Blast Mitigation

March 2006 – Explosives Engineering

Protecting personnel, vehicles and structures from the effects of an explosion is becoming more important due to the increased threat of random terrorist violence. Blast mitigation is the practice of making a blast less severe and is very much the selection of resistant materials, each having its own unique properties and application, in the attenuation and absorption of blast related effects.

Explosive blast has clear and definable elements, a high peak energy, short duration shock wave, high over-pressure, intense heat, a sustained impulsive pressure and high velocity primary and secondary fragments. The particular dominant effects of a blast threat change from circumstance to circumstance; a letter bomb is primarily a shock-wave event, a large vehicle bomb is an impulse dominated threat.

A brief survey of the materials shows each has its own properties, which are unique to certain blast mitigation situations. Materials and options for blast mitigation include:

• Separation distance or stand off, as blast overpressure decreases exponentially with increasing range, simply increasing the separation between the explosion and the object to be protected can significantly mitigate blast. Problems with this approach include the requirement for enough space to create the separation distance and the prospect of moving the blast effect closer to a different object.

• Solid walls or barriers of soil, sand, concrete, composites and steel materials mitigate blast effects through resistance, that is ‘bouncing’ the blast energy off in another direction through shear bulk or material strength. Using soil or sand to absorb the energy in a blast wave can be based on the mass or the tensile strength of the material. Physical properties of soil and sand may vary with age, weather, and cyclic loading. Wall can also be added to existing structures; with concrete walls an elastic/plastic covering can be added on the side opposite the blast (or inside a room) to prevent spalling. Concrete can also be covered with steel, glass fibre or contain metal fibre reinforcement to substantially increase its blast resistant performance. Barriers reflect the blast overpressure causing the incident overpressure to be multiplied, a worst case over eight times. Independent concrete or steel barriers may not be aesthetically pleasing and with the detonation of a large device could produce secondary fragmentation.

• Windows are a special case, as they are usually the first building component to fail in response to blast pressure. Their blast resistance depends on the size, fixing of the pane and frame, as well as the material and processing of the window e.g. annealed plate, polycarbonate, laminated and thermally tempered. Films and coatings have also been widely used to retrofit windows to improve their blast resistance and reduce fragmentation as well as automatic shutters and curtains. Extensive testing has been completed in recent years by both the US and UK governments to analyse the effects of blast on windows, as historically, failed window glazing due to blast has accounted for a large proportion of casualties and loss of use of the facility.

• Water walls or blankets are used to absorb blast and fragments and are normally in the form of modular plastic containers filled with water, or a water and gas mixture, sometimes with the addition of a fragment resistant screen. Attenuation of the blast wave is achieved by transforming it into kinetic energy. The water also suppresses the thermal blast effects by quenching and reduces the velocity of fragmentation. Water blankets have also been used to mitigate the overpressure from an explosion. The blanket is divided compartments that hold a quantity of water, which is dependent upon the type and quantity of explosives.

• Pumice, expanded perlite and vermiculite with an epoxy binder have all been used as barriers and blast absorbers with a number of US patents having been granted for their use for the storage of munitions, explosives transportation and IEDs. In reducing the possibility of sympathetic detonation, it was found that four inches of the pumice material inhibited fragment impact action on nearby warheads, the barrier consisting of hollow aluminium shells filled with the pumice/epoxy composite. Tests have also shown that filling a container with such materials may substantially increase the capacity of a confinement vessel.

• Blast chambers or total containment vessels, with full or partial containment (with venting). The lower weight option comprises of one or several layers of a ballistic material sandwiched between inner and outer layers of a Iightweight, rip stop, fabric material. Others are pressure vessel type construction of a heavy steel sphere or drum, approximately 1.5 inches thick, with a hatch to allow access to the interior and sealing of the vessel. Such vessels are designed to absorb and fully contain the effects of blast and fragmentation but are limited by their weight, ease of movement and size in some applications.

