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Effect of Blast Loading On a Person Protected By Body Armour

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  1. Executive Summary

The ballistic threat has been presented to military forces for hundreds of years in one form or another, from spears to bomb fragmentation. A lot of work has gone into countering this ballistic threat and the protection currently provided is very good. The standards of body armour ballistic testing is rigorous. Due to the modern asymmetric battlefield that modern military personnel now face, the threat has evolved. More often, the personnel are being subjected to blast from improvised explosive devices emplaced by irregular forces. This paper will review the some of the research that has been carried out into the effects of blast loading on a person protected by body armour. British Army studies show there was a higher incidence of primary blast injury in fatally injured soldiers wearing body armour that in civilian bystanders (Committee on Testing of Body Armor Materials for Use by the U.S. Army 2012) involved in IED incidents in Northern Ireland. This paper will explore the effects of blast loading on a person wearing body armour.

  1. Introduction.

Soft body armour is designed primarily to protect the wearer from the most dangerous battlefield threat, projectiles, and fragmentation, with the 3 main testing standards for body armour (NIJ (U.S. Department of Justice 2008), HOSDB (Croft & Longhurst 2007) and STANAG 2911 (North Atlantic Treaty Organization 2015)) only specifying testing criteria for ballistic protection. Due to the increasing use of improvised explosive devices (IEDs) in modern asymmetric warfare, there is a growing risk of personnel being subjected to blast associated injuries (Tranchemontagne 2016). This paper will review the effects of blast loading on a person protected by body armour; this will be broken down into:

  1. Theory of blast impact,
  2. Research into blast loading on different materials.
  1. Theory of blast impact.

Blast waves are generated from the rapid expansion of gases produced during an explosion during which a shock wave, travelling faster than the speed of sound, propagates from the source of the explosion omni-directionally. One of the most important characteristics of a blast to consider is peak overpressure, which is, in effect, the strength of the blast. A further and equally important characteristic is the positive impulse duration, which determines the time that the force will be imparted on any object or structure. Both the peak overpressure and the positive impulse duration need to be known to effectively calculate the effects of a blast on an object. If you have two blasts of equal peak overpressure but one is acting for longer, the total force imparted on the object will be greater for the longer impulse and will affect the object more. Figure 1 shows an ideal blast wave representation. The pressure increases almost instantaneously to the peak overpressure and decays over time, a negative pressure phase then follows.


Fig 1. Ideal blast wave resulting from an explosion in air. (Goel et al. 2012)

The effect a blast wave has on a person is complicated due to how the blast wave interacts with the irregular shape of the human body. Blast wave diffraction around the human body results in a complex pressure load on the body all of which need to be understood to successfully gauge how a wave will affect a person. Figure 2 shows how a blast wave interacts with an irregularly shaped object, such as a human body.

Fig 2. Illustrating the blast wave interaction with an irregularly shaped object. (Gibson 1989)

Initially, a portion of the blast wave is reflected from the front of the body. The outer parts of the shock wave continue and diffracts around to the rear of the body where they are weakened. Rarefaction waves move across the front of the body (1), reducing the peak pressure of the reflected wave whilst vortices form at the rear of the body (2) and (3). The complicated gas flow means that the body is loaded for an extended time (Gibson 1989). The injuries caused by this peak initial pressure/positive impulse duration are known as primary blast injuries (PBI). The organs most susceptible to PBI are the gas-filled organs such as the ears, the lungs, and the gastrointestinal tract. The mechanism of injury on these organs is a form of barotrauma, which is an injury, caused by the pressure differential of the internal organs and the outer surface of the body at the moment the pressure wave impacts.

Blast waves can be sub-divided into stress waves and shear waves. Shock waves are a special type of stress wave, which are longitudinal like sound waves but travel much faster, and cause primary blast injury (PBI) of the lung and small bowel. Shear waves are trans- verse waves, characterised by long duration, low velocity, and cause compression of visceral structures. In the abdomen, stress waves cause damage at the microscopic level, whereas the shear wave causes tearing of the tissues due to gross body wall and visceral motion. Laceration of solid abdominal viscera is related to very high blast loading (Housden 2012).

