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Interaction of Cylindrical Penetrators with Ceramic and Electromagnetic Armour
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Växelverkan mellan cylindriska penetratorer och keramiska och elektromagnetiska pansar (Swedish)
Abstract [en]

Armoured vehicles have traditionally used steel armour as protection against penetrators such as projectiles and shaped charge jets. The latter produce a thin stretching metal jet, usually of copper, with a tip velocity of about 7-8 km/s. In order to obtain more weight-efficient solutions, there is a search for lighter materials and other protection techniques. In this thesis, ceramic and electromagnetic armours are studied. Ceramic materials are lighter than steel, and their high compressive strength makes them useful as armour materials. Electromagnetic armour consists of two metal plates connected to an electric power supply capable of delivering a strong current pulse. A conductive penetrator passing through both plates is destroyed by the effects of the resulting current.

Tests of the ceramic armour materials alumina and boron carbide were performed with reverse impact technique, which signifies that a target assembly (ceramic confined in a metal cylinder) was launched by a gun towards a projectile placed in front of the gun barrel. By this technique yaw was eliminated, but the geometric scale had to be very small. Therefore, we studied scaling laws for ceramic armour through a series of tests with direct impact technique and projectile diameters from 2 to 10 mm. The small scale has the advantage that flash X-ray photography can be used to photograph the projectile inside the ceramic target. The phenomenon of interface defeat or dwell was also demonstrated. It signifies that the ceramic, at least for a short time, can withstand the impact pressure so that the projectile just flows out onto the target surface. A transition velocity, above which dwell does not occur, was determined. Simulations were performed with the continuum-dynamic code Autodyn and by use of a model for the brittle ceramic materials by Johnson and Holmquist. The simulations reasonably well represented the penetration behaviour above the transition velocity. They also did below, if under this condition the ceramic model was forced to remain undamaged.

The performance of electromagnetic armour was tested against a shaped charge jet. The jet was registered with shadowgraph flash X-ray photography between the plates and after passing through the plates. The current through the jet and the voltage over the plates were also registered. The current caused heating leading to melting and even vaporization. The magnetic Lorentz force compressed the jet radially, and as this effect increases with decreasing jet radius, instability may arise. Explosions in the compressed regions resulted in a fragmented jet with disk-shaped fragments which are less effective penetrators than the elongated fragments obtained in the absence of current. We also performed a theoretical study, in which the penetrator was subjected to small elastic strains only and the current was constant. The magnetic field was determined by FFT, and the stresses due to the Lorentz force were calculated with a semi-analytical method. The velocity skin effect was demonstrated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , 73 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1032
National Category
Interaction Technologies
Research subject
Engineering Science with specialization in Solid Mechanics
Identifiers
URN: urn:nbn:se:uu:diva-197563ISBN: 978-91-554-8633-4 (print)OAI: oai:DiVA.org:uu-197563DiVA: diva2:613844
Public defence
2013-05-17, Polhemssalen, Ångström, Uppsala, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2013-04-25 Created: 2013-03-27 Last updated: 2013-08-30Bibliographically approved
List of papers
1. Influence of scale on the penetration of tungsten rods into steel-backed alumina targets
Open this publication in new window or tab >>Influence of scale on the penetration of tungsten rods into steel-backed alumina targets
1996 (English)In: International Journal of Impact Engineering, ISSN 0734-743X, Vol. 18, no 4, 403-416 p.Article in journal (Refereed) Published
Abstract [en]

