Sensitivity of an explosive

The sensitiveness of an explosive to impact is measured by determining the minimum height from which a given weight must be dropped in order to initiate detonation. Many forms of fall hammer test have been devised, the most important point of the various designs being the means adopted for retaining the explosive. A simple and practical method, used at Ardeer for many years, is shown in Fig. 6.7. In this the explosive is put... 

The sensitivity of an explosive to impact and friction is a key factor in deciding whether it finds practical use. Most developed countries have an ongoing program to gradually replace current explosives and propellants with insensitive materials, a process which will greatly reduce the risk of accidental detonation. At present these programs are still in their early phases. 

Andrej Ma ek, "Sensitivity of Explosives , ChemRevs 62, 41-63(1962). "The sensitivity of an explosive can be defined as the minimum amount of energy that must be imparted to the explosive, within limited time and space, to initiate explosive decomposition (p 60). This definition can serve as a basis of quantitative fundamental treatments provided the imparted energy is thermal and provided its initial distribution in time and space is known. If the energy is not supplied directly as heat, but by mechanical means (such as a shock), there is the additional requirement of quantitative assessment of conversion of the stimulus into heat (p60)... 

The above classifications are not strictly applicable to the sensitivity of an explosive. Although it is true that most primary explosives are more sensitive than most secondary explosives, certain low explosives, eg Black Powder, are more sensitive than many secondary or even primary explosives... 

In other words, investigation of explosives involves a study of these aspects. For example, an investigation of the potential energy involves study of thermochemistry of the chemical compound in question. Further, the power and sensitiveness of an explosive depend on properties such as heat of formation and heat of explosion . An investigation of the feature (2) involves measurement of the rate of propagation of explosion waves and all phenomena in the proximity of detonating mass of the explosive. This rate of decomposition largely determines the pressure... 

In addition, the cap sensitivity of the emulsion decreases. The cap sensitivity is the sensitivity of an explosive to initiation by a detonator. Thus, the explosive becomes less sensitive to initial detonation. 

Desensitization or phlegmatizotion) of an explosive, means rendering it insensitive or less sensitive to the following actions heat, shock, impact, percussion, rifle bullet or friction Desensitizer (or pblegmatizer) is a substance which makes insensitive or reduces the sensitivity of an explosive. The term phlegmatizer is also used for a material added to a cannon propint to reduce its erosive effect in gun barrels Desensitized explosives are those which are made insensitive to the above listed actions. 

The sensitivity of an explosive to heat, mechanical stress, shock, impact, friction impact and detonation impact (initiability) determine its handling safety and its application potential. 

Which experimental test is suitable to assess the shockwave sensitivity of an explosive ... 

The sensitivity of an explosive is a measure of the ease or minimum amount of energy required to initiate fast decomposition in the material [1-5], It is not a quantity which can be defined precisely, and as indicated in the introduction to Volume 1, it is often expressed as a probability that an explosion will occur following the application of a given stimulus at a specified level. Very often the nature of the decomposition regime initiated is not determined with any precision, although in assessing the potential consequences it is clearly important to know whether the regime is stable and whether detonation is achieved. 

The nonreproducibility of data implies that the tests and the meaning of the term "sensitivity" need further study and understanding [6]. Questions arise as to whether certain tests actually measure the effect of the specified stimulus, even as one among several stimuli directed at the test sample by the apparatus. Kistiakowsky and Connor [1] concluded that a definite mechanical sensitivity of an explosive was not found. Koenen et al. [2] and Afanas ev and Bobolev [5] also concluded that an intrinsic impact sensitivity of an explosive does not exist, but is a function of the test method, container, compression, arrangement of the sample, state of the sample tested, its method of preparation, particle size and shape, and possibly the climactic conditions during testing. By extension these factors can also be expected to affect sensitivities to other stimuli. 

Sculhon [51] developed an equation to relate the sensitization of an explosive to the amount of grit added. The impact sensitivity (y/) of an adulterated explosive was related to the insensitivity of the uncontaminated explosive... 

The desirable sensitivity of an explosive is dependent on its use. Of course, it should not be so sensitive that it cannot be handled or stored for a few years without decomposition. Small amounts of a sensitive explosive are used to start the reaction in the detonator. With the modern initiation systems of flying plates or exploding bridge wires, even these detonator explosives can be less sensitive than those used in the past. Another use is as the booster explosive which requires moderate sensitivity. The largest use of explosives is in the main charge, which does the work . The least sensitive explosive that will do the work is used to prevent accidents during handling. 

The spark test is used to determine the sensitivity of an explosive to a spark. In this test the explosive is separated from an electrode by an air space or gap (there are many different devices to do this). The energy from a capacitor or other device is discharged through the elective, resulting in an arch or spark jumping to the explosive. The explosive s sensitivity is a measure of the energy dumped into the electrode. 

The adiabatic sensitivity of an explosive is determined by allowing a test weight to fall from a predetermined height onto the air-compressing piston. This causes the compression of the air between the compressing piston and the sample of the explosive. Its adiabatic heating may consequently lead to the initiation of the sample. 

The methods for the experimental determination of the fragment (or projectile) impact sensitivity of an explosive differ in details such as... 

Sensitivity is a complex problem and there is no single useful definition. Tests of the sensitivity of an explosive are devised with the idea of judging the potential safety of use in a particular application. This immediately raises two questions 1) How do different molecules rank within any given sensitivity test 2) How do different sensitivity tests compare in ranking molecules ... 

Explosive solids can be detonated by heat, mechanical impact, friction, or electrical spark or discharge. The sensitivity of an explosive is the effect of the stimulus on its spontaneous detonation ... 

To evaluate the potential shock sensitivity of an explosive for engineering purposes, one needs to determine experimentally the Arrhenius constants. Then one calculates the adiabatic explosion times for several assumed hot spot temperatures to determine the relative sensitivity of the explosive compared with explosives of known sensitivity. 

Sensitivity of an explosive to shock is a very reproducible quantity. Shocks generated by a donor explosive can cause detonation in another explosive material. The strength of the shock wave required is a relative measure of the sensitivity of the material under test. In practice, a strong shock is produced and attenuated in an inert medium. The width of the medium that will allow detonation in 50 percent of the trials is reported as the test result. These tests are called gap tests. Gap test results are much more reliable data than impact test results, although there is some dependence on the geometry of the test apparatus. Gap test procedures are covered in detail in Chapter 5. 

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