Friction initiation processes

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. 

Syllabus (1957-1958) pp 137-38 (Frank-Kamenetskii formulation) 151-62 [Session 13, entitled "Hetergeneity of the Initiation Process , includes initiation of solid expls by impact, friction, thermal effect, elastic waves by ultrasonic vibrations, electromagnetic energy and chemical influence. 

Studies at the Cavendish Labs in Cambridge (Refs 109 67b) suggest that mechanical props of an expl have important bearing on its initiation behavior. High speed photography was used to follow the processes preceding the impact and friction initiation of RDX and other expls. It was found that the sample may undergo plastic flow in bulk, show evidence of partial... 

For elastic materials, the contact problem is usually solved as a unilateral contact problem obeying Coulomb s friction law. The algorithms used here are based on those pioneered by Kalker [66]. The contact area, the stick and slip regions, the pressure and traction distributions are numerically determined first and then the stress and displacement distributions within the elastic bodies can be established at the various stages of the tangential cyclic loading. On the basis of these calculations, the occurrence of crack initiation processes can subsequently be analysed in the meridian plane of the contact, y = 0 (Fig. 12), where the cracks first initiate. As a first approach, parameters based on the amplitude of the shear stress, rm, along a particular direction and the amplitude of the tensile stress, [Pg.174]

Frictional initiation has been established as an important initiation mechanism, and the sizes of hot spots required for initiation of some explosives are now well known. However, many of the frictional tests used in practice involve shearing of a layer under load between two rapidly moving surfaces. It seems very likely that other processes, in addition to the frictional ones, are present. These tests, although practically very useful, could be improved if the true frictional sensitivity of the explosive is to be investigated. 

Fig 1 4 25 Plane-strain bending in 50 mm (2 in.) thick 5051-T5 Al. (a) Parent metal bent to 27°, with cracks initiating on the ten-" sile surface, (b) Friction stir processed 5051-T5 Al bent to 85° without cracking. Circle grid analysis of the surface strains showed that the negative minor strain at the crown was less than 1 %. 

Chapter 8 in this volume presents FSP of copper alloys, including NiAl bronze, an alloy frequently used to fabricate ship propellers. Data in Chapter 8 show mechanical and fatigue properties to be improved considerably by FSP. However, as shown previously for an aluminum alloy, the need may arise to friction stir process a prior-fusion repair within an NiAl bronze propeller. Thus, studies were initiated to evaluate procedures and properties for this unique combination of prior processing. Fusion welds were made at the Naval Surface Warfare Center, Carderock Division, using standard Navy weld procedures for NiAl bronze. Figure 14.43 illustrates a multipass 12.7 mm (0.5 in.) penetration fusion weld using Ampcotrode 46 weld wire of composition... 

Laser treatment is also an energy-efficient process. After treated by the C02-pulsed laser beam without photo initiator, the samples showed significant differences in hydrophilicity depending on the number of the laser pulses. The friction coefficient of the PDMS decreased drastically when the surface was treated with the laser, and low platelet spreading and aggregation was observed on the laser-treated PDMS surface. 

The activation factor in the first case is determined by the free energy of the system in the transitional configuration Fa, whereas in the second case it involves the energy of the reactive oscillator U(q ) = (l/2)fi(oq 2 in the transitional configuration. The contrast due to the fact that in the first case the transition probability is determined by the equilibrium probability of finding the system in the transitional configuration, whereas in the second case the process is essentially a nonequilibrium one, and a Newtonian motion of the reactive oscillator in the field of external random forces in the potential U(q) from the point q = 0 to the point q takes place. The result in Eqs. (171) and (172) corresponds to that obtained from Kramers theory73 in the case of small friction (T 0) but differs from the latter in the initial conditions. 

A rather general method of the calculation of the tunneling taking account of the dissipation was given in Ref. 82. The cases of rather strong dissipation were considered in Refs. 81 and 82, where it was assumed that a thermodynamical equilibrium in the initial potential well exists. The case of extremely weak friction has been considered using the equations for the density matrix in Ref. 83. A quantum analogue of the Focker-Planck equation for the adiabatic and nonadiabatic processes in condensed media was obtained in Refs. 105 and 106. 

In the simulations, the inner nanotube was initially displaced relative to the outer tube by a distance x. To reduce surface area, the inner nanotube was pulled into the outer tube and a potential energy minimum was found to exist when the inner tube is completely embedded. However, because of the accumulation of kinetic energy, the inner tube will move past this minimum and extend beyond the other end of the outer tube. In the absence of frictional forces, one would anticipate that this process would be repeated indefinitely, with the relative displacements of the two nanotubes oscillating between +x and —x. In the presence of frictional forces, the maximum displacement will decrease with time. This can be seen from the data shown in Figure 24, where the relative displacement of the nanotubes and frictional forces are shown as functions of time. 

Details of the instability conditions of the debond process and Zmax are discussed in Section 4.3.4. Further, the solution for the initial frictional pull-out stress, (T, upon complete debonding is determined when the debond length, , reaches the embedded length, L, and the crack tip debond stress, ai, is zero ... 

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