Norwegian Defence Estates Agency

Sentrum, Norway

Norwegian Defence Estates Agency

Sentrum, Norway
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Borvik T.,Norwegian University of Science and Technology | Borvik T.,Norwegian Defence Estates Agency | Olovsson L.,IMPETUS Afea AB | Hanssen A.G.,Norwegian University of Science and Technology | And 4 more authors.
Journal of the Mechanics and Physics of Solids | Year: 2011

The structural response of a stainless steel plate subjected to the combined blast and sand impact loading from a buried charge has been investigated using a fully coupled approach in which a discrete particle method is used to determine the load due to the high explosive detonation products, the air shock and the sand, and a finite element method predicts the plate deflection. The discrete particle method is based on rigid, spherical particles that transfer forces between each other during collisions. This method, which is based on a Lagrangian formulation, has several advantages over coupled LagrangianEulerian approaches as both advection errors and severe contact problems are avoided. The method has been validated against experimental tests where spherical 150 g C-4 charges were detonated at various stand-off distances from square, edge-clamped 3.4 mm thick AL-6XN stainless steel plates. The experiments were carried out for a bare charge, a charge enclosed in dry sand and a charge enclosed in fully saturated wet sand. The particle-based method is able to describe the physical interactions between the explosive reaction products and soil particles leading to a realistic prediction of the sand ejecta speed and momentum. Good quantitative agreement between the experimental and predicted deformation response of the plates is also obtained. © 2011 Elsevier Ltd. All rights reserved.


Borvik T.,Norwegian University of Science and Technology | Borvik T.,Norwegian Defence Estates Agency | Olovsson L.,Impetus Afea AB | Dey S.,Norwegian University of Science and Technology | And 2 more authors.
International Journal of Impact Engineering | Year: 2011

Normal and oblique impact on 20 mm thick AA6082-T4 aluminium plates are studied both experimentally and numerically. Two types of small arms bullets were used in the ballistic tests, namely the 7.62 × 63 mm NATO Ball (with a soft lead core) and the 7.62 × 63 mm APM2 (with a hard steel core), fired from a long smooth-bore Mauser rifle. The targets were struck at 0°, 15°, 30°, 45° and 60° obliquity, and the impact velocity was about 830 m/s in all tests. During testing, the initial and residual bullet velocities were measured by various laser-based optical devices, and high-speed video cameras were used to photograph the penetration process. Of special interest is the critical oblique angle at which the penetration process changes from perforation to embedment or ricochet. The results show that the critical oblique angle was less than 60° for both bullet types. A material test programme was also conducted for the AA6082-T4 plate to calibrate a modified Johnson-Cook constitutive relation and the Cockcroft-Latham failure criterion, while material data for the bullets mainly were taken from the literature. 3D non-linear FE simulations with detailed models of the bullets were finally run. Good agreement between the FE simulations and the experimental results for the APM2 bullets was in general obtained, while it was more difficult to get reliable FE results for the soft core Ball bullets. © 2011 Published by Elsevier Ltd. All rights reserved.


Borvik T.,Impact Lab | Borvik T.,Norwegian Defence Estates Agency | Hopperstad O.S.,Impact Lab | Pedersen K.O.,Impact Lab | Pedersen K.O.,Sintef
International Journal of Impact Engineering | Year: 2010

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. © 2009 Elsevier Ltd. All rights reserved.


Holmen J.K.,Norwegian University of Science and Technology | Johnsen J.,Norwegian University of Science and Technology | Jupp S.,Hydro Rolled Products GmbH | Hopperstad O.S.,Norwegian University of Science and Technology | And 2 more authors.
International Journal of Impact Engineering | Year: 2013

