Dependency of the Blast Wave Pressure on the Amount of Used Booster

Abstract

Most of the damage caused by an explosion is caused by a pressure effect. The magnitude of the pressure generated by the explosion is influenced by the external characteristics of the environment (surrounding objects, their arrangement, geometry, etc.) and internal characteristics (type of explosive, type of charge, booster and others). An effective combination of internal factors creates a symmetry that results in the highest possible value of pressure generated by the charge explosion. The paper focuses on the influence of the booster reaction on this symmetry. The scope of the paper is to understand the dependency of the blast wave pressure on the amount of used blaster to increase the efficacy of explosions on the environment and structures to increase the protection of affected structures. The open-air field tests were conducted using different types of explosives: trinitrotoluene and three different types of industrially made ANFO explosives (pure ammonium nitrate and fuel oil, ammonium nitrate and fuel oil plus aluminum powder, ammonium nitrate and fuel oil mixed with trinitrotoluene). The obtained data were compared with the analytical approach for setting the generated maximal pressure on the front of the blast wave.

1. Introduction

An increasing number of people killed in various public areas, or misuse of explosives and explosive substances (terrorist attacks, industrial accidents, removing of explosive remnants in post-conflict areas) forces us to seek the causes of this phenomenon and the possibilities of their prevention and mitigation of the consequences.

Generally, the soft targets, such as a specifically defined area with a high number and concentration of people, which is easily accessible, relatively unprotected and thus also relatively vulnerable, need to be protected from certain threats. Furthermore, it can be stated that from the point of view of their protection, it is necessary to take such security measures that minimize the risks of terrorist threats, which belong to the anthropogenic threats.

At the same time, it can be stated that the most common types of attacks are performed using explosive systems, weapons, vehicles, or incendiary substances. These attacks belong to the category of threats, with which soft targets have the opportunity to work especially in the phase before and partially after the attack. A possible immediate reaction to stop the attacker requires the intervention of professional teams, which in most cases are not available. For these threats, it is necessary to take such technical and organizational security measures to minimize potential risks.

Current approaches to determine the level of protection of soft targets use a qualitative approach based on expert estimates of the probability of an attack and the possible consequences. In principle, this is a qualitative approach to risk management. The result of the risk management process is the design of security measures, which include protection by planning and designing a layout solution for the protection of public spaces, also called the concept of Crime Prevention Through Environmental Design (e.g., blast protection design).

An essential part of blast protection design is to estimate the effect of the accidental loads—blast loads on the structures. To know the blast load—time pressure diagrams (blast wave), i.e., as shown in Figure 1, the blast wave propagation is recorded in the field test as described below. The blast wave of the explosive has a sudden peak of pressure which drops under the atmospheric value.

Various previous research on blast wave propagation has been done. Numerical analysis of blast wave propagation from several explosive charges was studied in [1].

The complex approach of blast wave propagation and its mitigation is described in [2]. Chauhan deals with the propagation of blast waves in non-ideal magnetogasdynamics in [3,4].

Various research on blast wave propagation was conducted by Stoller in [5,6] and resistance of structures against such loads was researched in [7,8,9]. A field test that focused on overpressure measurements of various types of explosives was described in [10,11,12,13]. Prediction software for blast wave properties and numerical approaches were explained in [14,15].

According to the statistics [16], many terrorist attacks are carried out using ANFO (ammonium nitrate—fuel oil) explosive in three different variants (ammonium nitrate with oil, ammonium nitrate with oil and aluminum powder, or ammonium nitrate with oil and TNT). A detailed description of the ANFO explosives can be found in [17]. To increase the efficacy of the blast effect of such an explosion, a booster (or ignition) explosive is used.

Generally, explosives are classified into three classes: (i) Low explosives, which can begin to deflagrate (or burn) when are unconfined (i.e., black powder), (ii) High explosives, that can begin to detonate with a N°8 blasting cap (i.e., dynamite, TNT, gelatins) and (iii) Blasting agents containing fuel and oxidizer, projected for blasting but then not an explosive (no possibility to be detonated with a No. 8 blasting cap) (i.e., ANFO—ammonium nitrate—fuel oil explosives). To increase the blast effect of an explosion a booster is used. Boosters are a type of highly sensitive explosives used to help to initiate less sensitive explosives—for example, explosives based on ammonium nitrate. It is also used for the rapid start-up of detonation to a high detonation velocity and the subsequent development of the maximum detonation velocity of the main explosive charge. Incremental charges are produced in sizes from a few grams to several kilograms. A typical shape is a hollow or solid cylinder. The efficacy of the booster depends on various factors, such as the grain size of the explosives or the forming [18]. Tang investigated the grain size effects on the performance of boosters in [19]. Research on the detonation of different shapes of plastic explosives was made in [20].

