Patent Application
20140311326
This application claims priority to FR 13002111.6 filed Apr. 22, 2013 the content of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to the manufacture of armor shielding panels for the protection of vehicles from perforating projectiles and fragments projected during an impact.
2. Description of Related Art
In general, an armor shield comprises a metal panel typically made of steel, aluminium, titanium or alloys of these metals. Such panels usually have an excellent capacity for absorption of the kinetic energy of the perforating projectile during an impact. However, in particular if they are made of steel or a titanium alloy, such panels are heavy and consequently have a low efficiency in terms of energy absorption relative to the weight transported by a vehicle. Titanium alloy panels usually give the best armor shielding protection but they are very expensive and heavy.
The armor shielding panel has a face exposed to impacts and a rear face. When there is an impact on a metallic armor panel, the armor-piercing projectile might be stopped completely in the panel but damage to the rear face of the panel can cause the formation of fragments which can become more dangerous than the projectile stopped by the panel when they are violently ejected from the panel (towards the inside of the vehicle).
Composite panels have been developed that have a greater projectile stopping capacity and lower sensitivity to fragmentation, thus giving better performances relative to the weight transported by the vehicle. But these are composite products comprising ceramic products placed on the face exposed to shocks of a support plate, itself composite, usually based on carbon, glass and polymers with a high molecular weight. Such products are very expensive.
The efficiency of armor panels is usually characterised by two types of tests. The first test is designed to quantify their capacity to stop piercing projectiles. This is referred to as “AP” (“Armour Piercing”) and characterises the resistance to perforation. The second test is designed to quantify their capability to withstand the impacts of fragmented debris. This second type of test is referred to as “FSP” (“Fragment Simulated Projectiles”). During these tests, the armor panels are the target of different shaped projectiles (spindle shape for the AP test, larger and more squat form projectiles for FSP tests). In each type of test, several projectile geometries are used depending on the thickness of the tested panel and the nature of the threats that said armor panel is designed to protect.
For both tests, the capacity to stop projectiles and absorb their kinetic energy without emitting dangerous debris is quantified by a velocity V50 that is defined for example in standard MIL-STD-66; V50 is the average velocity reached by projectiles at the time of the impact obtained using an equal number of results with the highest partial penetration velocities and results with the lowest complete penetration velocities, the velocity being imposed within a specified range.
In general, the material from which the armor panel is made rarely has a good AP-FSP compromise, regardless of whether it is a ceramic, steel, an aluminium alloy or titanium alloy. When it has good resistance to armor piercing, its FSP resistance is often mediocre. Conversely, a material with good FSP resistance often has mediocre AP resistance.
Patent application US2011/0252956 discloses metallic armor panels composed of at least two layers of different aluminium alloys that are metallurgically bonded together. The intimate metallurgical bond between these two layers typically results from transformation procedures such as co-rolling, multi-layer casting, or casting to obtain a controlled gradient of the concentration of an element such as magnesium within the plate thickness. Alloys are chosen and positioned within the plate thickness such that one alloy gives the plate good resistance to perforation and the other gives it good FSP resistance. However, making such panels requires the use of complex and expensive processes.
The applicant attempted to develop a armor shielding system particularly adapted to fast vehicles such as military vehicles, typically on wheels, with better efficiency in terms of AP and FSP protection relative to the transported weight, that is easier to make and less expensive than known products.
A first purpose of the invention is a armor panel comprising an aluminium alloy plate characterised in that:
a) said aluminium alloy has the following chemical composition expressed in percentages per weight:
5.1%≦Zn≦9.7%
1.5%≦Mg≦2.9%
1.2%≦Cu≦2.1%
Si≦0.4%
Fe≦0.5%
Mn≦0.3%
Cr≦0.28%
Ti≦0.2%
Zr≦0.15%
the remainder being aluminium and inevitable impurities, the content by weight of each element being less than 0.05%, and the sum being less than 0.15%;
b) said plate comprises a face oriented towards the projectiles and a face opposite said face oriented towards projectiles coated with a composite reinforcing layer comprising reinforcing fibres or bands with high mechanical performance that confers a high ballistic protection capability on them. Such reinforcing fibres or bands with a high ballistic protection capability may be made from one or several materials belonging to the group including:
glass with high mechanical performance such as R, H, S glass or preferably S2 glass;
aramids, preferably para-aramids such as Kevlar®;
High Performance PolyEthylenes (HPPE) or Ultra-High Molecular Weight PolyEthylenes (UHMWPE or UHMW), that are strongly oriented polyethylenes in the form of fibres, threads or bands, for example Tensylon®.
