Hot spot frictional

The treatment is for inert materials, and a rigorous treatment would need to bring in any heat of reaction from the explosive. As pointed out by Brown [48], this has not yet been done and would be an ambitious undertaking. However, the importance of this component to the hot spot temperature will depend on the time available as well as the temperatures involved. From their work on frictional initiation Bowden et al. [3,49,50] concluded that several explosives initiate if a temperature rise of 500°K is achieved in 10 sec. This kind of temperature rise was also measured for frictional hot spots between inert solids (see also below). The evidence suggests that the contribution from the explosive itself is, in most cases, likely to be a minor one. 

External fire. May be prompted by flammable gas or vapor liquid, solid, metal, wood, or waste material pyrophoric material and presence of ignition source such as sparks, static, friction, hot spots, welding, lightning, auto ignition, or furnace. Immediate consequence engulfment, thermal radiation, fire damage, smoke, domino effect (where one incident is the primary cause for several other incidents). 

IGNITION Sparks, static, friction, hot spots, wridine. decomoosMon... 

Impact and Friction Initiation of PETN have been extensively studied. This general subject is summarized in Bowden Yoffe (Refs 13 19), Afanas ev Bobolev (Ref 85a), and Vol 7, 135—55, which contains a rather complete description of impact initiation with many refs to the impact and friction initiation of PETN. There is general agreement that impact and friction initiation proceed via a hot spot mechanism (see Vol 7, HI 70—75). There is, however, considerable doubt and controversy about the mechanisms of hot spot formation... 

Aside from the question of how hot spots are formed, there is general agreement about the subsequent stages of the initiation of PETN by impact. Bowden Yoffe (Ref 13), and more recently Bobolev et al (Ref 70) have observed the following sequential stages in the impact (also friction and spark) initiation of PETN layers ... 

By the development of hot spots by friction. This is shown particularly by the effect of added materials of a gritty nature. For initiation to occur, the melting point of the grit must be above a limiting temperature dependent on the explosive. Initiation is favoured by a low thermal conductivity and also by a high hardness value. 

According to investigations of Bowden et al (Refs 6, 7, 8, 11, 17, 19 20), there is a good deal of evidence that the initiation of expls by impact and by friction is, commonly,thermal in origin. The mechanical energy of the blow (strike) or of rubbing is degraded into heat and concentrated to form, under certain conditions, the small hot spots. The size of these spots varies from 10 5 to 10 cm in diam and their... 

The following discussion on influence of hot spots in initiation of explosion by impact friction and heat is given in Dunkle s Syllabi (Refs 22 26) ... 

Ref 22, pp 153 162. In the case of granular materials the hot spots in which the reaction is initiated originate along the friction surfaces of neighboring grains, whereas in cast materials similar action occurs between separate crysts and the cryst fragments into which the substance is broken by the impact. Low density, due to voids, also makes the material less homogeneous and consequently distribution of... 

The minimum temperature developed by hot spots for initiation can be determined by compression of a known volume of air surrounding the expl. This type of test, described in Ref lp, p 60, gives results, shown in Table IX of Ref 22, p 162, which are in fairly close agreement with the friction test... 

Initiation by Friction. See Friction Sensitivity Tests in Vol 6, p F204-L and Hot Spots ... 

Tribochemistry is defined as decompn brought about by mechanical means, such as friction, impact, rapid shear or ultrasonic vibration See under Detonation, Spot or Hot Spot Initiation of in Vol 4, D563-R to D569-R Hot Spots in Vol 7, H170-L to H175-R and Impact, Initiation of Explosion by in Vol 7, 143-Lto I48-R (Theoretical Considerations) Addnl Ref FT. Bowden A. Yoffe, Tribo-... 

How can the actual contact surface be measured One possibility is to measure the electrical resistance between two conductors and calculate the contact area from the measured resistance and the specific resistivity of the materials. Another possibility is to use an IR sensitive microscope to measure hot spots of a transparent solid that is in contact with a hot surface. With these methods it was found that the friction force is, in fact, proportional to the actual contact area. This implies that the true contact area must increase linearly with load. To illustrate how this is possible, we consider two extreme cases. In the first case, purely elastic deformation is considered. In the second case, we assume plastic deformation of the microcontacts. 

Apart from oxidation of the lubricant and the metal surfaces, there can be complex tribo-chemical reactions. Chemical reactions at the surfaces can be stimulated by different factors. One factor is heating due to friction. This can either be a global effect (elevated mean temperature of surfaces and lubricant) or a localized phenomenon. Especially in situations where mixed or boundary lubrication exists, the direct contact of surface asperities can lead to high flash temperatures. At these hot spots temperatures in excess of 1000°C promote chemical reactions and surface melting. Other factors promoting chemical reactions are ... 

In a dichlorodifluoromethane system, frictional wear exposed fresh metal surfaces on an aluminium compressor impellor, causing an exothermic reaction which melted much of the impellor. Later tests showed similar results, decreasing in order of intensity, with tetrafluoromethane chlorodifluoromethane bromotrifluoromethane dichlorodifluoromethane 1,2-difluorotetrafluoroethane 1,1,2-trichlorotrifluoroethane [6], In similar tests, molten aluminium dropped into liquid dichlorodifluoromethane burned incandes-cently below the liquid [7]. Aluminium bearing surfaces under load react explosively with polytrifluoroethylene greases or oils. The inactive oxide film will be removed from the metal by friction, and hot spots will initiate reaction [8]. An attempt to scale up the methylation of 2-methylpropane with chloromethane in presence of aluminium chloride and aluminium went out of control and detonated, destroying the autoclave. The preparation had been done on a smaller scale on 20 previous occasions without incident [9]. 

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