Time-to-explosion tests

The question of the effects of iron impurities on the explosive properties, of lead azide was addressed by Hutchinson, who subjected pure and Fe" -doped lead azide (0.016 mole % iron) to thermal decomposition and explosive tests [67], Both samples pressed to the same density had identical detonation velocities, 4650 m/sec at a 3.5 g/ml. The iron-doped material was slightly more sensitive to impact (ball-drop test) and to heat (time to explosion tests) than was the pure material. However, the differences in results were close to the scatter in the data. 

The experiments were termed Fast-Heat-and-Hold/FTIR and can be used to simulate time-to-explosion tests of explosives [11]. Hence, Fast Thermolysis/FTIR can be used to study the explosion hazard of materials directly. Engineering tests, such as Henkin [30] and one-dimensional time-to-explosion (ODTX) [31], measure the time-to-explosion as a function of the sample temperature. As shown in Figure 10 for HMX, a similar measurement, the time-to-exotherm, is made in the Fast-Heat-and-Hold experiment [11], An apparent activation energy can be calculated from... 

Figure 6. Critical temperature of HMX and HMX—dg determined from isothermal time—to-explosion tests. ...

This test bridges the gap in the growth from thermal decomposition reaction to explosion and eventually involves fast oxidation reactions. A small sample of explosive is pressed into a blasting cap cup made of gilding metal. The cup is then inserted into a molten Wood s Metal bath. The time it takes from insertion in the bath until some noticeable reaction takes place (usually a mild explosion) is noted. The test is repeated at several different bath temperatures. See Table 6.3. A smooth curve is drawn through the data points (time to explosion versus bath temperature), and the temperatures that cause reaction in 1, 5, and 10 s are interpolated from the graph. 

The validity of Eq. (22.8) is demonstrated by the results of small-scale tests where a slab of explosive of known properties is held between the two heated anvils in a manner that seals in all evolved gases. The anvils are electrically heated and held at a constant temperature. The time it takes to explosion is measured. The test is repeated over a range of temperature, and the results are plotted as reciprocal anvil temperature versus log of time to explosion. The temperature at which the relationship become asymptotic (time approaches infinity) is defined as the critical temperature, Tc, corresponding to the conditions of Eq. (22.8). 

The time to explosion was significantly prolonged when lead azide was preheated in the presence of water and Freon before testing [18]. In this case the retardation may be due to the scavenging of lead nuclei over a period of time at an elevated temperature by the water and Freon. The presence of water for shorter periods of time at ambient temperature did not influence the induction time. It is of interest to note that the induction period of lead azide was lengthened during thermal decomposition in the presence of adsorbed water [19], but... 

P. A. Longwell, Calculation of Critical Temperatures and Time-to-Explosion for Propellants and Explosives , U. S. Naval Ordnance Test Station, China Lake, report NAVWEPS 7646 (1961). 

The following alternative procedure is recommended and it possesses the advantage that the same tube may be used for many sodium fusions. Support a Pyrex test tube (150 X 12 mm.) vertically in a clamp lined with asbestos cloth or with sheet cork. Place a cube (ca. 4 mm. side = 0 04 g.) of freshly cut sodium in the tube and heat the latter imtil the sodium vapour rises 4 5 cm. in the test-tube. Drop a small amount (about 0-05 g.) of the substance, preferably portionwise, directly into the sodium vapour CAUTION there may be a slight explosion) then heat the tube to redness for about 1 minute. Allow the test tube to cool, add 3-4 ml. of methyl alcohol to decompose any unreacted sodium, then halffill the tube with distilled water and boil gently for a few minutes. Filter and use the clear, colourless filtrate for the various tests detailed below. Keep the test-tube for sodium fusions it will usually become discoloured and should be cleaned from time to time with a little scouring powder. 

The Accelerating Rate Calorimeter (ARC ) is another adiabatic test instrument that can be used to test small samples. The ARC with the clamshell containment design can handle explosive compounds. It is a sensitive instrument that can indicate the onset of exothermicity where the reaction mixture can be accurately simulated (HSE 2000). ARC testing results can be used in determining a time to maximum rate of decomposition, as well as in calculating a temperature of no return for a container or vessel with specific heat removal characteristics. Further information and references related to the ARC are given in CCPS (1995a) and Urben (1999). 

Results from extensive test programs on molten aluminum-water explosions have been reported by Long (1957), by Hess and Brondyke (1969), and by Hess et al. (1980). In almost all experiments, molten aluminum, usuaUy 23 kg, was dropped into water from a crucible with a bottom tap (see Fig. 9). In only a few tests was there instrumentation to indicate temperatures, pressures, delay times, etc. The test results were normally reported as nonexplosive or explosive—and if the latter, qualitative comments were provided on the severity of the event. A large number of parameters were varied, and several preventative schemes were tested. Over 1500 experiments were conducted. Some of the key results are summarized below. ... 

In view of the above the following ignition tests ignition temperature tests are primarily of historical interest. They also serve to outline the difficulties encountered in trying to characterize quantitatively the response of explosives to heat. Quantitative treatment of explosion temperatures and delay to explosion (induction time) and the parameters of the explosive that affect these quantities will be presented in a future Vol under Thermal Explosions ... 

Special permitted explosives are tested under much more drastic conditions. As previously stated 1 kg of inversely-initiated explosive is fired creating a cloud of coal-dust. The number of test shots should not be less than 20. Safety in the. presence of coal-dust is also tested by a special method worked out by Cybulski, which consists in firing two charges, each weighing 1 kg, simultaneously from two opposite mortars. The distance between the mortars is 1 m. A cloud of coal-dust is obtained as described above. The charges are inversely initiated. The number of shots is 20. The test is considered exceptionally stringent. Such tests are repeated from time to time for inspection purposes. 

A small sample of AN in an evacuated tube was heated gradually to desired temps, and samples of the gas produced by decompn were pumped out, measured and tested. Decompn proceeded very quietly at temps below 200°, and only a small amt of gas was formed even on heating for several hours. The reaction proceeded more vigorously at higher temps and became rather violent at ca 260°. Between 260 and 269° gray smoke was produced and, after a time, an explosion took olacr. This also Orftirred after hearing... 

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