steel plate impacted by a blunt projectile. This strongly suggests that a phase transformation has occurred in the thicker Weldox 460 E steel plate.

Perforation of AA5083-H116 aluminium plates with conical-nose steel projectiles—experimental study

The interest regarding use of aluminium alloys in lightweight protective structures is today increasing. Even so, the number of experimental and computational investigations giving detailed information on such problems is still rather limited. In this paper, perforation experiments have been performed on AA5083-H116 aluminium plates with thicknesses varying between 15 and 30 mm impacted by 20 mm diameter, 98 mm long, HRC 53 conical-nose hardened steel projectiles. In all tests, initial and residual velocities of the projectile were measured and a digital high-speed camera system was used to photograph the penetration and perforation process. Based on these measurements, impact versus residual velocity curves of the target plates were constructed and the ballistic limit velocity of each target was obtained. An analytical perforation model from the open literature is then used to predict the ballistic limit velocity, and excellent agreement with the experimental data is found. The experimental results are finally compared to similar experiments on steel and concrete targets, and the capacity of the different materials is evaluated in relation to total weight.

Normal and Oblique Impacts of Hard Projectile on Single and Layered Plates-An Experimental Study

The phenomenon of ordnance velocity impact of projectile on single and layered plates is of interest for many applications. In this paper. an experimental study of normal and oblique impacts or an ogive shaped, hard steel projectile on single and layered plates of mild steel and aluminium is presented. The projectiles were fired at an impact velocity of about 820 ms-1. The plate thickness was varied in the range 10 mm to 40 mm and the ratio of plate thickness to the diameter of the projectile varied in the range 1.5 to 13.0. Observations on target damage and measurements of incident and residual velocities for different angles of impact are presented. Plate thickness t*, for which the incident velocity is the ballistic limit, is determined. Computer simulations were carried out using a hydrodynamic code to simulate the normal impact of a projectile and compared these with the experimental results. Experiments were performed to evaluate the response of these plates of intermediate thickness when layered. and the results were compared to the results of single plate of same total thickness.

High strength concrete response to hard projectile impact

High strength concrete (HSC) becomes more common in practice and may have advantageous implementations. According to existing penetration formulae HSC is expected to enhance the performance of structural elements that are designed to resist projectile impacts. However, scabbing at the rear face is expected to be more severe in elements that are made of HSC, because of the relatively high material brittleness. Therefore, it is important to enhance the ductility of HSC elements, and one possible direction is to use fibers or wire mesh reinforcement. In order to study the influence of the concrete strength and of the reinforcement type on the elements response, penetration tests were conducted on regular strength concrete (RSC) and on HSC plates, with the following types of reinforcement: 5 mm steel mesh, steel fibers, small diameter steel wire mesh, and woven steel fence mesh of various diameters. The plates were subjected to an impact of a cylindrical hard steel projectile, weighing 120 g, with a conical nose and a 1.5 aspect ration. The projectiles were accelerated by a laboratory gas gun to velocities that ranged between 85 and 230 m/sec, which were measured by an electro-optical device. By comparing the response of these plates to an impacting projectile, the effects of concrete strength and of the reinforcement were studied. Major trends of the elements behavior were studied, their responses were compared and are described herein.

Ballistic evaluation of ceramics: Influence of test conditions

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

Effect of target bending in normal impact of a flat-ended cylindrical projectile near the ballistic limit

The current work is concerned with the normal impact of flat-nosed cylindrical projectiles striking at a velocity in the vicinity of the ballistic limit on metallic targets. The ratio of target thickness to striker diameter ranges from ¼ to ½. The resulting penetration model extends existing theories to include the extensive deformation of the target resulting from the bending effect. The dynamic and static stress-strain relations of 2024-0 aluminum are obtained by means of the split-Hopkinson bar technique and an Instron machine, respectively. A series of tests with 0.8 mm diameter pre-drilled holes in each target is conducted to assess the relative effects of bending and shear on the plate deformation. A total of 20 shots are executed to examine the phenomenon of the plate response under impact loading. The 12.7 mm diameter hard-steel projectiles are fired with a pneumatic gun against 3.18, 4.76, and 6.35 mm aluminum targets at a velocity of up to 182 m s−1. A dynamic plastic bending theory is proposed that will be superposed on the previously developed one-dimensional phenomenological penetration model of Liss et at. (Int. J. Impact Engng 1, 321–324 (1983)), to permit a more accurate prediction of target response. This phenomenological model is numerically analyzed and compared with the experimental findings and the two-dimensional Lagrangian computational codes Dyna2d and Autodyn using a Cray X/MP-48 and PC-AT, respectively. Excellent correspondence with data is obtained for the projectile exit velocity when a higher impact speed is employed. The final deformation field computed from the model is not limited to a narrow zone and shows good correlation with the experimental data, especially for thinner targets.