As ballistic tests are often performed in reduced geometrical scale, the scaling laws are important for the interpretation of the results. In this study, we tested the validity of replica scaling, by which we mean that all geometrical dimensions are scaled uniformly, while the materials and the impact velocity are kept the same. Long tungsten projectiles with length-to-diameter ratio 15 were fired against unconfined alumina targets with steel backing. The tests were carried out with impact velocities 1500 m s−1 and 2500 m s−1, and in three different scales with projectile lengths 30, 75 and 150 mm (diameters 2, 5 and 10 mm). The alumina targets were photographed by means of a high-speed camera, and the tungsten projectiles were photographed inside the alumina targets by means of flash radiography. Also, the residual penetrations in the steel backings were measured. The Johnson-Holmquist model for ceramic materials was implemented into the AUTODYN code, which was used for simulation of the experiments. The agreement between results of experiment and simulation was fair, and over the tested interval of scales replica scaling was found to be valid with reasonable accuracy.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-92318 (URN)10.1016/0734-743X(95)00049-G (DOI)
Available from: 2004-11-03 Created: 2004-11-03 Last updated: 2013-09-18Bibliographically approved
2. High velocity penetration of homogeneous, segmented and telescopic projectiles into alumina targets
Open this publication in new window or tab >>High velocity penetration of homogeneous, segmented and telescopic projectiles into alumina targets
1997 (English)In: Int. J. Impact Engng, Vol. 20, 817-827 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:uu:diva-41209 (URN)
Available from: 2005-04-28 Created: 2005-04-28 Last updated: 2013-08-30
3. Tungsten long-rod penetration into confined cylinders of boron carbide at and above ordnance velocities
Open this publication in new window or tab >>Tungsten long-rod penetration into confined cylinders of boron carbide at and above ordnance velocities
2001 (English)In: International Journal of Impact Engineering, ISSN 0734-743X, Vol. 25, no 7, 703-714 p.Article in journal (Refereed) Published
Abstract [en]

The purpose was to investigate the influence of impact velocity and confinement on the resistance of boron carbide targets to the penetration of tungsten long-rod projectiles. Experimental tests with impact velocities from 1400 to 2600 m/s were performed using a two-stage light-gas gun and a reverse impact technique. The targets consisted of boron carbide cylinders confined by steel tubes of various thicknesses. Simulations were carried out using the AUTODYN-2D code and Johnson–Holmquist's constitutive model with and without damage evolution. The experimental results show that the penetration process had different character in three different regions. At low-impact velocities, no significant penetration occurred. At high-impact velocities, the relation between penetration velocity and impact velocity was approximately linear, and the penetration was steady and symmetrical. In between, there was a narrow transition region of impact velocities with intermittent and strongly variable penetration velocity. In the lower part of this region, extended lateral flow of the projectile took place on the surface of the target. The influence of confinement on penetration velocity was found to be small, especially at high-impact velocities. The simulated results for penetration velocity versus impact velocity agreed fairly well with the experimental results provided damage evolution was suspended below the transition region.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-92320 (URN)10.1016/S0734-743X(00)00072-5 (DOI)
Available from: 2004-11-03 Created: 2004-11-03 Last updated: 2013-09-18Bibliographically approved
4. Experimental Study of Electromagnetic Effects on Solid Copper Jets
Open this publication in new window or tab >>Experimental Study of Electromagnetic Effects on Solid Copper Jets
Show others...
2010 (English)In: Journal of applied mechanics, ISSN 0021-8936, E-ISSN 1528-9036, Vol. 77, no 1, 011010- p.Article in journal (Refereed) Published
Abstract [en]

In this paper we present a study of the interaction between all electric current pulse and a solid copper jet. Experiments were performed using a dedicated pulsed power supply delivering a current pulse of such amplitude, rise little, and duration that the jet is efficiently affected. The copper jet was created by using a shaped charge warhead. All electrode configuration consisting of two aluminum plates with a separation distance of 150 mm was used. The discharge current pulse and the voltages at the capacitors and at the electrodes were measured to obtain data oil energy deposition in and the resistance of the jet and electrode contact region. X-ray diagnostics were used to radiograph the jet, and by analyzing the radiograph, the degree of disruption of the electrified jet could be obtained. It was found that a current pulse with an amplitude of 200-250 kA and a rise time of 16 mu s could strongly enhance the natural fragmentation of the jet. In this case, the initial electric energy was 100 kJ and about 90% of the electric energy was deposited in the jet and electrodes. At the exit of the electrode region, the jet fragments formed rings with a radial velocity of up to 200 m/s, depending oil the initial electric energy in the pulsed power supply. [DOI: 10.1115/1.3172251]