Advantageously, said composite reinforcing layer comprises one or preferably several single-directional or woven fabrics made from threads comprising fibres with a high ballistic protection capability.
The threads or bands are preferably impregnated with a thermoplastic or thermosetting resin, typically a modified PVB (polyvinyl butyral) phenolic resin. The fabrics may be made by weaving several weaves (single-directional, basket weave, fabric stiffener, etc.). The composite reinforcing coating is obtained for example by stacking the fabrics on each other and then compressing them hot.
When values or ranges are listed herein, the value itself is included. For example, “more than X” can also include X.
The term “plate” is used to refer to a flat product that may actually be a sheet or a plate with a thickness of more than 5 mm, preferably more than 20 mm, typically close to 20-30 mm. The width/thickness ratio of the plate is preferably but not necessarily more than 10. The thickness of the composite panel is typically less than 50 mm, preferably less than 40 mm and it has a mass per unit area less than 125 kg/m2, preferably less than 110 kg/m2, and even more preferably less than 100 kg/m2. The advantage of such armor panels is that they provide the best possible AP and FSP protection with the lowest possible mass per unit area. Thus, a composite panel according to the invention has a mass per unit area of less than 90 kg/m2, or even less than 85 kg/m2, it might also be possible to achieve protection level 5 defined in STANAG 4569 (V50FSP=960 m/s with a 20 mm calibre projectile fired from 25 m).
The aluminium alloy plate includes one face oriented towards the projectiles that may receive the impact directly, or that on the contrary may be protected for example by ceramic tiles. It includes one face opposite said face oriented towards the projectiles that is covered by a composite reinforcing layer although there is not necessarily a continuous bond over the entire contact surface, for example by means of an adhesive binder. For example, all that is necessary is that the composite reinforcing layer is kept fixed to the plate around the periphery of the plate by gluing or any other attachment means, typically mechanical.
We carried out AP and FSP tests on aluminium alloy plates coated or not coated with a composite reinforcing layer comprising aramid fibres. AP perforation tests use 7.62 mm calibre and 35.6 mm long projectiles called “0.30 cal AP M2” that have a steel core, an intermediate lead layer and a copper ogive casing. For the panels according to the invention and the panels for which comparative tests were carried out, FSP tests use 23 mm long steel projectiles called “20 mm FSP”, the cylindrical part of which is 20 mm in diameter.
We observed firstly that in the target range of mass per unit area (50 to 125 kg/m2), the results of perforation tests on the tested panel structures (plates from 19 to 46 mm thick, panels between 30 and 50 mm thick, ratio of the weight of the composite reinforcing layer/total panel weight less than 25%), depended essentially on the alloy of the plate and the mean mass per unit area of the composite panel: a panel made of a given uncoated alloy gives a result (expressed by the velocity V50) practically identical to the result for a panel composed of the same alloy but thinner and coated with a composite reinforcing layer with a thickness such that the assembly has the same mass per unit area. A slight deterioration of AP properties was even observed when the ratio by weight of the layer of composite stiffener/total weight of the panel is less than of the order of 22%. Thus, considering only the criterion for resistance to perforation, an uncoated plate has a significant economic advantage and is smaller for exactly the same or even better performance. Among tested materials, aluminium alloys in the 7xxx series give better results than alloys in the 5xxx and 6xxx series and steels for a comparable mass per unit area.
