Ordnance – Shields – Shape or composition
Patent
1987-11-10
1991-10-29
Bentley, Stephen C.
Ordnance
Shields
Shape or composition
428911, F41H 504
Patent
active
050605534
DESCRIPTION:
BRIEF SUMMARY
This invention relates to armour materials, for example the type of armor used to protect personnel and instrumentation from ball round, armor piercing bullets and shrapnel. Such armor is either worn by personnel or used in the structure of a vehicle, helicopter or aircraft, such as in door panels or in the lining of a pilot's seat.
The prior art includes proposals for armor materials in which tiles of aluminium oxide or alumina are bonded on to an energy absorptive backing. The priniciple of operation of this prior art system is complex and not fully understood, but the generally accepted theory is that, in use, a ballistic projectile strikes the ceramic tile, and in transferring kinetic energy to this relatively dense and hard material, establishes shock waves in the tile which lead to multiple fractures in the ceramic material.
This prior art armor material has various shortcomings including the fact that it is somewhat heavy. The alumina ceramic material makes a significant contribution to the overall weight of the composite armor material, having a density in the range 3.4 to 3.9.times.10.sup.3 kg/m.sup.3. These qualities of relatively high density and hardness have been seen as an inevitable feature of a material which has to absorb the kinetic energy of a projectile by fragmentation, though there is of course a need (particularly in the aircraft industry) for a material which is lighter and preferably is softer and easier to machine and cut.
We have discovered that there is a range of materials which posses densities lower than 3.4.times.10.sup.3 kg/m.sup.3 yet which perform equally or better than alumina (on a weight/threat basis) in terms of preventing ballistic projectiles penetrating composite armor incorporating same, and despite having a hardness significantly less than alumina, the Vickers hardness numbers actually being one half to one third of that of alumina.
According to the invention there is provided an armor material as defined in the accompanying claims. The invention also provides a method of making an armor layer for use in such an armor material.
In an embodiment described below, a glass ceramic composition is produced by controlled crystallisation of a corresponding glass by heat treatment, thereby to promote during use in an armor material, multiple fracturing upon impact by a ballistic projectile.
The method of heat treatment comprises:
(a) maintaining the glass for up to 10 hours at a nucleation temperature which can be determined for the material in question by thermal analysis and/or electron microscopy, at which temperature nuclei form within the body of the glass; and
(b) raising the temperature from the nucleation temperature to a crystallisation temperature which is preferably somewhat below the temperature for maximum rate of crystal formation by an amount such as to produce a higher density of fine grain crystals than would be obtained at the said temperature for maximum rate of crystal formation; and
(c) maintaining the temperature at the crystallisation temperature for up to 10 hours; and
(d) cooling the glass ceramic at a rate such as to avoid undue residual stress, for example by cooling at the natural cooling rate of a kiln containing the glass ceramic.
The heat treatment step in the production of armor materials according to the invention has the effect of maximising, or at least significantly increasing, the velocity of sound in the material. We have discovered that this parameter is directly linked to the optimisation of the ballistic performance of an armor material, which is somewhat unexpected having regard to the above-mentioned prior art theories of operation of materials of this kind.
From the foregoing it will be understood that the ballistic properties of an armor material according to the invention are related to the velocity of sound therein, which in turn is related to the ratio of the square root of its elastic modulus to its density and thus is not connected to the hardness of the material in a simple direct way. The heat treatment process forming p
REFERENCES:
patent: 3444033 (1969-05-01), King
patent: 3671374 (1972-06-01), Kolarik
patent: 4310595 (1982-01-01), Beall et al.
patent: 4473653 (1984-09-01), Rudoi
patent: 4633756 (1987-01-01), Rudoi
patent: 4739690 (1988-04-01), Moskowitz
Bentley Stephen C.
Ceramic Developments (Midlands) Limited
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