Controlled tumbling of projectiles—I. Theoretical model

The present investigation is concerned with the generation of controlled tumbling of a blunt-nosed hard-steel projectile and, to some extent, the effect of this tumbling on targets struck subsequently. The mechanism for producing this motion from an initial state of translation in the direction of the projectile axis consisted of the impingement of the bullet on the edge of deflector plates of aluminum, steel or plastic of various thicknesses with various degrees of overlap. These initially plane rectangular plates were rigidly held on three sides and struck at the center of the fourth, or free, edge. This first portion of the study consists of the continuum-mechanical analysis of the impact of the projectile on the plate and the subsequent motion of the projectile. A mode shape analysis is employed to ascertain the deplection of the plate with the impact area modeled as a hexagon. The projection is considered as rigid and its motion as planar prior and subsequent to the impact with the deflector plate, which is modeled as a rigid-perfectly plastic material. The numerical predictions of the theoretical development, the experimental set-up and the data derived from the tests, as well as a comparison between the analytical and measured values, are presented in a companion paper [Ruiz and Goldsmith, Int. J. Impact Engng Vol. 7, in press (1988)].

Impact on Ceramic Targets

A series of approximately 150 tests were conducted on ceramic targets struck by cylindro-conical hard-steel projectiles at normal incidence up to velocities of about 1000 m/s. The primary objective was the determination of the effect of layering and the delineation of the ballistic limit of various combinations. In addition, a study of the erosive effect of the ceramic was executed. It was found that, on the basis of areal density, metal plates prefaced by ceramic materials are ballistically more inefficient than purely metallic targets in the low velocity range, while the reverse was found at speeds above 250 m/s. The eroded length was found to be related to the velocity of the projectile and the thickness of the ceramic layer. The projectile displacement data were found to be in very good accord with the results obtained from a previously utilized analytic force history taking into account the erosion process. The energy required to erode the projectile was found to be several orders of magnitude greater than that consumed in the fracture process of the frontal ceramic plate.

Petalling of thin, metallic plates during penetration by cylindro-conical projectiles

A theoretical and experimental investigation has been undertaken to study the processes of perforation at normal incidence of thin, soft aluminum plates by hard-steel cylindroconical projectiles of 30° half-cone angle. The analysis is executed by means of an energy balance for a rigid/perfectly-plastic or work-hardening material. For an intact plate, where petalling always occurs, consecutive stages of the process involve initial crack propagation followed by plastic hinge motion out to the position of crack arrest, followed by petal bending due to hinge rotation up to and beyond the point of projectile passage. In the case of central impact on an initial hole in the plate, the first stage constitutes an enlargement of the hole, followed by crack propagation and bending of trapezoidal regions of the plate until the projectile has either perforated or is embedded in the target.Tests conducted on 2024-0 aluminum with a thickness of 3.175 mm indicated a condition so that if the initial hole radius was greater than about 2/3 that of the 12.7 mm diameter hard-steel projectile, cracking did not occur and all the energy was absorbed by hole enlargement. Furthermore, for each initial impact velocity, an optimal hole radius was found to exist where a maximum energy absorption of the plate occurred, greater than for the intact target. This phenomenon was qualitatively substantiated by the theory; discrepancies in magnitudes are attributed to the neglect of certain energies in the analysis, particularly that of dishing. Excellent correlation was found between the theoretical prediction for the terminal velocity of a projectile striking the intact plate and test results when an estimate of the dishing energy for the plate was included.

Normal impact and perforation of thin plates by hemispherically-tipped projectiles—I. Analytical considerations

The present development constitutes a very elementary analysis of the penetration and perforation processes involved in normal impact of thin plates by hemispherically-tipped cylindrical hard-steel projectiles. It is principally directwed towards the determination of the force and central plate deflection histories both below and above the ballistic limit. These parameters are considered to be two of the most important facets of the event, and they can be evaluated rapidly and with negligible computational cost. The theory is based on a lumped-parameter system that includes the radial variation in axial target displacement and is completeley predictive below the ballistic limit, but requires either measured information from the plug separation process or corresponding reasonable geometric and kinematic assumptions for a solution in the perforation domain. Surprisingly, considering the simplicity of the theoretical approach, the calculated values were found to be in excellent agreement below and in reasonable correspondence above the ballistic limit with data for soft aluminium targets. Somewhat greater deviations between predictions and measurements for mild steel plates are attributed to the neglect of rate effects in the constitutive description of this material. In spite of the existence of a variety of more complicated phenomenological representations, the current model appears to be the only one that forecasts the drop in peak force upon initiation of perforation and the subsequent simultaneous motion of plug and target plate; both of these features were verified by experiment. These attributes and the case of evaluating the model constitute the predominant justification for its employment. A much more comprehensive continuum-mechanical model, which still awaits development, or else the widely known, but far more expensive, numerical procedures that are crucially dependent on a precise knowledge of all material behavior features, must be utilized for the evaluation of other system variables, such as target strain. A complete description of the test procedures and results are presented in a companion paper.

Normal impact and perforation of thin plates by hemispherically-tipped projectiles — II. Experimental results

The present investigation is concerned with the measurement of the forces, permanent deflections, strains and plugs produced by rigid projectiles during contact and perforation of thin aluminium and mild steel plates. Tests were generally executed just below and just above the ballistic limit; a few experiments were conducted at much higher velocity. The direct force measurements were performed by means of a special technique using a projectile containing a piezoelectric quartz crystal. Observations were carried out on more than two hundred 2024-0 aluminium plates, 0.51, 1.27 and 3.175 mm thick and 1.22 mm thick mild steel plates. The tests involved the use of hard-steel projectiles of 12.7 and 6.35 mm diameter with three different nose shapes. The residual strains, of the order of 40%, were measured by using a grid method while the strain history in the vicinity of the impact point was determined by a specially designed gage capable of measuring strains up to 100%.