The results of FSP tests led to a different and surprising finding. The applicant observed that if plates are covered with a composite reinforcing layer comprising reinforcing fibres or bands with a high ballistic protection capability, for example made of aramid fibres, with a mass per unit area more than 0.5 kg/m2, preferably 1 kg/m2, and even more preferably 2 kg/m2, the gain in terms of an increase in V50 as a function of the increase in the mass per unit area of the armor panel, is significantly more for aluminium alloys and particularly for alloys in the 7xxx series.
However, the best AP-FSP compromise is obtained with alloys in the 7xxx series that have a sufficiently high content of zinc and copper. Thus 7039 and 7020, if they are present, associated with a composite reinforcing layer of aramid fibres, have significantly improved FSP performances, but have a relatively poor performance in AP perforation tests.
Thus, the alloy for the plate according to the invention has the following composition, in which contents are expressed in percentages by weight:
5.1%≦Zn≦9.7% preferably 7.5%≦Zn≦8.7%.
1.5%≦Mg≦2.9% preferably 1.8%≦Mg≦2.7%.
1.2%≦Cu≦2.1% preferably 1.4%≦Cu≦2.1%
Si≦0.4% preferably Si≦0.12%
Fe≦0.5% preferably Fe≦0.15%
Mn≦0.3% preferably Mn≦0.2%
Cr≦0.28% preferably Cr≦0.05%
Ti≦0.2% preferably Ti≦0.05%
Zr≦0.15% preferably Zr≦0.05%
other elements≦0.05% individually and≦0.15% total.
Preferably, these alloys are treated to obtain a state not only with high instantaneous mechanical properties (strength UTS, conventional yield stress TYS, elongation at failure E %) but also good toughness. Advantageously, a solution treatment will be carried out followed by quenching and annealing to obtain states such as T6 (maximum UTS), T64 (quenched slightly under-annealed state), or preferably T651 (relaxed quenched by controlled moderate tension and annealing) or even T7651 (relaxed quenched by controlled moderate tension and over-annealing).
In practice, the mass per unit area of the composite reinforcing layer is between 2 and 25 kg/m2. It is preferably less than 20 kg/m2, even more preferably less than 15 kg/m2 mainly due to the cost.
The effect of this composite reinforcing layer on improvement of FSP properties is certainly more accentuated when the mass per unit area of the composite reinforcing layer is high, but it is remarkable that this effect that becomes manifest with aluminium alloy plates, particularly plates composed of the alloy according to the invention, even if the composite reinforcing layer is thin with a mass per unit area of the order of 1 kg/m2, in other words as soon as the panel is coated with three or four woven aramid fabrics.
When the mass per unit area is less than 90 kg/m2, aluminium alloys such as 7xxx have FSP performances lower than the performances of a steel for armor shielding, such as HHS (“high hardness steel”). But when these alloys are combined with a composite reinforcing layer comprising aramid fibres, the FSP results are quickly better than the results for steel, even if the steel is covered with the same type of composite reinforcing layer with a comparable mass per unit area. For example, to obtain the same improvement of FSP performances on a steel plate as is observed on 7xxx plates with a composite reinforcing layer with a mass per unit area equal to only 2 kg/m2, said steel plate needs to be associated with a composite reinforcing layer between 4 and 6 times thicker, all other things being equal.
The effect of the composite reinforcing layer on the improvement of FSP properties is particularly remarkable when the plate is 7449 T651.
Among the different tested composite reinforcing layers, the layer composed of a stack of woven fabrics using Kevlar® 129 threads gave good results regardless of the type of weaving made. Kevlar® 129 grade is known for its lightweight and its high mechanical performances and particularly its high toughness.
7449 T651 plates covered with fabric layers woven from Kevlar® 129 threads have the best AP and FSP performances. This alloy can give a V50 for the FSP test greater than 950 m/s with an armor panel for which the global mass per unit area is less than 95 kg/m2, or even less than 90 kg/m2.
Armor plates were made from thick plates made of different alloys. They were machined to different thicknesses between 25 and 40 mm. Table 1 shows the main constituents of their chemical compositions.
