Grit particle

Pulpstones. Improvements have been made in the composition and speed of the grinding wheel, in methods of feeding the wood and pressing it against the stone, in control of power to the stones, and in the size and capacity of the units. The first pulpstones were manufactured from quarried sandstone, but have been replaced by carbide and alumina embedded in a softer ceramic matrix, in which the harder grit particles project from the surface of the wheel (see Abrasives). The abrasive segments ate made up of three basic manufactured abrasive siUcon carbide, aluminum oxide, or a modified aluminum oxide. Synthetic stones have the mechanical strength to operate at peripheral surface speeds of about 1200—1400 m /min (3900 to 4600 ft/min) under conditions that consume 0.37—3.7 MJ/s (500—5000 hp) pet stone. 

Particles of grit such as carborundum, when added to an expl, can artifically create hot spots (see Vol 7 H170-L to H175-R) when the expl is suddenly compressed by impact as in an impact sensy device. Only grit particles of mp higher than that of the expl can initiate higher temp hot spots and thus increase the sensitivity of the expl... 

The spots may be formed in two main ways 1) By friction on the confining surfaces of expls, on grit particles, or on crystals of the explosive itself and 2) By the adiabatic compression of small occluded gas bubbles the presence even of a small bubble can render most expls very sensitive to impact (Ref 19)... 

Impact produces hot spots, the temperatures of which are (frequently) determined by melting of the solid, being effectively buffered at the melting point. Hence, the mp frequently determines the hot-spot temperature, T0 in the adiabatic-decomposition equation 8.8, listed on p 174 of Cook. If T0 is below a certain critical value, the reaction will not be adiabatic and, owing to heat loss, may not undergo reaction build-up. But above this critical value it becomes effectively adiabatic and expln then always results after a time T. The failure of grit to sensitize an expl may, however, depend simply on the ratio of the mp of the expl to that of the grit particle. 

If this ratio is greater than 1.0, no sensitization by the grit particle results, but if it is less than 1.0, the grit particle effectively sensitizes the explosive because it allows the formation of hot spots of higher temperature than those created in the pure explosive (Ref 23, p 180)... 

Mader (Ref 7) called this hot spot model the hydrodynamic hot spot and the development of this model is largely due to his work. In Ref 7 Mader considered the shock initiation of nitromethane containing inhomogeneities such as voids or grit particles. To quote Mader ... 

Friction between the impacting surfaces, explosive crystals, and/or grit particles in the explosive layer. 

Ref 112) detd the effect of grit particles for frictional initiation of LA... 

Detonation Rate, 1500 m/sec (unconfined and initiated by hot wire), 1700 m/sec (unconfined and initiated by impact with grit particle), and... 

Friction (a) on confining surfaces, (b) on extraneous grit particles, and (c) by intercrystalline movement of the expl particles... 

Friction Sensitivity. LA is more sensitive than MF (Refs 73,79 110). Bowden Gurtcn (Ref 112) detd the effect of grit particles for frictional initiation of LA... 

Explosive Pure Explosive Grit Particles/Cap Initiation Sensitization Gas Senslti... 

Bowden and Yoffe 79. 80] pointed out the role of small trapped gas bubbles in the sensitivity of both liquid and solid explosives. The adiabatic compression of small trapped gas bubbles creates hot spots manifested by high temperature. Also friction occurs at the surface of the containing walls, on grit particles or between the crystals of explosives and forms an additional factor governing the sensitivity of explosives to impact. 

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