  1. Research into the effects of blast loading on a person protected by body armour.

An interim report produced by the US Army Natick RDE Centre in 1989, titled "Response of Clothing Materials to Air Shock Waves" (Gibson 1989) provided remarkable insight into blast loading of high impact shock waves. The report focuses on testing the then, in-service body armour of the US Army (The Personnel Armor Systems, Ground Troops (PASGT)). The report covered four areas:

  1. Blast wave characteristics,
  2. Blast biology,
  3. Blast protection,
  4. Blast attenuation by porous and compressible materials.

The main objective of the report was to ascertain how blast waves interacted with PASGT and it stated there was previous evidence that the materials contained within the body armour were causing a "blast amplification effect". Initial thoughts were that this amplification might have been due to how the blast wave propagated through the multi-layered design of the armour.

The report described the construction of the body armour (13 layers of Kevlar® 29 cloth sandwiched between an inner and outer nylon shell fabric). The report explains that the armour was designed purely to provide ballistic protection and no thought had gone into providing protection against blast effects.

The report's main focus was the lack of protection afforded to the wearer of the body armour from enhanced blast weapons such as Fuel-Air Explosives (FAE). A person can sustain injuries from a blast with overpressures that result in ranges as little as 10 - 20 psi whereas a typical FAE event can produce much more substantial psi, as high as 300. In this circumstance, the lack of protection armour would provide to the wearer was highly concerning. Prior to this report, a lot of basic work on the response of mammals to blast waves had been conducted by The Lovelace Foundation, The Research Institute of National Defence (Sweden), and the Walter Reed Army Institute of Research. This work identified how various characteristics of the blast can vary the effect of the blast on a body, such as the orientation of the body in relation to the blast wave.

In one study conducted by Walter Reed Army Institute of Research, human volunteers were subjected to low level blast waves. The volunteers' internal lung pressure was measured during the study whilst wearing several different types of protective clothing. The study showed that the volunteers wearing the PASGT body armour gave test measurements showing the greatest increase of internal lung pressure. This would suggest that the body armour would increase the risk of lung damage at higher blast overpressures. This study was extended to higher blast overpressures using sheep instead of human volunteers. Half of the sheep were fitted with PASGT body armour. During this testing, the level of damage to the sheep's lungs was measured by the percent increase of the lung weight. An assumption was made that this would directly relate to the blast damage. This extended study showed that the sheep wearing body armour displayed far greater blast damage to their lungs than the unprotected sheep.

Analysis of the experimental data on how shock waves were transmitted through layers of material showed that there was a significant increase in the reflected pressure when compared to testing were no fabric was present. There was a trend of the pressure increasing in line with the number of layers of Kevlar or cotton present. This continued to a certain point. It was noted during these experiments that there was not significant difference between the increases of pressure when using layers of cotton compared to layers of Kevlar. This suggested that the type of material was not as significant as the number of layers and the density of the fabric.

Analysis of the computer modelling showed an increase in peak internal lung pressure of around 50% between the unprotected chest model and the model wearing the body armour. A point to note, the experimental data compiled shows a measured increase of 20% in the human volunteers wearing the body armour compared to the unprotected volunteers. This disparity could be due to the difficulty in modelling the complex reaction of blast waves influencing a human body.

The report concluded that soft body armour does not offer wearers protection from blast effects. On the contrary, the wearing of soft body armour may actually increase the blast effects on the personnel wearing the armour. The report does indicate that this counter -intuitive phenomena might be reduced with the introduction of hard armour plates to the body armour.

A more recent confirmation of these findings were detailed in a journal article titled, "Shock Enhancement Effect of Lightweight Composite Structures and Materials" (Zhu et al. 2011). This review looks at the broader range of lightweight materials that are used to provide protection (namely ballistic). The author concludes that the research into the effects of lightweight materials is still very limited but the evidence still points to shock enhancement when using multi layered soft armour as PPE.