Table 2 shows the state, thickness and average mechanical properties of these plates (tension, transverse longitudinal direction).
Some plates were covered with a composite reinforcing layer comprising a stack of a various number of fabrics woven in threads based on Kevlar® 129 fibres with a linear density of 1330 dtex, coated with polyvinyl butiral (PVB) resin, each fabric having a mass per unit area of about 275 g/m2. Composite reinforcing layers with different thicknesses were made by stacking a variable number of fabrics, and the stack was then hot compressed in a press.
Ballistic perforation tests (“AP tests”) Table 3 contains the results of “0.30 cal AP M2” tests carried out on thick plates coated or not coated with a composite reinforcing layer. When there was a composite reinforcing layer, it was placed on the side opposite the projectile. The mass per unit area of the stack of fabrics woven from Kevlar® 129 threads is given in the fourth column in table 3 below.
It is found that for masses per unit area typically less than 100 kg/m2, the performances of 5083 and 6061 alloys are not as good as the performances of steels such as RHA Steel, which has a lower performance than 7449. AP performances of 7039 plates relative to the mass per unit area are hardly better than steel and significantly lower than 7449. Known AP tests on the 7020 alloy were made with a different projectile and the results are not directly comparable. However, they show that the AP performances of 7020 are not better and are rather worse that the results of 7039.
Once the thick 7449 plates have been coated with woven fabric made of Kevlar® 129 threads, their behaviour is similar or slightly less good than uncoated thick plates, for equal mass per unit area.
Table 4 contains the results of the “20 mm FSP” tests carried out on thick plates coated or not coated with a composite reinforcing layer. When there was a composite reinforcing layer, it was placed on the face opposite the face that will receive the impact of the projectile.
Experimental points obtained with uncoated 7449 T651 show a trend curve parallel to curve for 7039 T64, but with slighter lower FSP performances. Experimental points obtained with 7020 T651 are also on a trend curve approximately parallel to the curve for 7039 T64 but with slightly higher FSP performances. The experimental point of uncoated HHS steel plate is slightly below the trend curve for “RHA Steel”.
Points for 7449 T651 plates coated with a composite reinforcing layer comprising aramid fibres are significantly higher than the curve that contains the FSP results for uncoated plates. The difference, significant even with a thin coat, is greater when the composite reinforcing layer is thicker. Thus, the combination of a 7449 T651 plate with a composite reinforcing layer comprising Kevlar® 129 fibres with a mass per unit area of 10.7 kg/m2 can give a V50 of more than 1100 m/s
The results for the coated 7020 T651 plate also show the significant influence of the composite reinforcing layer. This appears nevertheless lower than that observed on 7449 plates. Furthermore, known AP results on the 7020 alloy suggest that alloys with low copper content such as 7020 and 7039, even associated with a composite reinforcing layer comprising aramid fibres, cannot give a good AP-FSP compromise.
The FSP results for the coated steel plate also show an influence of the composite reinforcing layer, but this is significantly lower.
Table 5 shows FSP results on coated aluminium alloy plates and estimates the gains obtained in comparison with uncoated plates. For each composite panel, the 6th column contains the results obtained for the uncoated plate A composed of the same material as the core of the composite panel and with the same thickness as the composite panel, and the 7th column contains estimated values for an uncoated plate B composed of the same material as the core of the composite panel and with the same mass per unit area as the composite panel. It can be seen that the gain due to the presence of the composite reinforcing layer expressed in terms of an increase of V50, is higher by a factor of between 4.8 and 7.8 for aluminium alloys. This coefficient is of the order of 6.6 for aluminium alloy and only of 4.5 for steel, for the same thickness of the composite reinforcing layer.
For example, it can be seen in
Finally, an analysis of the results leads to the conclusion that protection level 5 as defined in standard STANAG 4569 [V50 greater than 960 m/s for 20 mm FSP tests] can be obtained with a composite panel made of an aluminium alloy with the composition coated by a composite reinforcing layer containing aramid fibres with a mass per unit area of less than 95 kg/m2.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13002111.6 | Apr 2013 | EP | regional |
