GB1578640A - Bullet-resistant armour - Google Patents

Abstract

A bulletproof laminate which can be used as armour for vehicles has an at least two-layered structure in which one layer arranged on the bullet impact side is formed from a hard material, in particular steel. The other layer consists of a cast polyamide obtained by the activated anionic polymerisation of monomeric laurolactam and having a Vicat-B softening point of no more than approximately 183 DEG C and has a spherulite-containing structure with an amorphous component and a crystalline component which is designed for transforming the kinetic bullet energy into heat of fusion and work of deformation.

Description

(54) A BULLET-RESISTANT ARMOUR (71) We, ELTEKA KUNSTSTOFF- TECHNIK GMBH a German Company of Birkenallee, 7950 Biberach (risks) Germany do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a bulletresistant armour.

In general weapon technology, prominence has been given for some time to seeking a replacement material for the high-cost special steels used for armoured vehicles, which up to the present time offer the only guarantee of sufficient safety against missiles. In this respect, within the western defence alliance, safety against missiles is generally estimated in relation to SmK shells of calibre 7.62 mm X 51 fired by a NATO rapid firing gun. These shells require for armour plates of HH steels an average thickness of about 14.5 mm so that they do not shoot through and do not lead to splintering of the material as compared with smaller material thicknesses, which would carry the fear of greater injury and death risk for the tank crew and an increased danger of damage to the instruments on board.

With regard to the suitability of such replacement material for armoured vehicles, it is also desirable to attain a substantial lessening in weight so that the vehicle has greater mobility over difficult countryside from the military point of view. In this respect it is preferable that the shot goes smoothly through rather than becomes held but at the same time splinters the material, or at least is substantially braked and possibly deviated as such material splintering substantially multiplies the injury and death risk.

In following these concepts, it has been found that cast polyamide obtained by active anionic polymerisation of lauric lactam monomer, hereinafter known as "polyamide 12" for short, proves to be a very effective missile resisting material.

The missile resisting action of such polyamide 12 is mainly due to the fact that the kinetic shot energy may be transformed by this material into melting heat and work of deformation.

The extent of these two factors is dependent on the material structure. In this respect, by further tests it was determined that with reference to spherulite a structure with a higher crystalline proportion leads to a lower work of deformation than a structure with a higher amorphous proportion, by which the particular advantage of complete or at least substantial avoiding of any material splintering is obtained. This action can also be explained in that the polyamide 12 attains a temporary plasticity in the penetration channel of the penetrating shell if the structure is sufficiently amorphous. By such a temporary plasticity a type of retaining cushion is provided for the shell, which destroys its kinetic energy.For complete destruction of the kinetic energy of the shell, a certain path length is therefore necesary, determined by the material thickness, for which a shell with a given penetration force can be braked to such an extent that it remains inserted in the polyamide 12. The shell remaining inserted can be therefore likened to congealing, which sets in at the moment in which the remaining kinetic energy of the shell is no longer sufficient to melt any more of the polyamide 12.

In order to optimise the missile resisting action of the polyamide 12, different process parameters can be varied during the anionic polymerisation of the lauric lactam monomer, such as the proportioning of the additives which initiate polymerisation, or the temperature control and/or by suitable heat treatment of the finished casting, in order to obtain the highest possible proportion of amorphous material in the polyamide 12, or to hold its crystalline proportion correspondingly small relative to spherulite. This relatively between amorphous and crystalline proportions, which can also be controlled by the gravity, is here to be judged from the point of view of what missile resisting action can be obtained for a given material thickness.Thus by also including the splinter action of the material or its tendency to form cracks, a respective optimum material structure can be found empirically.