The ballistic properties of the aluminium alloy AA6070 in different tempers are studied, using target plates of 20 mm thickness in tempers O (annealed), T4 (naturally aged), T6 (peak strength) and T7 (overaged). The stress-strain behaviour of the different tempers was characterised by quasi-static tension tests and was found to vary considerably with temper in regards to strength, strain hardening and ductility. Ballistic impact tests using 7.62 mm APM2 bullets were then carried out, and the ballistic limit velocities were obtained for all tempers. In the material tests it was shown that the O-temper was most ductile and almost no fragmentation took place during the ballistic impact tests. The T6-temper proved to be least ductile, and fragmentation was commonly seen. The experiments show that despite fragmentation, strength is a more important feature than ductility in ballistic impact for this alloy, at least for the given projectile and within the velocity range investigated. A thermoelastic-thermoviscoplastic constitutive relation and a ductile fracture criterion were determined for each temper, and finite element analyses were performed using the IMPETUS Afea Solver with fully integrated 3rd-order 64-node hexahedrons. The numerical simulations predicted the same variation in ballistic limit velocity with respect to temper condition as found in the experiments, but the results were consistently to the conservative side. In addition, analytical calculations using the cylindrical cavity expansion theory (CCET) were carried out. The ballistic limit velocities resulting from these calculations were found to be in good agreement with the experimental data.© 2013 Elsevier Ltd. All rights reserved.


Forrestal M.J.,1805 Newton P1 | Borvik T.,Norwegian University of Science and Technology | Borvik T.,Norwegian Defence Estates Agency | Warren T.L.,3804 Shenandoah P1
Proceedings of the Society for Experimental Mechanics, Inc. | Year: 2010

We conducted an experimental and analytical study to better understand the mechanisms and dominant parameters for 7.62 mm APM2 bullets that perforate 7075-T651 aluminum armor plates. The 7.62-mm-diameter, 10.7 g, APM2 bullet consists of a brass jacket, lead filler, and a 5.25 g, ogive-nose, hard steel core. The brass and lead were stripped from the APM2 bullets by the targets, so we conducted ballistic experiments with both the APM2 bullets and only the hard steel cores. These projectiles were fired from a rifle to striking velocities between 600 and 1,100 m/s. Targets were 20 and 40-mm-thick, where the 40-mm-thick targets were made up of layered 20-mm-thick plates in contact with each other. The measured ballistic-limit velocities for the APM2 bullets were 1% and 8% smaller than that for the hard steel cores for the 20 and 40-mm-thick targets, respectively. Thus, the brass jacket and lead filler had a relatively small effect on the perforation process. Predictions from a cylindrical cavity-expansion model for the hard steel core projectiles are shown to be in good agreement with measured ballistic-limit and residual velocity data. The results of this study complement our previous paper with 5083-H116 aluminum target plates in that the ultimate tensile strength of 7075-T651 is about 1.8 times greater than that of 5083-H116. We also present a scaling law that shows a square root relationship between ballistic-limit velocity and plate thickness and material strength. © Society for Experimental Mechanics 2010.


Pedersen K.O.,Norwegian University of Science and Technology | Pedersen K.O.,Sintef | Borvik T.,Norwegian University of Science and Technology | Borvik T.,Norwegian Defence Estates Agency | Hopperstad O.S.,Norwegian University of Science and Technology
Materials and Design | Year: 2011

The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20. mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event. © 2010 Elsevier Ltd.


Borvik T.,Norwegian University of Science and Technology | Borvik T.,Norwegian Defence Estates Agency | Forrestal M.J.,1805 Newton Pl | Warren T.L.,3804 Shenandoah Pl
Proceedings of the Society for Experimental Mechanics, Inc. | Year: 2010

