GB2242193A - Material for protective clothing - Google Patents

Abstract

A material suitable for use in protective clothing includes a layer of lightweight and flexible material having distributed therethrough a hard dense additive material having a high acoustic impedance. The flexible material can be plastic foam and the additive material can be glass spheres or metallic (e.g. iron) powder. The clothing can be used for protection against blast, in conditions where explosions might occur.

Description

MATERIAL FOR PROTECTIVE CLOTHING The present invention relates to protective clothing, and in par ticular to protective clothing for prztection against blast.

Many designs of protective clothing have been suggested for the use of personnel working in conditions where explosions might occur. An explosion (for example the detonation of a bomb) may cause injuries in a number of ways. Damage, particularly pulmonary and bowel contusions, caused by the impact of a pressure wave (blast) upon the body wall; penetrating and non-penetrating injuries resulting from the impact of debris; and skeletal injuries, often associated with soft tissue injury, produced by gross displacement of the body. These three effects are commonly known as primary, secondary and tertiary effects.

It is known that some lightweight and flexible materials, such as plastic and rubber foams, which are generally useful in protective clothing, can actually exacerbate the extent of blast injury. This fact reduces the usefulness of these materials in protective clothing.

There is, therefore, a requirement for a material suitable for use in protective clothing which gives improved protection against blast damage.

According to the present invention a material suitable for use in an article of protective clothing includes a layer of lightweight and flexible material having distributed there-through a hard dense additive material having a high acoustic impedance.

The flexible material may be, for example, a foam plastic. The additive material may be, for example, glass spheres or a metallic (for example, iron) powder.

Direct impact of a pressure wave on a person's chest wall (blast) has two effects; gross deformation of the body, and the transmission of a high velocity stress wave through the body tissue. A great deal of research work has been carried out in an attempt to provide protection against the transient gross deformation of the body caused by blast.

Much of this research work has been centered around the transient deformation of the thoraco-abdominal system under load, on the assumption that lung damage is linked to the gross inward movement of the chest wall.

This assumption fails, however, to explain fully some of the phenomena observed when different materials are used as potentially protective systems for the body. More recent research efforts have found that primary blast injury does not seem to depend on an increase in intrathoracic pressure due to the gross deformation of the body but that such injury appears to result from stress wave interactions. The transmission of stress waves through lung parenchyna may lead to pressure differentials across the microvasculature which result in failure and haemorrhagic contamination.

The manner in which the Applicant investigated the effects of high velocity stress waves, arrived at the present invention, and some embodiments of the present invention are now described, by way of example only, with reference to the accompanying diagrammatic figures, of which: Figure 1 is an elevation, in section, of the test equipment used, Figure 2 is a graph illustrating blast wave effects without and with various forms of protection, Figure 3 is a graph of blast effects using various types of metal sheet protection, Figure 4 is a graph of stress wave transmission against material acoustic impendances, Figure 5 is a histogram of maximum transmitted stress for various materials, including protective materials according to the invention, and; Figure 6 is a graph of second peak over-pressure against weight of protective sample.

An experimental apparatus (Figure 1) forming part of a shock tube facility includes a cylinder 10 closed by a soft tissue simulant 11. A first pressure transducer 12 is positioned on the surface of the tube 10, and a second transducer 13 at the centre of the soft tissue simulant 11 external to the tube. A sheet of sample material 14 can be fitted to the surface of the soft tissue simulant 11 within the tube 10. Tests are carried out by subjecting the inside of the tube 10 to shock waves as illustrated by arrows 15. The first transducer 12 monitors incident and reflected shock waves, and the second transducer 13 measures stress transmitted into the soft tissue simulant 11.

As stated above, it is known that rubber foam exacerbates primary blast injury. It is also known that rigid metallic plates (which are, of course, usually impracticable in protective clothing) reduce primary blast injury.

Samples of these materials were therefore placed within the experimental configuration of Figure 1 to provide an experimental model and an initial appraisal of the stress transmitted. The pressure-time histories from the second transducers 13 were compared, figure 2, for the following three conditions: (a) Material sample: None (continuous line) (b) Material sample: Rubber foam of a type known to exacerbate primary blast injury (dotted line) (c) Material sample: Copper plate knownto reduce primary blast injury (chain dotted line) From Figure 2 it can be seen that the pressure recorded at transducer 13 5 strongly affected by the material sample present in the experimental configuration.The waveform recorded in the absence of material sample demonstrates two phases; an initial sharp pressure peak considered to represent transmission of the stress wave and a subsequent lower frequency oscillation considered to be the gross compression of the system. The same two phases are also present when the material sample was rubber foam but the maximum values of over-pressure reached are much greater. However, only the lower frequency response is present when the copper plate was placed in the experimental configuration. The materials used in these experiments varied considerably in many aspects, including mass, stiffness and acoustic impedance. It is therefore unwise to speculate on the damage causing mechanism from these preliminary results.However, there is evidence of twodistinct phenomena, which can be supposed to be gross move- ment and stress wave transmission. Further controllcd experimentation was required in order to confirm this supposition.

It was assumed that the movement of this system could be reasonably approximated by a linear oscillator. Hence if the mass of the material samples were the same in each case the gross deformation should be similar in each experiment. The next phase of work therefore investigated transmission through different metal plates of identical areal densities.

