BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0001] This invention relates generally to armor systems for structural protection against
ballistic impact or explosive blast, and more particularly to the use of a metallic
foam as the shock energy-absorbing element in a multi-layer armor system.
DESCRIPTION OF THE RELATED ART
[0002] With increasing terroristic violence and military action, there is a need for improved
structural protection against ballistic impact from projectiles or blast protection
from explosives. Such structural protection can be built into the infrastructure of
a building to reinforce the building, or certain rooms within a building, against
attack. Structural protection is also useful in vehicles, illustratively military
vehicles, such as tanks, or civilian VIP vehicles. Presently, a multi-layer armor
system is employed in known vehicular applications.
[0003] A typical configuration for the armor system in medium weight military vehicles,
for example, consists of a high strength strike face (either a metal or a ceramic
plate), bonded to a ceramic tile, which is subsequently bonded to a metallic backing
plate. In this configuration, the ceramic tile breaks-up or deforms an incoming projectile,
and the metallic backing "catches" the extant penetrator and ceramic fragments. The
high strength strike plate aids the ceramic tile by providing front face confinement,
and may, in some cases, protect the ceramic tile from field damage.
[0004] Upon projectile impact at typical ordnance velocities, a stress wave is generated
and propagates through the ceramic tile. Reflections from boundaries and subsequent
stress wave interactions result in tensile stress states and attendant microcracking.
Microcracking due to these impact-induced stress waves weakens the ceramic tile, allowing
a projectile to penetrate more easily. In armor system designs utilizing a metal strike
plate over ceramic tile, stress waves from a projectile impact on the metal strike
plate can run ahead into the ceramic, and failure may initiate prior to contact of
the projectile with the ceramic tile. In an attempt to overcome the abovementioned
problem, a three layer system has been proposed and is disclosed in DE9007336U. The
three layer system includes a strike plate, a backing plate and an intermediate layer
to reduce the risk of failure of the strike plate in order to limit backface deformation.
However, provision of the intermediate layer is of only limited benefit and there
is therefore a need for an armour system to be provided having an improved shock absorbing
element, and more particularly a shock absorbing element that gives more control of
behind the target effects, such as backface deformation and spalling.
[0005] Metallic foams with a high fraction of porosity are a new class of materials which
have attributes that lend themselves to various engineering applications, including
sound and heat isolation, lightweight construction, and energy absorption. In particular,
the unique characteristics of a metallic cellular material include its comparatively
high specific strength and its characteristic non-linear deformation behaviour. An
example of the use of metallic foams in an armour system is disclosed in DE2039343
which discloses an armour wall for a vehicle comprising two layers, one of which is
a metal foam layer. However, such a two-layer armour system suffers from the same
problem as mentioned above of a high failure rate and little or no control of behind
the targets effects, such as backface deformation and spalling.
[0006] As will be described more completely hereinbelow, certain metal foams are effective
in containing rearward deformation of a target under high-speed impact, and therefore
are useful in controlling backface deformation and spalling. Moreover, metal foams
are capable of mitigating impact-induced stress waves thereby delaying damage to ceramic
layers in armour systems employing same.
[0007] It is, therefore, an object of the invention to provide an armour system incorporating
metal foam as a shock energy-absorbing element to improve protection of equipment
and personnel behind the target.
[0008] It is a further object of the invention to provide an armour system incorporating
metal foam as a shock energy-absorbing element to control behind-the-target effects
as a result of backface deformation caused by the high energy impact of a projectile.
Summary of the Invention
[0009] The foregoing and other objects, features and advantages are achieved by this invention
which provides a multilayered armour system comprising a strike plate, a deformable
back plate and one or more intermediate layers there-between, characterised in that
the one or more intermediate layers include a shock absorbing element comprising a
metallic foam having cells distributed therethrough and, on application of a force
on said strike plate, the shock absorbing element is urged to undergo progressive
modes of deformation corresponding to a substantially linear elastic deformation,
a cellular collapse deformation and a densification deformation, in response to the
magnitude of the force applied to the strike plate.