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-127434 (URN)10.1115/1.3172251 (DOI)000271574200010 ()
Available from: 2010-07-14 Created: 2010-07-13 Last updated: 2017-12-12Bibliographically approved
5. Interaction between solid copper jets and powerful electric current pulses
Open this publication in new window or tab >>Interaction between solid copper jets and powerful electric current pulses
Show others...
2011 (English)In: Journal of applied mechanics, ISSN 0021-8936, E-ISSN 1528-9036, Vol. 78, no 2, 021006- p.Article in journal (Refereed) Published
Abstract [en]

The interaction between a solid copper jet and an electric current pulse is studied. Copper jets that were created by a shaped charge device were passed through an electrode configuration consisting of two aluminum plates with a separation distance of 150 mm. The electrodes were connected to a pulsed power supply delivering a current pulse with amplitudes up to 250 kA. The current and voltages were measured, providing data on energy deposition in the jet and electrode contact region, and flash X-ray diagnostics were used to depict the jet during and after electrification. The shape of, and the velocity distributions along, the jet has been used to estimate the correlation between the jet mass flow through the electrodes and the electrical energy deposition. On average, 2.8 kJ/g was deposited in the jet and electrode region which is sufficient to bring the jet up to the boiling point. A model based on the assumption of a homogenous current flow through the jet between the electrodes underestimates the energy deposition and the jet resistance by a factor five compared to the experiments, indicating a more complex current flow through the jet. The experimental results indicate the following mechanism for the enhancement of jet break-up. When electrified, the natural-formed necks in the jet are subjected to a higher current density compared to other parts of the jet. The higher current density results in a stronger heating and a stronger magnetic pinch force. Eventually, the jet material in the neck is evaporated and explodes electrically, resulting in a radial ejection of vaporized jet material.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-197561 (URN)10.1115/1.4002568 (DOI)
Available from: 2013-03-27 Created: 2013-03-27 Last updated: 2017-12-06Bibliographically approved
6. Stresses in a long cylindrical conductor moving axially through a pair of electrode plates under stationary conditions
Open this publication in new window or tab >>Stresses in a long cylindrical conductor moving axially through a pair of electrode plates under stationary conditions
2013 (English)In: Journal of applied mechanics, ISSN 0021-8936, E-ISSN 1528-9036, Vol. 80, no 2, 021013- p.Article in journal (Refereed) Published
Abstract [en]

In a conductor carrying electric current, the Lorentz force gives rise to mechanical stresses. Here, we study a long elastic cylindrical conductor that moves axially with constant velocity through two electrode plates. The aims are to explore how the stresses in the conductor depend on the velocity in the stationary case of constant current and to assess the validity of the analytic method used. The diffusion equation for the magnetic flux density is solved by use of Fourier transform, and the current density is determined. The stresses, due to the Lorentz force, are found by use of an analytic method combining the solutions of a quasi-static radial problem of plane deformation and a dynamic axial problem of uniaxial stress. They are also determined through FE analysis. Radial field profiles between the plates indicate a velocity skin effect signifying that the current and the magnetic field are concentrated near the cylindrical surface up-stream and are more uniformly distributed downstream. The radial and hoop stresses are compressive, while the axial stress is tensile. The von Mises effective stress increases towards the symmetry axis, in the downstream direction, and with velocity. There are circumstances under which a large current can produce an effective stress in a copper conductor of the order of the yield stress without causing a significant temperature rise. The stresses obtained with the two methods agree well, even relatively near the electrode plates. The analytical method should be useful in similar cases as well as for the provision of test cases for more general simulation tools.

National Category
Applied Mechanics
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-187308 (URN)10.1115/1.4007221 (DOI)000326175400026 ()
Available from: 2012-12-04 Created: 2012-12-04 Last updated: 2017-12-07Bibliographically approved

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