Experimental and numerical study on fragmentation of steel projectiles (original) (raw)
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An experimental study on the deformation and fracture modes of steel projectiles during impact
2013
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 20 mm diameter, 80 mm 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 100-350 m/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.
On the influence of constitutive relation in projectile impact of steel plates
International Journal of Impact Engineering, 2007
In this paper the influence of constitutive relation has been studied in numerical simulations of the perforation of 12-mm thick Weldox 460 E steel plates impacted by blunt-nosed projectiles in the sub-ordinance velocity regime. A modified version of the well-known and much used constitutive relation proposed by Johnson-Cook and both the bcc-and hcpversion of the Zerilli-Armstrong constitutive relation were combined with the Johnson-Cook fracture criterion. These models were implemented as user-defined material models in the non-linear finite element code LS-DYNA. Identification procedures have been proposed, and the different models were calibrated and validated for the target material using available experimental data obtained from tensile tests where the effects of strain rate, temperature and stress triaxiality were taken into account. Perforation tests carried out in a compressed gas gun on 12-mm-thick circular Weldox 460 E steel plates were then used as base in a validation study of plate perforation using LS-DYNA and the proposed constitutive relations. The numerical study indicated that the physical mechanisms during perforation can be qualitatively well predicted by all constitutive relations, but quantitatively more severe differences appear. The reasons for this are discussed in some detail. It was concluded that for practical applications, the Johnson-Cook constitutive relation and fracture criterion seems to be a good choice for this particular problem and excellent agreement with the experimental results of projectile impact on steel plates were obtained under the conditions investigated.
2006
Due to the lack of the classical local continuum formulation to produce physically meaningful and numerically converging results within large deformation and strain localization computations, a thermodynamically motivated micro-damage formulation is proposed for highrate and temperature dependent materials. This model uses a combined viscosity and nonlocal gradient localization limiters in order to regularize the dynamic strain localization problems. The enhanced nonlocal gradient-dependent theory formulates a constitutive framework on the continuum level that is used to bridge the gap between the micromechanical theories and the classical (local) continuum theories. They are successful in explaining the size effects encountered at the micron scale and in preserving the well-posedeness of the initial boundary value problem governing the solution of material instability triggering strain localization. Moreover, viscosity (rate dependency) allows the spatial difference operator in the governing equations to retain its hyperbolicity and the initial boundary value problem is well-posed. This is due to the incorporation of either explicit (via nonlocal theory) or implicit (via viscosity) intrinsic material length scale parameter in the constitutive description. Model capabilities are preliminarily illustrated for the dynamic localization of inelastic flow in adiabatic shear bands and the perforation of Weldox 460E steel plates with different thicknesses by deformable blunt projectiles at various impact speeds. The simulated shear band results well illustrated the potential of the proposed model in dealing with the well-known mesh sensitivity problem. Consequently the introduced implicit and explicit length scale measures are able to predict size effects in localization failures.
International Journal of Impact Engineering, 2004
The effect of target strength on the perforation of steel plates is studied. Three structural steels are considered: Weldox 460 E, Weldox 700 E and Weldox 900 E. The effects of strain hardening, strain rate hardening, temperature softening and stress triaxiality on material strength and ductility are determined for these steel alloys by conducting three types of tensile tests: quasi-static tests with smooth and notched specimens, quasi-static tests at elevated temperatures and dynamic tests over a wide range of strain rates. The test data are used to determine material constants for the three different steels in a slightly modified version of the Johnson-Cook constitutive equation and fracture criterion. Using these three steel alloys, perforation tests are carried out on 12 mm-thick plates with blunt-, conical-and ogivalnosed projectiles. A compressed gas gun was used to launch projectiles within the velocity range from 150 to 350 m/s. The initial and residual velocities of the projectile were measured, while the perforation process was captured using a digital high-speed camera system. Based on the test data the ballistic limit velocity was obtained for the three steels for the different nose shapes. The experimental results indicate that for perforation with blunt projectiles the ballistic limit velocity decreases for increasing strength, while the opposite trend is found in tests with conical and ogival projectiles. The tests on Weldox 700 E and Weldox 900 E targets with conical-nosed projectiles resulted in shattering of the projectile nose tip during penetration. Finally, numerical simulations of some of the experimental tests are carried out using the non-linear finite element code LS-DYNA. It is found that the numerical code is able to describe the physical mechanisms in the perforation events with good accuracy. However, the experimental trend of a decrease in ballistic limit with an increase in target strength for blunt projectiles is not obtained with the numerical models used in this study.
Archive of Mechanical Engineering, 2015
This paper presents a numerical investigation of fracture criterion influence on perforation of high-strength 30PM steel plates subjected to 7.62x51 mm Armour Piercing (AP) projectile. An evaluation of four ductile fracture models is performed to identify the most suitable fracture criterion. Included in the paper is the Modified Johnson-Cook (MJC) constitutive model coupled separately with one of these fracture criteria: the MJC fracture model, the Cockcroft-Latham (CL), the maximum shear stress and the constant failure strain models. A 3D explicit Lagrangian algorithm that includes both elements and particles, is used in this study to automatically convert distorted elements into meshless particles during the course of the computation. Numerical simulations are examined by comparing with the experimental results. The MJC fracture model formulated in the space of the stress triaxiality and the equivalent plastic strain to fracture were found capable of predicting the realistic frac...