2. Materials and Methods

2.1. Filed Test

To study the dependency of the blast wave pressure on the booster, open airfield tests were conducted. The field tests were conducted at the development and testing set of the Ministry of Defence of the Slovak Republic, called Military Technical and Testing Institute Zahorie. Blast pressure sensors type 137A23 and 137A24 PCB Piezotronics (see Figure 2) were used to measure the maximum overpressure.

Thirteen field tests using various explosives were conducted. For the research the industrially made explosives ANFO (called DAP-2, DAP-E and POLONIT) produced by Slovak company Istrochem Explosives a.s., Bratislava was selected as well as TNT as a reference sample. Their characteristics and the type of ANFO explosive which they represent are in

Table 1. Characteristics of used explosions.

Explosives	Ingredients	Explosive Velocity [m/s]	Heat of Combustion [kJ/kg]	Density [g/cm3]	Explosive Pressure [GPa]	Factor KTNT	Factor Kv
DAP-E	AN, oil, aluminum	3100	4200	0.65	4.58	1.00	0.60
DAP-2	AN, oil,	2650	3830	0.65	2.95	0.91	0.39
Polonit-V	AN, oil, TNT	4000	5138	0.9	6.93	1.23	0.66
PLNp10	PETN	7411	4982	1.45	21.7	1.19	1.27
TNT	TNT	6800	4200	1.58	18.4	1.00	0.99

. All used explosives were produced industrially, according to the standards valid for the production technology. Homemade explosives differ from industrially made explosives: the mixing process is lower, the quality of raw material (nitrogen content) is worse, the content of chemical impurities or water is present. For this reason, it is presumed that the efficiency is 70–90 % of standardly produced explosives. The explosive DAP-E is composed of ammonium nitrate, methyl esters of higher fatty acids, vegetable oil and red dye. It is characterized by loose consistency and by red-grey color. Explosive is used in blasting operations made on the surface or underground. The only condition is that the environment is without the danger of gas, vapor and dust explosions. DAP-2 explosives consist of ammonium nitrate, kerosene and dye. The explosive PLONIT is mixed using ammonium nitrate, kerosene, charcoal and ground TNT with water-resistant additives. The consistency is loose, the explosive has a white to yellowish color.

As a booster plastic explosive PlNp 10 was used. It belongs to the category of high-efficiency explosives. Its color is dirty gray; it contains pentrite Np 10 and chemical binders. It is about 50% more effective than trinitrotoluene. It is completely safe and reliable up to −30 °C during handling. It is not sensitive to impact, friction and shot. At temperatures of −10 °C and lower, its ductility deteriorates slightly and it is necessary to knead it more and heat it with the hands. The used form of the booster was a cuboid positioned in the center of the top on the explosives (as is shown in Figure 2b). The form of TNT explosive was a cube (as is shown in Figure 2b) and ANFO explosives were placed in plastic bags of an approximately rectangular shape.

The explosive charges were placed 1.6 over the ground. Sensors measuring maximum overpressure were located in four distances, i.e., 2, 5, 10 and 20 m away from the source of the explosion (Figure 3). The maximal registered overpressure of various types of explosives with the different types of booster weight is reported in (Figure 4).

Three different types of explosives were used, all of the same weight of 1000 g, the booster weights increased from 5 g to 50 g (precisely 5 g, 10 g, 15 g or 20 g, 30 g and 40 g).

2.2. Estimation of the Maximum Blast Wave Pressure

For the estimation of the maximum blast wave, pressure can be conveniently used (approach described in [21]). Another approach is published by Wharton et al. who studied the dependence of peak overpressure and positive phase impulse on scaled distance, the results were compared to that of TNT [22]. Normally the TNT coefficient is expressed in the form of the dependence of detonation heat and density. But practically there are more explosives having the same detonation pressure, but their overpressure is really different. For these reasons it is more realistic to determine a new TNT coefficient k_v mainly for ANFO explosives, containing explosive pressure P_cj and density ρ:

$$k_v=0.085\left(\frac{P_{Cj}}{\rho }\right)$$

(1)

When we change k_v with k_TNT the scaled weight can be set as follows:

$$W_R=W_{exp} k_E k_G k_v$$

(2)

where the real weight of charge W_R [kg] and scaled distance Z is:

$$Z=\frac{R}{\sqrt[3]{W_R}}$$

(3)

For the distances R ≤ 1 Z ≤ 10, the maximal blast pressure can be calculated according to the relationship:

$$P_{+}=\left(\frac{0.202}{Z}+\frac{0.224}{z^2}+\frac{1.182}{z^3}\right) 0.5 e^{0.035R}$$

(4)

If the scaled distance Z > 10 is convenient to use the relationship (4) where the value e^0.035R will be equal to 1. The maximal negative blast pressure (in negative phase) can be obtained as follows:

$$p_{-}=\frac{0.03}{z}\left[\mathrm{kPa}\right]$$

(5)

3. Results

The obtained data from conducted field tests were analyzed and compared with the analytical approach described above. In

Table 2. Comparison of calculated and recorder maximal blast pressure obtained in field test.