Three metals were chosen as the next material samples. The samples were selected so as to exhibit a wide range of acoustic impedances yet have the same areal densities. Table 1 lists the samples used in this experiment.

MATERIAL WEIGHT THICKNESS ACOUSTIC IMEDANCE DESIGNATED Kg mm x 1G6 kg/m .s IN Fig 3 BY: Aluminium 0.978 10 17.1 Full line Copper 0.967 3 42.0 Dotted line Steel 1.135 4 46.6 Chain dotted line Table 1 The pressures recorded at transducer 13 for these three samples are shown in Figure 3. It can be seen from the pressure-time histories that the two peaks on each waveform again indicate the occurrence of two distinct phenomena. However, the second peak in each case was coincidental and of the same size and shape indicating that this pressure rise was probably due to the gross movement of the system. Figure 4 shows the maximum value of this first peak, considered to represent stress wave transmission, against the acoustic impedance of the material sample.

As the acoustic impedance of the sample increased then the maximum value of this first pressure peak decreased. This behaviour is consistent with the assumption that the first pressure rise in each case is due to stress wave transmission.

The experimental simulation studies desciibed above provide a means of separately identifying gross deformation and stress wave transmission effects. The combination of these studies and biological trials supports the hypothesis that the direct stress wave transmission is the main c=,e of primary lung injury. Furthermore a means is provided for identifying materials which may ameliorate rather than exacerbate lung injury.

Various materials according to the invention were then tested. These comprised a layer of lightweight and flexible material having distributed therethrough a hard dense additive material having a high acoustic impedance. The incident stress wave in these materials is disrupted sufficiently to significantly reduce the risk of primary blast injury without the disadvantages involved in using rigid metallic plates.

First, a material according to the invention was formed by impregnating a foam material with varying concentrations (0.5%, 1%, 2% and 3%) of glass spheres. The characteristic double peak was again present in the pressure-time histories recorded at transducer 13. Figure 5 shows the maximum value of the first, stress related,peak compared to the material sample used.

The acoustic impedance of the glass used in this experiment was approximately 13 x 106 kg/m 2.s and it was thought probable that the value of this first peak could be further reduced by the use of an additive with a higher acoustic impedance without incurring a very great weight penalty.

In order to ascertain that the same concentration of a material with a higher acoustic impedance was effective against the stress wave, foam samples, according to the invention, were manufactured containing varying concentrations of iron powder. Iron has an acoustic impedance of 46.6 x 106 kg/m Again in all the resultant pressure-time histories recorded by transducer 13 two pressure peaks were present. Table 2 shows the maximum values of the first and second peaks for the different concentrations.

MAXIMUM VALUE SAMPLE MATERIAL WEIGHT FIRST PEAK SECOND PEAK (stress) (movement) g bar bar None - 0.608 1.048 Foam 34.6 0.752 1.135 Foam + 0.5% Fe 44.0 0.480 1.472 Foam + 1.0% Fe 63.3 0.184 1.104 Foam + 2.0% Fe 103.2 0.152 j 1.056 Foam + 3.0% Fe 139.6 - 0.200 1.024 Table 2 It can be seen from Table 2 that the maximum value of the first, stress related, peak was significantly reduced at 1% Fe.

The experiment also lent support to the theory that the second peak was due to the gross deformation of the system. If the movement was approximated by a linear oscillator as the weight of the system increased less movement would take place and this would be reflected by a lower second peak over-pressure at transducer 3. Figure 6 shows the weight of the sample against the maximum over-pressure of the second peak. This figure shows quite clearly that as the weight of the sample increased then the value of the second peak decreased.

It will be realised that other materials can be used to form materials according to the invention. For example materials other than plastic foam might be used as the basic material, and alternative hard dense additive materials might be used, the transmitted stress being reduced, in general, more as the acoustic impedance of the second material increases, as can be seen from Table 3 below.

ACOUSTIC IMPEDANCE PEAK OVER-PRESSURES/bar MATERIAL x 106 kg/m2.s 0.25% 1 0% Tungsten 105.01 0.440 - Nickel 53.7 0.440 0.064 Iron 46.6 0.520 - Copper 42.0 0.448 0.120 Zinc 30.0 0.504 0.128 Titanium 27.3 5.520 0.088 Lead 22.3 0.464 0.060 Aluminium : 17.1 0.480 0.688 Foam 0.552 0.720 None - 0.540 0.632 Table 3 These experiments have successfully demonstrated that if the incident stress wave is sufficiently disrupted in its path then the first pressure peak, considered to represent stress transmission - the probable mechanism of primary lung injuries, can be reduced by a material according to the invention.

Claims (6)

1. What is claimed is: 1. A material suitable for use in an article of protective clothing including a layer of lightweight and flexible material having distributed therethrough a hard dense additive material having a high acoustic impedance.
2. 2. A material as claimed in Claim 1 wherein the additive material is in the form of glass spheres.
3. 3. A material as claimed in Claim 1 wherein the additive material is in the form of a metallic powder.
4. 4. A material as claimed in Claim 3 wherein the metallic powder is iron.
5. 5. A material, for use in an articleofprctective clothing, substantially as herein described with reference to Figures 5 and 6 of the Accompanying Drawings.
6. 6. An article of clothing containing a material as claimed in any one of Claims 1 to 5.