Impact Sensitivity Data

A more serious problem, however, is that the initiation of detonation, as already mentioned and to be further discussed, depends upon a complex interplay of various molecular, crystal and physical factors. It can therefore be viewed as remarkable that relatively good correlations have been established between impact sensitivity and a number of different properties, although they are generally limited to a particular class of compounds, e.g. nitroaromatics. The existence of these relationships certainly does not mean that all of these properties (or perhaps any of them) play important roles in detonation initiation, as was pointed out by Brill and James [12,13]. Many of them may be symptoms of some more fundamental factor others may happen to correlate with a more relevant property. 

In this chapter, we shall present an overview of some of these sensitivity correlations. We shall try, as much as possible, to relate them to a conceptual framework. 

The formation of hot spots is generally attributed to the presence of lattice defects [11,17,19-23], which could include vacancies, voids, dislocations, misalignments, cracks, impurities, etc. One explanation is that defects induce strain in the lattice, which is relieved, via structural relaxation, by the externally-introduced energy this results in a disproportionate localization of energy in the neighborhood of the defect, a portion of it being in lattice vibrations [21,22]. The thermal energy of hot spots must be efficiently transferred to appropriate molecular vibrational 

The fact that solids composed entirely of very small particles are less sensitive [7,30] can now be explained on the grounds that these will have smaller hot spots and thus, as shown by Tarver et al [16], require higher temperatures for ignition. Once this has been achieved, however, these higher temperatures will result in more rapid reaction, as has been observed [31]. 

It should be noted that initiation of detonation can occur even in a homogeneous (defect-free) solid [6,24,32-36] (although this is of less practical significance since energetic compounds typically do contain lattice defects of various sorts). This can occur, for example, if there is efficient anharmonic coupling to channel energy from lattice into the critical molecular vibrations [24,25,36]. 

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In what follows we will describe some impact machines and impact tests, ie the apparatus and methods for measuring impact sensitivity. We will then present and discuss impact sensitivity data for common explosives obtained with these machines by different laboratories. Then we will briefly consider how impact sensitivity tests have contributed to the development of the theory of initiation of explosives. Finally, we will examine impact testing from a theoretical point of view... 

Impact Sensitivity Data Obtained with the Rotter Apparatus at ARDE (5kg falling weight 30mg samples)... 

A comparison of the impact sensitivity data of explosives reported by various teams from different laboratories suggests that the results obtained generally vary and it is due to (i) differences in the ways in which the experiments are conducted and (ii) use of different type of impact sensitivity apparatuses. However, when all influencing factors are considered, it appears that there is a similarity between the results obtained by different testing apparatuses. 

The following precautionary measures should be taken in order to minimize variation in the impact sensitivity data from laboratory to laboratory and operator to operator. 

Impact Sensitivity Data for Wettad Lead Azidas... 

This inorganic azide has been investigated as a constituent of a binary explosive. The impact sensitivity data obtained using the Picatinny Arsenal apparatus (Figure 9) include that for barium azide made with 1-2% rhodamine B dye. 

W. Seals, "Tabulation of Impact Sensitivity Data," Laboratory Report EMI.. 1... 17, Reaction Motors Division, Thiokol Chemical Corporation, Den- e, New Jersey (March 4, 1959),... 

In this case, the impact sensitivity data were mostly taken from the literature. Figure 10 represents the relationship between the impact sensitivity detected by sound and NMR chemical shifts of nitramines [6]. This relationship, in comparison with Fig. 6, is mathematically different. The difference could be interpreted by the different mechanisms of transfer of drop energy to the reaction centre of the molecule in the case of the first reaction as compared with the impact sensitivity detected by sound. This difference is evident also in the relationship between the sensitivity and the Affm.tr values (Fig. 11 taken from [147]). 

Impact Sensitivity Data for Three Pyrotechnic Compositions... 

The results obtained by the BAM friction test refer to the smallest load on the pin under which deflagration, crackling or explosion of the sample is observed, at least once in six consecutive trials. Other test procedures provide friction energy values on the basis of a 50% initiation probability. Therefore, absolute friction sensitivity values that are measured might be subject to certain fluctuations depending on the specific properties of a sample (e.g. purity, particle size, etc.) and the test equipment used. Nevertheless, friction sensitivity measurements provide useful safety information and allow direct comparison with other sensitive or less sensitive materials, and are thus of the same importance as impact sensitivity data. 

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