Industrial Explosion Simulation in Testing

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Justification of the need to model industrial accident explosions with large condensed explosive charges. 

Abstract: 

Over the past few years, a large number of explosion-proof barriers (windows, doors, screens) have appeared on the Russian market, which “protect” from what is unclear. 

Any garage cooperative, which was lucky enough to conclude a contract with the appropriate laboratory and detonate next to its product a pair of ammonite checkers, loudly begins to call its apartment door "explosive" and participate in state tenders. 

At the same time, due to the absence, until January 2017, of any standards regulating the procedure for such tests, the main criterion for suitability is the principle of “not collapsed and well”. 

Comparative testing of a fully certified product from such a manufacturer showed its complete inconsistency from anything to protect. 

Emergency explosions at explosion-prone chemical petrochemical and oil refineries are especially impressive mass of detonating (deflagrating) substance and, accordingly, large (several tens of milliseconds) duration of the blast wave. 

For their correct imitation during field tests, it is necessary to use hundreds of kilograms of condensed explosives.  

Scaling, in accordance with the Hopkinson-Krantz similarity formula, the shock wave from an emergency explosion (500 kg - 5 tons) with an order of magnitude smaller amount of TNT equivalent (10-50 kg), is correct only for structures that as a result of such a test will remain in the area of elastic deformations. The appearance of plastic deformations in the test design - a deflection of the metal structure, the occurrence of cracks in the glass, raises doubts about the suitability for protection from an emergency explosion, since the speed of propagation of irreversible deformations in the material is finite, and the duration of the impact of the shock wave from a small charge of explosives may not be enough to destroy the material, while when exposed to an emergency explosion, even a smaller amplitude, but a longer duration, the material will have time to collapse. 

Consider the issues of experimental determination of the admissible load-bearing capacity of protective structures when exposed to air shock waves (VSW) from charges of condensed explosives (VV).  

It is known that the effects of VW on objects and people depend on two parameters: the maximum pressure - P and the pulse of the compression phase - I (see, for example, [7,8]). At the same time, the role of the maximum pressure or compression phase impulse in the destruction of building structures depends on the relationship between the period of intrinsic oscillations of the construction structure - T0 and the duration of the compression phase of the VUV - TNAGRUZKI. When TNAGRUSH <<T0, the compression phase impulse plays a decisive role in the degree of structural destruction (CP): \(\mathrm{CP}=\mathrm{CP}(I)\). At TNAGRUSKI>>T0, the loading of the structure occurs almost in a static mode, so the degree of structural destruction is determined by the maximum pressure: \(\mathrm{CP}=\mathrm{CP}(P)\). With a load duration comparable to the period of eigen oscillations of the TNAGRUSKI ~ T0 design, the degree of structural destruction depends on both the compression phase pulse and the maximum pressure in the VUV, i.e. \(\mathrm{CP}=\mathrm{CP}(P,I)\). 

The following methodology is used to calculate the effects of dynamic impact on building structures. The dynamic load shall be replaced by an equivalent static load, which shall be determined by the formula [1-4]: 

According to the above method for modeling an explosion of 500 kg TNT at a distance of R1 = 17.8 m, when using a charge of 50 kg TNT, it should be located at a distance of R2 = 3.86 m; and when charging 5 kg TNT at a distance of R3 = 0.83 m. This dependence is based on the equality of impulses of the positive phase of VUV. 

It should be remembered that the above methods are conditionally reliable for structures that, as a result of tests, remained in the field of elastic deformations. If the structure or its parts were subjected to plastic deformation (which occurs in 90% of tests), such modeling is unreliable due to many factors: the influence of the dynamic fluidity coefficient, the rate of propagation of the deformation remaining finite at any loading speed, etc. 

From the above it follows that the tests of protective structures carried out on low-power charges do not give a real assessment of their explosion resistance in relation to charges of greater power.  

It should be noted the following feature of the impact of VOV from TNT explosions on the structure. As can be seen from Figure 1, the compression phase in the VUV is accompanied by a dilution phase. The consequence of this is, for example, the rejection of fragments of structures in the direction of the source of the explosion. The compression phase breaks the structure, and the dilution phase pulls fragments of the structure in the direction of the charge location. This phenomenon quite often puzzles employees conducting investigations of terrorist attacks related to explosive devices. For example, the presence of fragments of cladding tiles in the center of the underground passage, where the explosion was made, has long given rise to various hypotheses of their origin.  