As the material behaviour of non-hardened plastics is mostly estimated by the so-called Vicat softening point, tests were also carried out to determine which relationships exist between such different levels of amorphous and crystalline proportions and the said para meter for polyamide 12 for its use as a missile resisting material. The Vicat softening point was measured for various test pieces in accor dance with the requirements of DIN standard 53 460, and it was determined that the Vicat B softening point must have a value not exceeding 183"C and preferably a value of about 161"C to about 168 C, in order for a given material structure with a sufficiently high amorphous content to have adequate missile resisting action.If the Vicat B soften ing point is higher, then there is a correspond ing crystalline structure with reference to spherulite, such that the cast body has greater brittleness and is so allergic to cracks that it has reduced missile resisting action and is subject to material splintering.

The present invention is based on the use of cast polyamide obtained by the activated anionic polymerization of lauric lactam mono mer and having a Vicat B softening point of not more than 183 C in bullet-resistant armour.

In accordance with the present invention there is provided a bullet-resistant armour comprising at least two layers including a hard protective layer and at least one shock absorb ing layer comprising a cast polyamide obtained by the activated anionic polymerization of lauric lactam monomer and having a Vicat B softening point of not more than 183"C, the shock absorbing layer or at least one of the shock absorbing layers being disposed more remotely from a bullet impact side of the armour than the hard protective layer.

The hard protective layer enables the thick ness of the bullet-resistant armour to be reduced and the Vicat B softening point is such that the cast polyamide has a proportion 'of amorphous and crystalline structure with respect to spherulite.

With reference to the accompanying draw ings, each of Figures 1 to 3 shows a micro tome section through a test piece of cast polyamide obtained by the activated anionic polymerization of lauric lactam monomer. The microtome sections were taken by a Zeiss polarization microscope of 320 times magnification.

In this respect, Figure 1 shows a microtome section through a test piece, taken from a cylindrical casting with a diameter of 200 mm.

The casting had a Vicat B softening point of 192"C and a gross density of 1.027 g/cm3.

In comparison with that of Figure 2, taken from a microtome section through a test piece with a Vicat B softening point of 1730 C and a gross density of 1.018 g/cm3, the polished surface shows a higher proportion of crystalline material with respect to spherulite, while in Figure 2 the amorphous proportion is higher.

Figure 3 is a microtome section through a test piece taken at a shot penetration channel, the test piece coming from the same casting as the test piece of Figure 2. This micrograph shows clearly that in the shot penetration channel a material modification has taken place, connected with the temporary plasticis ing of the polyamide 12. After this plasticising, congealing of the material takes place but without the structure still visible in Figure 3 being again attained.

The hard protective layer may comprise a material selected from glass fibres, carbon fibres and steel.

One of the shock absorbing layers may be disposed between said bullet impact side of the armour and the hard protective layer.

Alternatively one surface of the hard pro tective layer may constitute said bullet impact side of the armour.

The hard protective layer acts to effect heating of the shell before it penetrates into the polyamide 12. This preheating of the shell then leads to quicker development in the polyamide 12 of the melting heat and consequently a reduction in the distance through which the shell is braked before it comes to a complete halt. A further method of reducing the material thickness is by providing an air gap between an externally applied layer of harder material and the polyamide 12, this air gap having a shell deviating action so that the shell no longer penetrates in the shot direction into the polyamide 12 but instead rather in a direction slightly deviating therefrom.A shell deviated by such an interface converts its kinetic energy more quickly into melting heat and work of deformation, so that the plasticised state is reached in the polyamide 12 and the shell congealed therein after a shorter depth of penetration.

The following test data was obtained for a plate of polyamide 12 and examples of armour according to the invention. A 50 mm thick plate of polyamide 12 with a Vicat B softening point of 1730C and a gross density of 1.018 g/cm3 was bombarded at a distance of 50 m with a SmK shell of calibre 7.62 mm X 51. The shell shot completely through smoothly without any subsequent visibility of the shell penetration channel. A 9 mm thick HH steel plate was then laid fully on the shell impact side of this plate to form two-layer armour (example 1) and further bombardment of the two-layer armour was undergone under the same conditions. The shell remained inserted in the polyamide 12 after a penetration depth of 44 mm.For a further bombardment the 9 mm thick HH steel plate was replaced by a 7 mm thick HH steel plate and an air gap of about 2 mm was provided between the HH steel plate and the polyamide 12 plate (example 2). For this bombardment the penetration depth was only 22 mm, showing that in contrast to the previous bombardment in which the shell had penetrated into the polyamide 12 in the shot direction orthogonal to the armour, in this case the shell was deviated.