Amount of PLNp10 Booster [g]	Field Test [kPa]	Analytical Approach [kPa]	Difference [%]
0	10.10	13.80	26.81
5	12.90	16.52	21.91
10	13.30	17.24	22.85
15	14.80	17.76	16.67
20	14.00	18.17	22.95
30	14.70	18.84	21.97

and Figure 5 the comparison of both approaches of the maximal pressures of 1000 g charge of the ANFO explosive Polonit, DAP-2 and DAP-E at the distance of 5 m can be observed.

It can be seen that the analytical approach cannot cover the evolution of blast pressure from the real field test, because of the fact that the maximal blast pressure does not increase equally.

4. Discussion

The efficiency of the explosive depends on whether we use only a standard detonator or also a booster. As was scoped in the presented research, unexpectedly the booster size is important only up to a certain weight. In this case, the direct proportion of the statement that more booster means more efficiency is not valid. As is obvious from the presented recorded experimental testing, if the weight of the booster is greater than 20–25 g, it does not have an impact on the final overpressure. The increase of the efficacy of blaster is around 25%.

Further research may be focused on experimental measurements of the ignition effect on the pressure generated by the explosion as well as on individual types of boosters and their effect on various explosives. The expected continuation of the research are experiments focused on TNT as a reference explosive and later measurements focused on homemade ANFO explosives. The transition from TNT to homemade ANFO explosives is in line to protect people and assets from blast effects.

With a sufficient amount of data, it will be possible to derive a mathematical equation to express this effect. The inclusion of the bust impact on the pressure generated by the explosion, to the process of risk assessment of critical infrastructure objects or other objects important for the state and society and soft targets, allow this process, such an approach is described in detail in [23,24,25,26]. In this way, the risk assessment can be adapted to the variability of how the attack is carried out and the possibilities of the offender. The influence of ignition on the pressure generated by the explosion can be practically incorporated into the modification of the environment surrounding the protected objects, e.g., using the CPTED method to ensure a safe distance between the protected area and the area in which an explosive can be placed.

Protection against explosives and the effects of an explosion on persons and objects in the vicinity of the explosion is a multidisciplinary area. For this reason, there is a wide scope for research, development and collaboration of individuals and teams of researchers, academics and practices.

The protection of assets, not only against the effects of an explosion, is aimed at ensuring the symmetry system of asset protection. If the symmetry is achieved during the explosion, and if it is known when this symmetry can be achieved, then the symmetry of the asset protection system must respond to this fact. This means that their relationship is dynamic. If the symmetry of the asset protection fails to respond to the symmetry of the explosion, then in the event of an actual attack, then losses of human lives and property will occur that could have been prevented.

Knowledge of the details of the influence of each factor on the symmetry of pressure generated during the explosion of explosives and improvised explosive devices creates space for planning, design and implementation of assets and their protection systems so that they are in line with real security threats and especially their real ability to compromise security (symmetry) of the elements. For a comprehensive perception of the issue, it is necessary to know not only the effect of ignition but also the effect of the properties of the charge. The position of the charge relative to the protected assets and the direction of the explosion or explosion of the cumulative charge are facts that include a number of other factors influencing the symmetry of the pressure generated by the explosion and thus, the effect of the explosion on persons and elements in the environment. These factors are e.g., the method and material of sealing of the charge or its directing, the shape of the charge, the place of initiation, and in the case of cumulative charges, the properties of the cumulative current, etc.

The presented results disprove the dogma that as the booster weight increases, it automatically increases the performance efficiency of explosives, i.e., the pressure at the front of the blast wave. The results show that the size of the primary charge of the booster does not affect the explosive properties of ANFO. Based on this, a critical mass is necessary to initiate the explosive, but then there is no linear, exponential, or logarithmic dependence due to the increase of the weight of the booster. Published results show the amount of optimal relevant booster charge, which still has an impact on the increased effect of the base charge, and is considerable for practical use. The research deals with the study of the effects of ANFO explosives, used in terrorist attacks as well as in mining works, therefore the results have important innovative implications for the field of security management, in addition to mining. In the field of security management, it is, therefore, possible to count on a standard mean value of the effect of the explosive and it is not necessary to consider the adoption of significantly more expensive measures to protect important buildings and human lives. Also in mining, the results have an economic nature, it is not necessary to consider the use of a higher amount of booster because higher weight does not affect the performance of the explosive and in this way the costs are reduced.