Fibrous armour material

Abstract

According to the invention there is provided a fibrous armor material for dissipating the kinetic energy of a moving object which is impregnated with a shear thickening fluid, in which the shear thickening fluid includes particles of a thickening agent suspended in a liquid, and the volume fraction of the thickening agent in the shear thickening fluid is selected so that the shear thickening fluid has a viscosity-shear stress characteristic substantially corresponding to curve B or lying between curve B and curve D as shown in FIG. 2.

Claims

1. A fibrous armour material for dissipating the kinetic energy of a moving object, said fibrous armour material being impregnated with a shear thickening fluid, in which the shear thickening fluid includes particles of a thickening agent suspended in a liquid, and the volume fraction of the thickening agent in the shear thickening fluid is selected to be in the range 47 to 50% so that the shear thickening fluid has at least one of: a viscosity of 0.11 to 0.21 PaS at a shear stress of 200 Pa, a viscosity of 0.13 to 0.27 PaS at a shear stress of 400 Pa, a viscosity of 0.14 to 0.37 PaS at a shear stress of 600 Pa, and a viscosity of 0.16 to 0.45 PaS at a shear stress of 800 Pa.

2. A fibrous armour material according to claim 1, in which the volume fraction of the thickening agent in the shear thickening fluid is selected so that the shear thickening fluid has at least one of: a viscosity of 0.13 to 0.21 PaS at a shear stress of 200 Pa, a viscosity of 0.15 to 0.27 PaS at a shear stress of 400 Pa, a viscosity of 0.19 to 0.37 PaS at a shear stress of 600 Pa, and a viscosity of 0.22 to 0.45 PaS at a shear stress of 800 Pa.

3. A fibrous armour material according to claim 2, in which the volume fraction of the thickening agent in the shear thickening fluid is selected so that the shear thickening fluid has at least one of: a viscosity of about 0.13 PaS at a shear stress of 200 Pa, a viscosity of about 0.15 PaS at a shear stress of 400 Pa, a viscosity of about 0.19 PaS at a shear stress of 600 Pa, and a viscosity of about 0.22 PaS at a shear stress of 800 Pa.

4. A fibrous armour material according to claim 1 in which the volume fraction of the thickening agent in the shear thickening fluid is in the range 48 to 50%.

5. A fibrous armour material according to claim 4 in which the volume fraction of the thickening agent in the shear thickening fluid is in the range 49 to 50%.

6. A fibrous armour material according to claim 1 in which the particles are inorganic particles.

7. A fibrous armour material according to claim 1 in which the particles are silica.

8. A fibrous material according to claim 1 in which the liquid is an organic liquid, a silicone based liquid or aqueous.

9. A fibrous armour material according to claim 1 in which the liquid is ethylene glycol.

10. A fibrous armour material according to claim 1 which contains aramid fibres.

11. A fibrous armour material according to claim 1, incorporated into a protective material for dissipating the kinetic energy of a moving object including a plurality of layers of fibrous armour material, in which at least one of said layers is impregnated with the shear thickening fluid.

12. A fibrous armour material according to claim 1, incorporated into an article of body armour.

13. A fibrous armour material according to claim 1, incorporated into a vehicle.

14. A fibrous armour material according to claim 13, incorporated into an aircraft.

15. A fibrous armour material according to claim 14, wherein the fibrous armour material is present in the aircraft as an engine lining.

16. A fibrous armour material according to claim 1, incorporated into a flexible structure for mitigating the effects of blast.

17. A fibrous armour material according to claim 16, wherein the flexible structure includes a tent or blanket.

18. A fibrous armour material according to claim 1 in which the volume fraction of the thickening agent in the shear thickening fluid is 49%.

19. A fibrous armour material according to claim 1 which includes poly paraphenylene terephthalamide fibres.

Description

(1) Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows penetration depths at various impact velocities obtained from ballistic tests on layers of Kevlar® impregnated with STF's of various volume fractions of silica; and

(3) FIG. 2 shows viscosity vs shear stress characteristics of STF's having various volume fractions of silica.

(4) A number of STF's were prepared by suspending silica particles in ethylene glycol at varying volume fractions. STF's were prepared with silica volume fractions of 43%, 46%, 49%, 52%, 55% and 57%. Samples of protective material were prepared by impregnating structures made up of 4 layers of Kevlar® with the STF's. In each instance, a 4 layer Kevlar® structure was impregnated with 40 g of a STF of a chosen volume fraction of silica.

(5) Ballistic tests were performed on the impregnated protective materials according to methodologies which will now be described. The impregnated layers were stacked on top of each other and retained within a polyethylene bag. These samples were then intimately held against the surface of a witness clay block with strips of elastic. The clay block was conditioned prior to testing in a 30° C. oven for three hours and the face of the block was smoothed to ensure a flat surface was provided. A 4.1 g, 10 mm diameter steel spherical projectile was fired at the samples from a gas gun, which is positioned with respect to the clay block to provide a projectile free flight of ca. 2 m. Careful alignment of the gas gun and target system ensured that the impact on the target was better than ±5 mm of the specified impact point. Prior to impact, the steel projectile passed through a velocity measurement apparatus in the form of two magnetic induction coils. The passage of the projectile through the magnetic field induces a current in the coils. The distance between the coils is known accurately, and hence an estimate of the projectile velocity can be made from the time taken for the projectile to travel between the coils. The method has an accuracy of better than ±2%.