Modeling and Simulation of Perforation of Steel Plates by Blunt Projectiles
Due to the lack of the classical local continuum formulation to produce physically meaningful and numerically converging results within large deformation and strain localization computations, a thermodynamically motivated micro-damage formulation is proposed for highrate and temperature dependent materials. This model uses a combined viscosity and nonlocal gradient localization limiters in order to regularize the dynamic strain localization problems. The enhanced nonlocal gradient-dependent theory formulates a constitutive framework on the continuum level that is used to bridge the gap between the micromechanical theories and the classical (local) continuum theories. They are successful in explaining the size effects encountered at the micron scale and in preserving the well-posedeness of the initial boundary value problem governing the solution of material instability triggering strain localization. Moreover, viscosity (rate dependency) allows the spatial difference operator in the governing equations to retain its hyperbolicity and the initial boundary value problem is well-posed. This is due to the incorporation of either explicit (via nonlocal theory) or implicit (via viscosity) intrinsic material length scale parameter in the constitutive description. Model capabilities are preliminarily illustrated for the dynamic localization of inelastic flow in adiabatic shear bands and the perforation of Weldox 460E steel plates with different thicknesses by deformable blunt projectiles at various impact speeds. The simulated shear band results well illustrated the potential of the proposed model in dealing with the well-known mesh sensitivity problem. Consequently the introduced implicit and explicit length scale measures are able to predict size effects in localization failures.
Engineering Fracture Mechanics, 2008
In this paper, a numerical study of normal perforation of thin steel plates impacted by different projectile shapes is reported. The numerical simulations of this problem have been performed using a finite element code, ABAQUS Explicit with a fixed and an adaptive mesh for the plate. To define the thermoviscoplastic behaviour of the material constituting the plate, the Johnson Cook model has been used. This homogeneous behaviour has been coupled with the Johnson Cook fracture criterion to predict completely the perforation process. Three kinds of projectile shape (blunt, conical and hemi spherical) have been simulated with a large range of impact velocities from 190 to 600 m/s. The analysis considers the influ ence of adiabatic shear bands, plastic work and the gradient of temperature generated in the plate. The numerical results predict correctly the behaviour projectile plate in agreement with experimental data published by other authors.
There is an urgent need to develop light-weight protective structures with a sufficient protection to prevent the damage occurring during extreme loading events such as blast and ballistic impacts. This study is a part of ongoing research to develop light weight amour materials which can sustain under those severe conditions. Numerical modelling with explicit finite element code LS-DYNA has performed with realistic geometries. Ballistic protection class BR7 in European norm EN 1063 considered, thus penetration of different shaped projectiles through thick steel plates was examined. Since the geometries and materials of the projectiles have a very significant influence on the outcome of this research detail modelling of the projectiles was performed. For the purpose of this paper, perforation mechanism of 7.62mm APM2 bullet through 6mm thick Weldox 460E high strength structural steel plate was examined. Largrangian methods combined with Johnson-Cook material model available in the LS-DYNA were used for the numerical simulations. Finally the ballistic limit curve for the 6mm thick Weldox 460E plate perforated by APM2 bullet was obtained. Results were compared with the analytical models.
International Journal of Impact Engineering, 2009
Thin plates of high-strength steel are frequently being used both in civil and military ballistic protection systems. The choice of alloy is then a function of application, ballistic performance, weight and price. In this study the perforation resistance of five different high-strength steels has been determined and compared against each other. The considered alloys are Weldox 500E, Weldox 700E, Hardox 400, Domex Protect 500 and Armox 560T. The yield stress for Armox 560T is about three times the yield stress for Weldox 500E, while the opposite yields for the ductility. To certify the perforation resistance of the various targets, two different ballistic protection classes according to the European norm EN1063 have been considered. These are BR6 (7.62 mm Ball ammunition) and BR7 (7.62 mm AP ammunition), where the impact velocity of the bullet is about 830 m/s in both. Perforation tests have been carried out using adjusted ammunition to determine the ballistic limit of the various steels. In the tests, a target thickness of 6 mm and 6 þ 6 ¼ 12 mm was used for protection class BR6 and BR7, respectively. A material test programme was conducted for all steels to calibrate a modified Johnson-Cook constitutive relation and the Cockcroft-Latham fracture criterion, while material data for the bullets mainly were taken from the literature. Finally, results from 2D non-linear FE simulations with detailed models of the bullets are presented and the different findings are compared against each other. As will be shown, good agreement between the FE simulations and experimental data for the AP bullets is in general obtained, while it was difficult to get reliable FE results using the Lagrangian formulation of LS-DYNA for the soft core Ball bullet.
Journal of Pressure Vessel Technology, 2008
This article studies the perforation of mild steel square and rectangular plates struck normally by cylindrical projectiles having blunt, hemispherical, and conical impact faces. Experimental results are obtained in a drop hammer rig for the perforation of 4 mm and 8 mm thick plates struck by relatively heavy projectiles weighing between 11.9 kg and 200 kg and traveling at an initial velocity up to about 13 m / s. The plates were struck at the center and at several positions near the fully clamped supports. The effect of the aspect ratio on the perforation energies of rectangular plates is examined, and comparisons are made with the perforation behavior of fully clamped circular plates. The predictions of several empirical equations are compared with the corresponding experimental values of the perforation energies. Simple design equations are also presented for predicting the maximum permanent transverse displacements of square plates prior to any cracking or perforation.