In addition, in this last bombardment there was no crack formation in the polyamide 12 plate in the region of the shell nose which had remained inserted therein. In contrast, crack formation had occurred in the case of the armour with the 9 mm thick HH steel plate, but without any material breakage or splintering occuring.

It can therefore be stated that the polyamide 12 in combination with such HH steels, which up to the present time were practically exclusively used for armoured vehicles of military use, provides excellent missile resisting action. Because of its low gross density, it also leads to considerable reduction in the weight of a protective construction. Thus the prerequiste for attaining greater mobility and enlarged suitability for cross-country work for armoured vehicles exists if these are built incorporating polyamide 12.

The use of polyamide 12 also has the advantage that because of its castaliily, larger castings of any desired geometrical shape can be obtained, so that for example vehicles can be more freely conceived. Polyamide 12 also has the advantage that in comparison to metals it has a substantially lower coefficient of thermal conductivity, and this is of particular significance where infrared orientation is used in the military sphere. Polyamide 12 also has a higher damping capacity in comparison to metals for any type of vibration, so that orgonometric conditions may be improved as can for example the instrument mounting for a land vehicle, which is subject to strong vibration.

According to a preferred aspect of the invention, the fuller-resistant armour may comprise a further cast polyamide layer to the rear side of the cast polyamide layer relative to the steel plate, and with several fabric sheets consisting of aramide fibres fixed therebetween. Such aramide fibres, known in particular under the trademark KEVLAR of Messrs. Du Pont, are characterised by a high resistance to extension and a low specific gravity, and have already been used for soft protective constructions such as bulletproof shirts, vests and jackets and other protective clothing, and for rigid protective constructions such as protective helmets, bulletproof shields, armoured plates and seats for vehicles, helicopters and aircraft.

If this intermediate applicarion of fabric sheets consisting of aramide fibres is made then on bombardment the fabric sheets exercise a particularly positive retention action on the penetrating shell, and because of the tem porary plasticity of the polyamide 12 due to the conversion of the kinetic energy of the shell into melting heat and work of defor mation, the fabric sheets are drawn into the shell penetration channel together with the shell. As the aramide fibres have practically the same resistance to stretching as the polyamide 12, and this resistance to stretching is very high, a correspondingly high resistance to flow is set up against the penetrating shell.

This resistance to flow is surprisingly not of the same order if under the same conditions armour is bombarded in which the steel plate is combined with only a single cast polyamide layer having the same material thickness as the two cast polyamide layers with the intermediate fabric sheets of the armour according to the invention. Thus it is possible under comparable conditions to use a further reduced material thickness, so that the armour can also be considered for the soft protective constructions of the aforesaid type. Alternatively, a steel of lesser quality than the HH steels may be chosen, but a sufficient strength must be maintained in order to offer the impacting shell a sufficiently high resistance.

In practical tests on such armour it was found that the vibration damping properties of the polyamide 12 in this combination with the fabric sheets of aramide fibres is particularly satisfactory and thus the suitability of this armour for soft protective constructione is optimum. In soft protective constructions the usefulness of which is judged not only by their impact resistance to bodies projected at them but also by their inherent vibration behaviour, the impacting shell leads to greater elastic deformation than in the case of rigid protective constructions, and this causes impact injuries by transmission to the protected part of the body. However, because polyamide 12 gives a greater braking action, its use in such a soft protective construction prevents these impact injuries.The polyamide 12 also, by its vibration-damping behaviour, prevents transmission of an excessive vibration energy to the body portion protected by the pro tective construction concerned, so that it protects against corresponding injury danger.