A further study into shock attenuation was made following clinical studies into the increasing reporting of brain injury and not pulmonary injury following blast exposures. The article titled, "Attenuation of Blast Pressure Behind Ballistic Protective Vests" (Wood, 2012). This article concludes that following shock tube testing on two variants of body armour (NIJ Level 2 soft armour and NIJ Level 4 hard armour); results show a substantial increase in protection to the torso against blast injuries whilst wearing these armours. The behind armour overpressures were reduced by a factor of 14.2 and 56.8 for the NIJ 2 and NIJ 4 body armour, respectively.

  1. Conclusion

This paper presents a review of the effects of blast loading on a person protected by body armour. A vast majority of the experimental data shows that the wearing of soft body armour enhances shock waves and the person is subjected to much higher peak overpressures. The combinations of differences in impedance between the atmosphere, the layers of fabric, the body itself, and the gas filed organs is likely to cause this phenomenon. Figure 3 shows the effect of interfaces in relation to shock waves. Using the shock impedance equation, you can show what is likely to be causing the enhancement of the blast. Stress is conserved across the interface between the air, the body armour, and the person:

Z1 = 0 (Air) ∴ σ2 = 2 x σ1

Assuming that the torso was of a similar impedance to the body armour, the next interface would be the gas-filled organs, which again would be of a lower impedance. This would result in the release of a tensile wave within the chest cavity, which could cause spalling of tissue with an increase of up to a factor of 2.

Fig 3. Illustration of the effect of interfaces (Appleby-thomas 2017).

Although there are many reviews on this subject, most of them seem to be focused on historical work. Teland (2012) noted that most of the blast injury prediction models are based on the Bowen curves, the Bass curves, and Axelsson BTD model. The report mentioned that there were many disadvantages and limitations to using these models and there is a need for further experimentation produce more reliable data. This may be due to the Computational models of how blast waves interact with the body armour and the wearer can be used but the theoretical models are limited. Using extrapolated data from experiments involving humans has limited uses because of the restriction on the size of blast you can subject a volunteer to without them becoming injured. The use of animals is of limited use due to the form fit of the body armour and the different anatomical makeup of the animals.

Throughout the reports, reviews and articles studied during the writing of this paper, it is clear that a need for further study has been identified. Whether these recommendations have been carried out or not is not clear, there are no open source reports providing evidence; it may be that the information is classified.

 

References

  1. Committee on Testing of Body Armor Materials for Use by the U.S. Army, 2012. Testing of Body Armor Materials: Phase III
  2. U.S. Department of Justice, 2008. Ballistic Resistance of Body Armor. NIJ Standard-0101.06.
  3. Croft, J. & Longhurst, D., 2007. HOSDB Body Armour Standards for UK Police (2007) Part 2: Ballistic Resistance.
  4. NATO Standardization Agency, 2003. STANAG 2920 PPS (Edition 2) - Ballistic test method for personal armour materials and combat clothing. , pp.1-F2.
  5. Tranchemontagne, M., 2016. The Enduring IED Problem. Why we need doctrine.
  6. Goel, M.D. et al., 2012. An abridged review of blast wave parameters. Defence Science Journal, 62(5), pp.300-306.
  7. Gibson, P.W., 1989. Response of Clothing Materials to Air Shock Waves.
  8. Housden, S., 2012. Blast injury: A case study. International Emergency Nursing, 20(3), pp.173-178.
  9. Zhu, F., Chou, C.C. & Yang, K.H., 2011. Shock enhancement effect of lightweight composite structures and materials. Composites Part B: Engineering, 42(5), pp.1202-1211.
  10. Wood, G. W. (2012). Attenuation of blast pressure behind ballistic protective vests. Injury Prevention 19(1):19-25. Injury Prevention, 19(1): 19-25.
  11. Appleby-thomas, G.J., 2017. Introduction To Shockwaves - Part 3 - MSc EOE Transition To Detonation. , pp.1-45.
  12. Teland, J.A., 2012. Review of blast injury prediction models.
  13. Kashuk, J.L. et al., 2009. Bomb Explosions in Acts of Terrorism: Evil Creativity Challenges Our Trauma Systems. Journal of the American College of Surgeons, 209(1), pp.134-140.

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