[0010] In preferred embodiments, the metallic foam has a closed-cell pore structure and
a high fraction of porosity, preferably ranging from about 50-98 percent by volume.
[0011] Metallic foams useful in the practice of the present invention may be, but are not
limited to, metal foams of aluminum, steel, lead, zinc, titanium, nickel and alloys
or metal matrix composites thereof. Metal foams can be fabricated by various processes
that are known for the manufacture of metal foams, including casting, powder metallurgy,
metallic deposition, and sputter deposition. Exemplary processes for making metal
foams are set forth in U.S. Patent Nos. 5,151,246; 4,973,358; and 5,181,549, the text
of which is incorporated herein by reference.
[0012] U.S. Patent 5,151,246, for example, describes a powder metallurgy process for making
foamable materials using metallic powders and small amounts of propellants. The process
starts by mixing commercially available metal powder(s) with a small amount of foaming
agent. After the foaming agent is uniformly distributed within the matrix material,
the mixture is compacted to yield a dense, semifinished product without any residual
open porosity. Further shaping of the foamable material can be achieved through subsequent
metalworking processes such as rolling, swaging or extrusion.
[0013] Following the metalworking steps, the foamable material is heated to temperatures
near the melting point of the matrix metal(s). During heating, the foaming agent decomposes,
and the released gas forces the densified material to expand into a highly porous
structure. The density of the metal foams can be controlled by adjusting the content
of the foaming agent and several other foaming parameters, such as temperature and
heating rate. The density of aluminum foams, for example, typically ranges from about
0.5 to 1 g/cm
3.
[0014] Strength, and other properties of foamed metals can be tailored by adjusting the
specific weight (or porosity), alloy composition, heat treatment history, and pore
morphology as is known to those of skill in the art. In advantageous embodiments,
the metallic foam will have high mechanical strength.
[0015] Metal foams are easily processed into any desired shape or configuration by conventional
techniques, such as sawing drilling, milling, and the like. Moreover, metal foams
can be joined by known techniques, such as adhesive bonding, soldering, and welding.
[0016] In certain preferred embodiments of the invention, the shock-absorbing element is
closed-cell aluminum foam, and in a specific illustrative embodiment, the shock-absorbing
element is closed-cell aluminum foam with a porosity of 80 percent by volume.
[0017] In device embodiments of the present invention, a multi-layered armor system, suitable
for structural protection against ballistic impact or explosive blast, such as armor
systems used in connection with military armored vehicles, includes one or more layers
of a metal foam as a shock energy-absorbing element.
[0018] As used herein, the term "multi-layer armor system" means at least two plates of
metal, metal foam, ceramic, plastic, and the like, known or developed, for defense
or protection systems. In the present invention, the multi-layer armor system includes
at least a strike plate, or buffer plate, bonded or otherwise held in communication
with, a shock-absorbing element that is a layer of metallic foam.
[0019] As described hereinabove, the metallic foam preferably has a closed-cell pore structure
and a high fraction of porosity. Illustratively, the metallic foam may be aluminum,
steel, lead, zinc, titanium, nickel and alloys or metal matrix composites thereof,
with porosity ranging from about 50-98 percent by volume. In a particularly preferred
embodiment of the invention, the metallic foam is a closed-cell aluminum foam having
a porosity of 80 percent by volume.
[0020] The term "strike plate" refers to a high strength metal or ceramic plate that has
a front face surface that would receive the initial impact of a projectile or blast.
The back surface of the strike plate is adjacent to a first surface of the shock-absorbing
element that, in the present invention, is a sheet or layer of metallic foam. It is
to be understood that the term "strike plate," as used herein, refers to any buffer
plate of a high strength material that receives impact or impact-induced stress waves
prior to a shock-absorbing element.
[0021] The strike plate may be a flat sheet of a high strength metal, ceramic or polymer-based
composite, such as a fiber-reinforced polymer composite.