• Liquid and rigid (plastic or metal) foams and sprays, cellular structures possess a great capacity for energy absorption through the collapse of the cells which comprise the structure. These cells can be of random size and shape or honeycomb. Materials used vary from paper and plastic to metallic expanded material or foam. With metallic foams properties such as the size, shape and size distribution of the open cells in the structure can be controlled with precision and the scope for alloy selection covers a wide range of materials. Water foams and sprays suppress blast by a physical and thermal interaction with the expanding blast wave’s peak pressure and impulse. Unfortunately their application and interaction can be somewhat inefficient and therefore the amount of energy that can be effectively removed from the blast is limited.

The AIGIS methodology is to manage the individual blast elements. In managing them, make them do work and thereby reduce the overall energy to sustainable levels. This method of attenuation and absorption allows more elegant solutions, using materials that offer both aesthetic and practical benefits in the design and protection against blast.

TABRE is a simple yet highly specific material being in essence a porous resin bonded aggregate. How it is used in combination with other materials gives the system cross-section that exhibits the significant blast protection capability. The system deals with each of these elements and has the design flexibility to be tailored to meet the principal blast effect, without compromising the total protection value of the system. The TABREshield system works through attenuating the peak over pressure from a blast. Through the systematic disintegration of the material, first with gross cracking, then with the breaking of the inter-particulate bonds and finally with the pulverising of the aggregate particles, blast energy is absorbed. Attenuating and absorbing the energy over the spectrum of the blast event substantially reduce incident and reflected pressures.

The system consists of the material facing the blast threat, backed with an intermediate material layer, to optimise attenuation capability and establish a tortuous load path to the final support structure. The support structure not only holds the material in place to act against the blast, but also is designed to act as the final, high velocity fragment catcher. The material is used extensively in the storage and transportation of explosives, detonators and grenades, the protection of vehicles and buildings, rubbish bins, protection against terrorist attack and in personal equipment against the land mine threat. In each application TABRE out-performs conventional protection methods in terms of practicality, weight and blast performance.

Independent tests have shown the effectiveness of the material for applications such as the storage and transportation of explosives, detonators and grenades. Pictures 1 to 6 are a series of photographs from testing for the French Gendarme. In this UN 6 (b) propagation test, one grenade (75g TNT equivalent) was detonated in the middle of the tray holding 20 blast grenades or 1.5kg of armed explosives per tray. There was no sympathetic detonation of the other grenades or any fragments outside the unit, enabling the unit to be given a UN Hazard Classification of 1.4S for transport of primed grenades. These capabilities make the unit ideal for Military, Police, Gendarme and Canine units for the transport of grenades, small munitions and the storage of explosives samples. The unit can be transported by land, sea or heli vehicles, is easy to fit and remove and can be adapted to other forms of explosives transport or used for static storage.

One of many other AIGIS products is the Detonator Containment Unit (DCU) an application of the material for the safe transport of detonators in the same vehicle as explosives (provided it complies with national legislation), which saves transport costs. The DCU (standard NATO H83 ammunition box), houses 1 to 18 detonators with an individual NEQ of 1.7g. Each DCU has full certification with a UN packaging number and a 1.4S Hazard Classification. The DCU is currently used by demining companies, NATO in Iraq, Afghanistan and Bosnia and has application in demolition, quarrying, mining, construction, EOD, Bomb Disposal and IEDD. Propagation test show no danger of sympathetic detonation, allowing electric and non-electric detonators to be carried together and no danger to personnel even when being carried by hand. Pictures 6 shows the effect of three No. 8 detonators on a standard H83 ammunition box, whilst Picture 7 shows that with L2A1 detonators and TABRE, the only effect was to momentarily lift the handle.

To conclude, hopefully this article has illustrated the application of materials for blast mitigation and that mitigation requires an understanding of explosion effects and the way shock waves interact with materials, the object to be protected and issues such as weight, formability and cost. It is imperative that engineers develop an appreciation for the context of the problem before offering a potential solution.