Burning to detonation

The usual burn under confinement proceeds on the surface of the explosive with a rate that increases with pressure to some power. It is important to describe both the surface area available for burning and the pressure-dependent rate law of burning. This is called the bulk burn model. 

Charles Forest constructed the model for bulk burning, assuming that the mass of propellant burns on a surface area S such that the burn proceeds normal to the surface according to a linear burn rate. 

He also assumed that the density of the burning explosive is constant. 

An approximation for (S/So) is needed since there is no direct way to follow the burning surface area, except in idealized cases. For this purpose then, let 

The motivation for this approximation is found in the following cases a. For all polyhedral volumes containing an inscribed sphere of radius r, for some k 5 Zkr f (pkr / M  

This delay will vary according to the nature of the explosive composition, its particle size, density and conditions of confinement. This principle of burning to detonation is utilized in delay fuses and blasting detonators. 

Explosive substances which on initiation decompose via the passage of a shockwave rather than a thermal mechanism are called detonating explosives. The velocity of the shockwave in solid or liquid explosives is between 1500 and 9000 m s , an order of magnitude higher than that for the deflagration process. The rate at which the material decomposes is governed by the speed at which the material will transmit the shockwave, not by the rate of heat transfer. Detonation can be achieved either by burning to detonation or by an initial shock. 

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D. Price, J. E. Wehner, and G. E. Robertson, Transition from Slow Burning to Detonation Kole of Confinement, Pressure Eoading and Shock Sensitivity, TR68-138, Naval Surface Weapons Center (NSWC), White Oaks, Md., 1968. 

The studies of Bobolev et al (Ref 70) on the transition from burning to detonation have already been described in Section VIII under Initiation by Impact Friction... 

High explosive. Literally any explosive which detonates. In practice, the term is usually confined to explosives which do not normally burn to detonation but which require a detonator for use. 

Secondary explosive. Alternative name for high explosive indicating that the explosive does not burn to detonation but is detonated by suitable devices. 

Primary explosives differ from secondary explosives in that they undergo a rapid transition from burning to detonation and have the ability to transmit the detonation to less sensitive (but more powerful) secondary explosives. Primary explosives have high degrees of sensitivity to initiation through shock, friction, electric spark, or high temperature, and explode whether confined or unconfined. Some widely used primary explosives include lead azide, silver azide, tetrazene, lead styphnate, mercury fulminate, and diazodinitrophenol. Nuclear weapon applications normally limit the use of primary explosives to lead azide and lead styphnate. 

Tetrazene (C2H8N10O) is a pale yellow crystalline explosive generally used in ignition caps, where a small amount is added to the explosive composition to improve its sensitivity to percussion and friction. Tetrazene is not suitable for filling detonators because its compaction properties make the transition from burning to detonation very difficult. This primary explosive is stable in ambient temperatures. Its ignition temperature is lower and it is slightly more sensitive to impact than mercury fulminate. 

An explosive device is initiated or detonated by an explosive train — an arrangement of explosive components by which the initial force from the primary explosive is transmitted and intensified until it reaches and sets off the main explosive composition. Most explosive trains contain a primary explosive as the first component. The second component in the train will depend on the type of initiation process required for the main explosive composition. If the main explosive composition is to be detonated, the second component of the train will burn to detonation so that it imparts a shockwave to the main composition. This type of explosive train is known as a detonator. Detonators can be initiated by electrical means, friction, flash, or percussion. 

Very insensitive substances having a mass explosion hazard which are so insensitive that there is very little probability of initiation or of transition from burning to detonation under normal conditions of carriage. 

Detonation from Burning, Transition of. See under Detonation (and Explosion) Development (Transition) from Burning (Combustion) or Deflagration and the following paper by A. Macek, "Transition from Slow Burning to Detonation. A Model for Shock Formation in a Deflagrating Solid , NOLNavOrd Rept 6105(1958) [See also Andreev Belyaev(1960), 141-44]... 

V.K. Bobolev (Bobylev) et al, ZhPrifel-Mekhan i TekhnFiz 1963(4), 99-101 CA 59, 13762(1963) (Transition from normal burning to detonation in porous explosives under conditions of slowly increasing pressure) 31) I- Shanfield, NASA Accession No... 

Transition of burning to detonation) 7) Zel dovich Kompaneets (i960), 135-36, 186-87 191-96 (Development of detonation from combustion) 8) A.S. Sokolik, Samovosplameneniye, Plamia i Detonat-siya v Gazaldb", IzdatAkadNauk, Moscow (I960) translated under the title Self-... 

Schweikert s theory differs radically from the conventional thermohydrodynamic Chapman-Jouguet theory in that it provides for a continuous transition from burning to deton. In Section I entitled "Introduction , the author criticizes the validity of the C-J theory for condensed expls. In Section II the burning rate constants of a colloidal propint are related to fundamental parameters such.as specific surface vol of the powd, the most probable molecular vel, and the collision efficiency c. Schweikert arrives in Section III at the conclusion that burning deton differ primarily in the magnitude of c i.e. c l in a deton and is a much. smaller value in a burning process A surprisingly simple relation is derived in Section IV for the upper boundary of the deton vel Dm of a condensed expl ... 

Lead azide passes very rapidly from burning to detonation. When used in very small amounts, it is therefore capable of initiating detonation in other explosives, hence it is very suitable for use in detonators though it cannot be employed in caps. 

Thus, at a pressure of 200 kg/cm2 the substance nears the condition of being dead pressed . In spite of the fact that burning under this condition passes to detonation with difficulty, when greatly compressed the material maintains its ability to be detonated by a cap. Thus, 0.4 g of tetrazene, pressed under a pressure of 200 kg/cm2, develops its maximum power, i.e. 21.1 g of sand crushed, when initiated with 0.4 g of mercury fulminate. The difficulty in passing from burning to detonation makes tetrazene unsuitable for detonators and its application is thus limited to... 

Elliot and Brown [62] made extensive studies of the inflammability of mixtures of perchloric acid with oxidizable substances. Most of the mixtures of 60% perchloric acid when ignited in a confined space burned to detonation. The mixtures with 70% perchloric acid and some of them with 60% perchloric acid could be ignited by impact. Explosion was induced under action of No. 6 detonator on mixtures with 60% perchloric acid with wood meal or cotton and the rate of explosion was found to be 3000 m/sec. 

The reaction zone of a secondary expl can be subjected to an impulsive rise of pressure leading to deton within tens of microseconds by a technique termed ACP (augmented by collision pressure). This ACP method is claimed to have practical advantages of simplicity and reliability when compared with expl bridgewires and the known procedures for burning to deton. It has been applied to RDX (Ref 45a)... 

Because Tetryl was widely used as a booster charge, its initiation behavior has been studied extensively. In particular, the reaction of Tetryl to shock has been the subject of many investigations. We shall now summarize the initiation characteristics of Tetryl with emphasis on its shock initiation behavior. The transition from burning to detonation will be described in the next section... 

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