In these comparisons it is obviously always presupposed that each bombardment is carried out under comparable conditions. Consequently armour according to the invention withstands larger and stronger shell calibres for the same material thickness than any other presently known material, or in other words for a given shell a thinner material thickness is necessary to reliably prevent complete penetration.

In accordance with this latter embodiment, such armour may for example comprise six layers, namely a 2 mm thick steel plate on the shell impact side, a 20 mm thick cast polyamide layer behind it, a layer of five fabric sheets consisting of aramide fibres, a further 20 mm thick cast polyamide layer, a further 2 mm thick steel plate and a final layer of fifteen fabric sheets consisting of the same aramide fibres (example 3). Bombarding this armour at 10 m with a shell of calibre 7.62 mm X 51 gives the useful result that the shell is retained in the last layer of the fifteen fabric sheets, but a noticeable impact affect occurs.Such an impact effect however is no longer determinable in the case of armour consisting of a 2 mm thick steel plate, an intermediate layer of eight fabric sheets of aramide fibres, an 8 mm thick cast polyamide layer, a further intermediate layer of six fabric sheets of aramide fibres, an 8 mm thick cast polyamide layer, an intermediate layer consisting of six fabric sheets of aramide fibres an 8 mm thick cast polyamide layer and a final layer consisting of fifteen fabric sheets of aramide fibres (example 4). The shell is then retained in the penultimate layer, i.e. in the 8 mm thick cast polyamide layer. In both of these examples a high strength steel was used.

These bombardment results are less critical in the case of less strong ammunition. Thus in this regard the armour may give sufficient missile resistance under such other require menus by a different construction. Thus for example, the individual cast polyamide layers of a multi-layer armour may be of different thickness and/or the number of intermediate fabric sheets may be different. The desirably particularly thickly woven fabric sheets may be impregnated as the fabric sheets should then give a greater reliability of retention.

WHAT WE CLAIM IS:- 1. A bullet-resistant armour comprising at least two layers including a hard protective layer and at least one shock absorbing layer comprising a cast polyamide obtained by the activated anionic polymerization of lauric lactam monomer and having a Vicat B softening point of not more than 1830C, the shock absorbing layer or at least one of the shock absorbing layers being disposed more remotely from a bullet impact side of the armour than the hard protective layer.

2. A bullet-resistant armour according to claim 1, wherein said polyamide has a Vicat B softening point in the range from 161"C to 168"C.

3. A bullet-resistant armour according to either preceding claim, wherein said hard protective layer comprises a material selected from glass fibres, carbon fibres and steel.

4. A bullet-resistant armour according to any of claims 1 to 3, wherein one of said shock absorbing layers is disposed between said bullet impact side of the armour and the hard protective layer.

5. A bullet-resistant armour according to any of claims 1 to 3, wherein one surface of the hard protective layer constitutes said bullet impact side ot the armour.

6. A bullet-resistant armour according to any preceding claim, wherein said shock absorbing layers comprise at least two layers of said cast polyamide, said layers being disposed more remotely from said bullet impact side of the aromur than the hard protective layer, a plurality of fabric sheets composed of aramide fibres being disposed between said layers of polyamide.

7. A bullet-resistant armour according to claim 6, further comprising a plurality of fabric sheets of aramide fibres on the cast polyamide surface most remote from said bullet impact side of the armour.

8. A bullet-resistant armour according to claim 6 or 7, wherein the hard protective layer is a steel plate.

9. A bullet-resistant armour according to claim 8, as appendant to claim 7, comprising a second steel plate disposed adjacent to and forward to said additional fabric sheets most remote froin, said bullet impact side of the armour.

10. A bullet-resistant armour according to any preceding claim, comprising in addition a plurality of fabric sheets composed of ara mide,fibres betwen said hard protective layer and the shock absorbing layer or one of the shock absorbing layers disposed more remotely from said bullet impact side of the armour than the hard protective layer.

11. A bullet-resistant armour according to claim 1, wherein the cast polyamide is substantially as described herein with reference to and as illustrated in Figures 2 and 3 of the accompanying drawings.