Therefore, not only the compression phase but also the dilution phase should be taken into account when considering the impact of VSW on design. 

Consider the issues related to the methodology of testing products for their explosion resistance.  

When placing and fixing the product, it is necessary to take into account the following features of the distribution of wave flows, including VUV. The nature of the interaction of the wave with the barrier is determined by the ratio between the wavelength - L and the linear size of the barrier - D. At L>>D, the wave "does not notice" the barrier and practically does not distort on it. At L<<D, the wave is completely reflected from the barrier. The process of interaction of the VUV from a charge of 50 kg with a barrier illustrates Figure 2, which shows isolines of equal pressure in VUV for several points in time. The step in time was 10ms. Levels of isolines from 10 kPa to 100 kPa in increments of 10 kPa. For clarity and to illustrate the linear dimensions of the problem in figure 2 on scale, the contours of the average person and passenger car are given. 

From the above picture it follows that the shock wave of the explosion flows behind the obstacle of a sufficiently large size, unloading it from the rear side. When testing protective structures, such phenomena should be excluded. To ensure this, it is necessary to place the test sample in an impermeable vertical screen of sufficient dimensions (in which case the tests will simulate the effect on the product of the reflected VOV) or to install solid impermeable barriers at the edges of the article to prevent the penetration of VOV beyond the test structure (in which case the tests will simulate the effect on the product of the passing VOV).  

When testing products for explosion resistance with small TNT charges, the following problem arises, leading to the additional impossibility of spreading the obtained data to high-power charges. In order to ensure the necessary pressure levels in the VHV, small charges are placed close enough to the test structure. This leads to the fact that the design runs VUV, which has a spherical shape. High-power charges create the necessary pressure levels at a considerable distance from the sample, when the spherical VUV degenerates into a practically flat shock wave. The loading of structures with a flat and spherical wave occurs in different ways, as illustrated by Figure 3.  

Figure 3 shows the instantaneous deflections of the panel when exposed to a flat and spherical VHV, which have the same amplitudes at the time of their approach to the panel. 

From the above figure it follows that the loading and deformation of the structure differ qualitatively when exposed to a flat or spherical VHV. From this follows the conclusion that it is impossible to extend the experimental data obtained on low-power charges to the state of explosion resistance of the structure in relation to high-power charges.  

The next problem relates to the nature of the spread of VHV and its interaction with the surface of the earth. Reflected from the ground, the UV forms a third wave with the falling UV, called a head shock wave, a passing shock wave or a Mach wave, in the form that the propagation speed of the reflected UV is higher.  

velocity of propagation is falling. Rice 4 

The pressure in the Mach wave is calculated by the formula: 

 

Where R is the distance to the barrier, m; m-mass of charge, kg; α-angle between the propagation of UV and the normal to the barrier, hail; 

ΔPf(R,m)- Sadovsky's formula for an air explosion. 

The pressure in the front of the head shock wave can be several times greater than the pressure in the front of the falling UV. Accordingly, the barrier immersed in the Mach wave experiences several times greater loads than the same barrier washed by the falling UV. When testing charges of low power, which are located close enough to the barrier, the resulting head HV or does not affect the barrier at all or acts on its lower part (Fig. 5, Fig. 6), while in a real emergency explosion, the barrier and the structure are completely immersed in the Mach wave, which is designed to reflect the test explosion of high power, when the distance from the center of the explosion to the barrier is many times greater than the barrier itself (Fig. 7).  

Figure 5: 

Pressure profile from elevated

charge weighing 50 kg TNT, t = 3.442 ms. 

 Figure 6: 

Explosion resistance tests of the BOPVZ-2 window. Undermining 50.4 kg of TNT, at an altitude of 1.3 meters from the ground, at a distance of 5.2 meters to the center of the structure. The same composition and size of armored glass, equally fixed. On the upper glass, no penetration was recorded, while the lower one completely collapsed. 

Figure 7  

Findings 

The analysis conducted in the article showed that testing of explosion-proof structures with small charges of explosives is unacceptable to extend to high-power charges, referring to the equality of excess pressures in the passing VUV. This contradicts both theoretical and regulatory provisions concerning the effects of blast waves on building structures. 

It is shown that during the tests it is unacceptable to use conventional frames as structural anchorages, which leads to leakage of VUV from the rear side of the structure and unloads it. As a result, data on the level of explosion resistance of the structure are significantly underestimated.  

List of sources used 

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