[0022] In a preferred embodiment, the multi-layer armor system of the present invention
further includes a deformable backing plate bonded to, or otherwise held in communication
with, a face surface of the metallic foam sheet or layer opposite, or distal, to the
surface contiguous to the strike plate. The backing plate illustratively is a sheet
of a deformable metal, such as titanium, aluminum, or steel.
[0023] In a specific illustrative embodiment of a multi-layer armor system in accordance
with the invention, a shock-absorbing layer of metallic foam is sandwiched between
a high strength strike plate and a deformable backing plate. Of course, the multi-layered
armor system may comprise additional elements, in any sequence, and the embodiments
presented herein are solely for the purposes of illustrating the principles of the
invention.
BRIEF DESCRIPTION OF THE DRAWING
[0024] Comprehension of the invention is facilitated by reading the following detailed description,
in conjunction with the annexed drawing, in which:
Fig. 1 is a schematic representation of an illustrative armor system incorporating
metallic foam as a shock energy-absorbing element in accordance with the principles
of the present invention;
Fig. 2 is a photomicrograph of a high porosity, closed-cell aluminum foam showing
the typical microstructure in cross-section;
Fig. 3 is a graphical representation of the typical behavior of a metal foam, of the
type shown in Fig. 2, under a uniaxial load; and
Fig. 4 is photomicrograph of the aluminum foam of Fig. 2 showing a cross-sectional
view of the microstructure following deformation by high energy impact.
DETAILED DESCRIPTION OF THE INVENTION
[0025] Fig. 1 is an illustrative schematic representation of an improved armor system 10
of the type having a high strength strike plate 11, at least one shock energy-absorbing
element 12, and a backing plate 13. In the embodiment of Fig. 1, a closed-cell metal
foam is used as shock energy-absorbing element 12. High strength strike plate 11 may
be ceramic or metal. Backing plate 13 is typically a highly deforming metal, such
as titanium, aluminum, or steel. However, backing plate 13 may comprise one or more
layers of metal and/or ceramic, as well as polymer-based composites. In armor system
10, the closed-cell metal foam is effective in containing rearward deformation of the
strike plate 11 in a ballistic target structure. The metal foam has the ability to
control backface deformation, without sacrificing ballistic efficiency behind targets
with highly deforming back plates, via a mechanism that will be discussed more completely
hereinbelow.
[0026] The shock energy-absorbing element 12 preferably comprises a closed-cell metallic
foam which, illustratively, may be aluminum, steel, lead, zinc, titanium, nickel,
and alloys or metal matrix composites thereof. Preferred metal foams have a high fraction
of porosity, typically ranging from about 50-98 by volume percent. In a specific preferred
embodiment, shock energy-absorbing element 12 is a closed-cell aluminum foam having
a porosity of 80 by volume. Fig. 2 shows the microstructure (
i.e., the pore structure) of this particular aluminum foam material.
[0027] This type of pore structure provides a substantial increase in the stiffness/weight
ratio (SWR) of the material with a low fractional density. Under deformation, this
microstructure features localized cell collapse and rapid compaction energy dissipation,
which leads to unique deformation behaviors and material properties including high
SWR and energy absorption in the material.
[0028] During deformation, metal foams of the type shown in Fig. 2, exhibit the universal
deformation behavior shown in Figure 3 as they move from the quasi-elastic regime
to the plastic regime. Fig. 3 is a graphical representation of the behavior of the
metal foam of Fig. 2 under uniaxial load referred to as a "loading curve." The vertical
axis of Fig. 3 represents stress and the horizontal axis represents strain. The loading
curve of Fig. 3 is divided into three regions: linear elastic region 31, collapse
region 32 (where plateau stress remains relatively constant) and densification region
33. In linear elastic region 31, the elastic portion of the stress-strain curve is
only partially reversible. During loading, small-scale localized plastic deformation
has already taken place within the sample. These small-scale plastic deformations
also contribute to the mechanical damping of metal foams. In collapse region 32, the
cell wall-buckling event occurs and the foam progressively collapses until densification
region 33. The deformation in densification region 33 is highly localized and is preceded
by the advance of a densification front from deformed to undeformed regions of the
sample. For strain rate insensitive materials such as aluminum, the deformation behavior
at the high strain rates remain the same. The area under the loading curve represents
the deformation energy absorbed by the metal foam.