(6) Optical images of the projectile and the deformation of the samples upon impact were captured using a high speed camera positioned obliquely to one side of the target to enable observation of the front face of the sample during impact. The performance of the samples was investigated by comparing the penetration depth and the profile of the penetration of the sample and/or projectile into the clay block. The profile of the penetration is also referred to herein as the back face trauma signature. Measurements of the penetration depth were made from plaster casts of the witness clay using Vernier height callipers. An error of ±1 mm was assigned to each measurement of penetration depth.

(7) FIG. 1 shows the results of the ballistic tests. It can be seen that there is little difference in the impact performance around 160 ms.sup.−1. However, a difference in performance is more noticeable upon perforation. For impacts at velocities approaching 260 ms.sup.−1 and above, there is a marked difference in performance, and the 49% volume fraction STF impregnated sample gives rise to a significantly lower penetration depth. This indicates that the 49% volume fraction STF impregnated sample absorbs most energy upon impact.

(8) The flow properties of a number of the STF's were investigated. More particularly, measurements were made on the STF's having silica volume fractions of 46%, 49%, 52% and 55%. The prepared STF samples were analysed for their flow properties using a controlled stress Malvern Bohlin Gemini rheometer, fitted with a 60 mm diameter/1° taper cone and plate geometry. The plate was set at a temperature of 25° C.

(9) To determine the flow properties of the fluids, the tests were performed in stress controlled mode, whereby an increasing shear stress was applied to the samples, measuring the corresponding viscosity and shear rate.

(10) The following parameters were set to perform these tests:

(11) TABLE-US-00001 Geometry CP1°/60 mm Controlled Mode Stress controlled Shear Range (25-900) Pascals Measurement Time 310 seconds Temperature 25° C. Thermal Equilibrium 60 seconds

(12) The samples were measured at least three times to check the reproducibility of the results. Runs were rejected when the plate had not been completely covered by sample.

(13) The measured viscosity-shear stress characteristics for the STF's are shown in FIG. 2. Curve A corresponds to the 55% volume fraction STF, curve B corresponds to the 52% volume fraction STF, curve C corresponds to the 49% volume fraction STF, and curve D corresponds to the 46% volume fraction STF, The associated ballistic performance of protective materials impregnated with the 46% and 55% volume fraction STF are not highly desirable, and therefore the viscosity-shear stress characteristic curves A and D are not preferred. Good ballistic results can be obtained using STF's with a viscosity-shear stress characteristic curve B, or with a viscosity-shear stress characteristic curve lying between curves B and D as shown in FIG. 2. A preferred group of STF's have viscosity-shear stress characteristic curves corresponding to curve B, curve C or lying between curves B and C as shown in FIG. 2. The best ballistic results are obtained with the 49% volume fraction STF, which corresponds to curve C in FIG. 2. Without wishing to be bound by any particular theory or conjecture, it is believed that the excellent ballistic properties are a consequence of the rheological properties of the STF, in particular the viscosity-sheer stress characteristic. Accordingly, STF's with viscosity-sheer stress characteristics close to curve C as shown in FIG. 2 are most strongly preferred. Table 1 shows the measured viscosities for the 52%, 49% and 46% volume fraction STF's at a number of applied sheer stresses.

(14) TABLE-US-00002 TABLE 1 Viscosity (PaS) at specified shear stress values STF volume Shear Stress (Pa) fraction/% 200 Pa 400 Pa 600 Pa 800 Pa 52 0.21 PaS 0.27 PaS 0.37 PaS 0.45 PaS 49 0.13 PaS 0.15 PaS 0.19 PaS 0.22 PaS 46 0.10 PaS 0.12 PaS 0.13 PaS 0.15 PaS

(15) Fibrous armour material and protective material of the invention can be used in a variety of soft body armour systems. The advantageous property of flexibility can be exploited in order to provide body armour to protect regions of the body which are difficult to protect using conventional materials. For example, it is difficult to provide protection for the neck region due to interference between body armour and any headwear worn by an individual, particularly when in a prone position. Protective material of the invention may be used to provide an anti-ballistic and/or spike resistant collar which is sufficiently flexible to address this problem. Protective material of the invention may be combined with other protective systems. For example, the protective material may be placed behind another armour system such as ceramic armour plates to reduce back face trauma. Such systems could increase the extent of the protection offered and/or reduce the thickness of the armour pack. Pouches of protective material may be provided for this purpose. Spike resistant or anti-ballistic body armour can be made using protective material of the invention. A multiple threat armour which provides spike and ballistic protection can be produced using two or more different protective materials, in which an outer structure is configured to mitigate spike threats and an inner structure is configured to provide ballistic protection.

(16) Protective material of the invention can be used for purposes other than body armour. Examples include spall liners for vehicles, blast tents or like structures for blast containment, and engine or turbine linings, especially linings for aircraft engines, for containing detached moving parts or fragments.