We conducted an experimental and analytical study to understand the mechanisms and dominant parameters for ogive-nose rods and 7.62 mm APM2 bullets that perforate 5083-H116 aluminum armor plates. The 20-mm-diameter, 95-mm-long, ogive-nose, 197 g, hard steel rods were launched with a gas gun to striking velocities between 230-370 m/s. The 7.62-mm-diameter, 10.7 g, APM2 bullet consists of a brass jacket, lead filler, and a 5.25 g, ogivenose, hard steel core. The brass and lead were stripped from the APM2 bullets by the targets, so we conducted ballistic experiments with both the APM2 bullets and only the hard steel cores. These projectiles were fired from a rifle to striking velocities between 480-950 m/s. Targets were 20, 40, and 60-mm-thick, where the 40 and 60-mm-thick targets were made up of layered 20-mm-thick plates in contact with each other. The measured ballistic-limit velocities for the APM2 bullets were 4, 6, and 12% smaller than that for the hard steel cores for the 20, 40, and 60-mm-thick targets, respectively. Thus, the brass jacket and lead filler had a relatively small effect on the perforation process. In addition, we conducted large strain, compression tests on the 5083-H116 aluminum plate material for input to perforation equations derived from a cavity-expansion model for the ogive-nose rods and steel core projectiles. Predictions for the rod and hard steel core projectiles are shown to be in good agreement with measured ballistic-limit and residual velocity data. These experimental results and perforation equations display the dominant problem parameters. © Society for Experimental Mechanics 2009.


Rakvag K.G.,Norwegian University of Science and Technology | Borvik T.,Norwegian University of Science and Technology | Borvik T.,Norwegian Defence Estates Agency | Westermann I.,Norwegian University of Science and Technology | And 2 more authors.
Materials and Design | Year: 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 to 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. © 2013 Elsevier Ltd.


Gruben G.,Norwegian University of Science and Technology | Fagerholt E.,Norwegian University of Science and Technology | Hopperstad O.S.,Norwegian University of Science and Technology | Borvik T.,Norwegian University of Science and Technology | Borvik T.,Norwegian Defence Estates Agency
European Journal of Mechanics, A/Solids | Year: 2011

In this study, the fracture characteristics of a cold-rolled, low-strength, high-hardening steel sheet (Docol 600DL) under quasi-static loading conditions are established using five different test set-ups. In all the tests, the sheet material is initially in plane-stress states. Optical field measurements with digital image correlation were used to determine the strain fields to fracture, to calibrate the material model for the sheet material and to validate the finite element models of the tests. Based on the field measurements, a novel method for experimental determination of the stress triaxiality and the Lode parameter is presented for isotropic materials and plane-stress states. These parameters were also obtained from finite element simulations. Comparisons show that the two methods give approximately the same average values of the stress triaxiality and the Lode parameter up to fracture. The sheet material displays only moderate variation in ductility as a function of the stress triaxiality and the Lode parameter within the investigated range of these parameters. The most critical through-thickness position in the specimens was found to be in the centre where the strains and the stress triaxiality are highest. © 2011 Elsevier Masson SAS. All rights reserved.


Fagerholt E.,Norwegian University of Science and Technology | Borvik T.,Norwegian University of Science and Technology | Borvik T.,Norwegian Defence Estates Agency | Hopperstad O.S.,Norwegian University of Science and Technology
Optics and Lasers in Engineering | Year: 2013

This paper evaluates mesh adaptation techniques in two-dimensional digital image correlation (2D-DIC) analyses of specimens with large deformations and a single propagating crack. The finite element formulation of DIC is chosen as basis for this study. The focus has been on the challenges caused by high-gradient and discontinuous displacement fields in the region of a propagating crack, and the aim has been to improve both the robustness and the accuracy of the correlation in such regions. Mesh adaptation based on node splitting and a novel overlapping-mesh technique have been implemented in a DIC code and used to capture the discontinuous displacement fields of cracked specimens. In addition, a procedure for optimization of the location of the crack path is presented. Synthetic image series with known displacement fields, generated from finite element simulations, have been analyzed to evaluate the performance of the presented techniques. Additionally, an experimental image series of a modified Arcan test has been analyzed using the proposed mesh adaptation and crack-path optimization. The paper contains a detailed description of the proposed techniques and results from the evaluation. It is found that both the node-splitting and overlapping-mesh techniques can be applied to successfully capture the discontinuous displacement field of a propagating crack. In the latter technique, the crack path is described down to pixel level. The crack-path optimization is capable of locating the crack path with sub-pixel accuracy, reducing correlation residuals and thus increasing the robustness of the DIC analysis. In addition, a certain filtering of pixels based on high correlation residual is found to increase the robustness of the correlation in areas affected by a propagating crack. © 2012 Elsevier Ltd.

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