[0029] Metal foams can be fabricated to maximize the energy absorption capability by adjusting
foam parameters including alloying elements, density level, cell size, wall thickness,
and uniformity. Improvements in modulus and plateau stress via heat treatment of the
metal foam, or via addition of particulate or whisker reinforcements to the metal
foam, are additional techniques known to increase the energy absorption capability.
[0030] Metal foams are capable of mitigating the impact-induced stress waves from the strike
plate, thereby delaying or eliminating damage to underlying layers, which in some
embodiments might be a ceramic tile, and improving protection of the personnel and
equipment behind the target. The deformation energy due to shock impact first densifies
the front portion (in the loading direction) of the metal foam layer that forms the
shock energy-absorbing element. Subsequent deformation introduces tearing and shearing
of the cell walls, an effect of core shearing deformation for energy dissipation in
the cellular structure. Thus, the deformation energy is redirected and dissipated
sideways. This is best illustrated in Fig. 4 which is a cross-sectional view of the
microstructure of the aluminum foam of Fig. 2 showing deformation following high energy
impact. This type of deformation mechanism reduces the transmitted deformation energy
behind the target in the loading direction. The energy of the impact-induced stress
waves is also dissipated efficiently within the cellular network. The high degree
of porosity in metal foam is beneficial for the absorption of the wave energy, and
the cellular network generates the cavity effect for scattering the wave energy within
the network.
[0031] The armor systems of the present invention would be useful as protection systems
for ballistic impact and for blast. Moreover, while the illustrative embodiment presented
herein is directed to a three element system, it is to be understood that invention
contemplates the use of closed-cell, high strength metal foams having a high fraction
of porosity, as a shock energy-absorbing element in any other configuration developed,
or to be developed, wherein its ability to contain rearward deformation under high-speed
impact, would be useful.
[0032] Although the invention has been described in terms of specific embodiments and applications,
persons skilled in the art can, in light of this teaching, generate additional embodiments
without exceeding the scope of the invention as defined by the following claims. Accordingly,
it is to be understood that the drawing and description in this disclosure are proffered
to facilitate comprehension of the invention, and should not be construed to limit
the scope thereof.
1. A multi-layered armour system comprising a strike plate, a deformable back plate and
one or more intermediate layers there between, characterised in that the one or more intermediate layers include a shock absorbing element comprising
a metallic foam having cells distributed therethrough and, on application of a force
on said strike plate, the shock absorbing element is urged to undergo progressive
modes of deformation corresponding to a substantially linear elastic deformation,
a cellular collapse deformation and a densification deformation, in response to the
magnitude of the force applied to the strike plate.
2. A multi-layered armour system according to claim 1 characterised in that the strike plate is selected from the group consisting of high strength metals, ceramics
and polymer based composites.
3. A multi-layered armour system according to claim 1 characterised in that the metallic foam is a closed cell metallic foam.
4. A multi-layered armour system according to claim 3 characterised in that the closed cell metallic foam is selected from the group consisting of aluminium,
steel, lead, zinc, titanium, nickel and alloys, or metal matrix composites thereof.
5. A multi-layered armour system according to claim 3 characterised in that the closed cell metallic foam has a porosity that ranges from about 50-98 percent
by volume.
6. A multi-layered armour system according to claim 1 characterised in the deformable backing plate comprises a metal selected from the group consisting
of titanium, aluminium or steel.
7. A multi-layered armour system according to claim 3 characterised in that the closed cell metallic foam is formed from aluminium and has a porosity of 80 percent
by volume.