12. A bullet-resistant armour according to claim 1, substantially as described herein with reference to any of examples 1 to 4.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. the shell impact side, a 20 mm thick cast polyamide layer behind it, a layer of five fabric sheets consisting of aramide fibres, a further 20 mm thick cast polyamide layer, a further 2 mm thick steel plate and a final layer of fifteen fabric sheets consisting of the same aramide fibres (example 3). Bombarding this armour at 10 m with a shell of calibre 7.62 mm X 51 gives the useful result that the shell is retained in the last layer of the fifteen fabric sheets, but a noticeable impact affect occurs.Such an impact effect however is no longer determinable in the case of armour consisting of a 2 mm thick steel plate, an intermediate layer of eight fabric sheets of aramide fibres, an 8 mm thick cast polyamide layer, a further intermediate layer of six fabric sheets of aramide fibres, an 8 mm thick cast polyamide layer, an intermediate layer consisting of six fabric sheets of aramide fibres an 8 mm thick cast polyamide layer and a final layer consisting of fifteen fabric sheets of aramide fibres (example 4). The shell is then retained in the penultimate layer, i.e. in the 8 mm thick cast polyamide layer. In both of these examples a high strength steel was used. These bombardment results are less critical in the case of less strong ammunition. Thus in this regard the armour may give sufficient missile resistance under such other require menus by a different construction. Thus for example, the individual cast polyamide layers of a multi-layer armour may be of different thickness and/or the number of intermediate fabric sheets may be different. The desirably particularly thickly woven fabric sheets may be impregnated as the fabric sheets should then give a greater reliability of retention. WHAT WE CLAIM IS:-

1. A bullet-resistant armour comprising at least two layers including a hard protective layer and at least one shock absorbing layer comprising a cast polyamide obtained by the activated anionic polymerization of lauric lactam monomer and having a Vicat B softening point of not more than 1830C, the shock absorbing layer or at least one of the shock absorbing layers being disposed more remotely from a bullet impact side of the armour than the hard protective layer.

2. A bullet-resistant armour according to claim 1, wherein said polyamide has a Vicat B softening point in the range from 161"C to 168"C.

3. A bullet-resistant armour according to either preceding claim, wherein said hard protective layer comprises a material selected from glass fibres, carbon fibres and steel.

4. A bullet-resistant armour according to any of claims 1 to 3, wherein one of said shock absorbing layers is disposed between said bullet impact side of the armour and the hard protective layer.

5. A bullet-resistant armour according to any of claims 1 to 3, wherein one surface of the hard protective layer constitutes said bullet impact side ot the armour.

6. A bullet-resistant armour according to any preceding claim, wherein said shock absorbing layers comprise at least two layers of said cast polyamide, said layers being disposed more remotely from said bullet impact side of the aromur than the hard protective layer, a plurality of fabric sheets composed of aramide fibres being disposed between said layers of polyamide.

7. A bullet-resistant armour according to claim 6, further comprising a plurality of fabric sheets of aramide fibres on the cast polyamide surface most remote from said bullet impact side of the armour.

8. A bullet-resistant armour according to claim 6 or 7, wherein the hard protective layer is a steel plate.

9. A bullet-resistant armour according to claim 8, as appendant to claim 7, comprising a second steel plate disposed adjacent to and forward to said additional fabric sheets most remote froin, said bullet impact side of the armour.

10. A bullet-resistant armour according to any preceding claim, comprising in addition a plurality of fabric sheets composed of ara mide,fibres betwen said hard protective layer and the shock absorbing layer or one of the shock absorbing layers disposed more remotely from said bullet impact side of the armour than the hard protective layer.

11. A bullet-resistant armour according to claim 1, wherein the cast polyamide is substantially as described herein with reference to and as illustrated in Figures 2 and 3 of the accompanying drawings.

12. A bullet-resistant armour according to claim 1, substantially as described herein with reference to any of examples 1 to 4.