1. Mehrschichtiges Panzerungssystem, das eine Auftreffplatte, eine verformbare Trägerplatte
und eine oder mehrere dazwischen befindliche Zwischenschichten umfasst, dadurch gekennzeichnet, dass die eine oder mehrere Zwischenschichten ein schockabsorbierendes Element einschließen,
das einen Metallschaum mit darin verteilten Zellen umfasst, und das schockabsorbierende
Element bei Ausübung einer Kraft auf die Auftreffplatte gezwungen wird, progressive
Verformungsmodi zu durchlaufen, die einer im wesentlichen linearen elastischen Verformung,
einer Zelleinsturzverformung und einer Verdichtungsverformung in Reaktion auf die
Größenordnung der auf die Auftreffplatte ausgeübten Kraft entsprechen.
2. Mehrschichtiges Panzerungssystem nach Anspruch 1, dadurch gekennzeichnet, dass die Auftreffplatte aus der Gruppe bestehend aus hochwiderstandsfähigen Metallen,
Keramikwerkstoffen und Verbundstoffen auf Polymerbasis ausgewählt ist.
3. Mehrschichtiges Panzerungssystem nach Anspruch 1, dadurch gekennzeichnet, dass der Metallschaum ein geschlossenzelliger Metallschaum ist.
4. Mehrschichtiges Panzerungssystem nach Anspruch 3, dadurch gekennzeichnet, dass der geschlossenzellige Metallschaum aus der Gruppe bestehend aus Aluminium, Stahl,
Blei, Zink, Titan, Nickel und Legierungen oder Metallmatrixverbundstoffen davon ausgewählt
ist.
5. Mehrschichtiges Panzerungssystem nach Anspruch 3, dadurch gekennzeichnet, dass der geschlossenzellige Metallschaum eine Porosität im Bereich von etwa 50 bis 98
Volumenprozent aufweist.
6. Mehrschichtiges Panzerungssystem nach Anspruch 1, dadurch gekennzeichnet, dass die verformbare Trägerplatte ein Metall ausgewählt aus der Gruppe bestehend aus Titan,
Aluminium oder Stahl umfasst.
7. Mehrschichtiges Panzerungssystem nach Anspruch 3, dadurch gekennzeichnet, dass der geschlosszellige Metallschaum aus Aluminium besteht und eine Porosität von 80
Volumenprozent aufweist.
1. Système de blindage multicouche comprenant une plaque d'impact, une plaque de renforcement
arrière déformable, et une ou plusieurs couches intermédiaires entre les deux,
caractérise en ce que
la ou les couches intermédiaires comportent un élément d'absorption de chocs comprenant
une mousse métallique à travers laquelle sont réparties des cellules, de façon que,
lorsqu'on applique une force à la plaque d'impact, l'élément d'absorption de chocs
soit poussé à subir des modes de déformation progressifs correspondant à une déformation
élastique essentiellement linéaire, à une déformation d'affaissement cellulaire, et
à une déformation de densification, en réponse à l'amplitude de la force appliquée
à la plaque d'impact.
2. Système de blindage multicouchè selon la revendication 1,
caractérisé en ce que
la plaque d'impact est sélectionnée dans le groupe comprenant des métaux à haute résistance,
des céramiques à haute résistance et des composites à base de polymères à haute résistance.
3. Système de blindage multicouche selon la revendication 1,
caractérisé en ce que
la mousse métallique est une mousse métallique à cellule fermées.
4. Système de blindage multicouche selon la revendication 3,
caractérisé en ce que
la mousse métallique à cellules fermées est sélectionnée dans le groupe comprenant
l'aluminium, l'acier, le plomb, le zinc, le titane, le nickel, ainsi que des alliages
ou des composites à matrice métallique de ceux-ci.
5. Système de blindage multicouche selon la revendication 3,
caractérisé en ce que
la mousse métallique à cellules fermées présente une porosité se situant entre environ
50 % et 98 % en volume.
6. Système de blindage multicouche selon la revendication 1,
caractérisé en ce que
la plaque de renforcement arrière déformable comprend un métal sélectionné dans le
groupe comprenant le titane, l'aluminium ou l'acier.
7. Système de blindage multicouche selon la revendication 3,
caractérisé en ce que
la mousse métallique à cellules fermées est constituée d'aluminium et présente une
porosité de 80 % en volume.