CASR
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In Detail |
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by T.S. Rea |
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Defence Technology — January 2005
Protecting Canadian Soldiers In The Field: “... short, sharp, shock ...” – Shock wave Behaviour in Ceramic-like Materials There is an easily demonstrated model of how a shock wave behaves in ceramic- like material. The use of various weights of chisels and hammers on natural stone is analogous to the effect of armour-piercing projectiles upon ceramic armours. A violently hammered chisel can deliver energies in the range of a pistol bullet at maximum range. The value of the chisel-and-stone model is that it demonstrates the roles mass and inertia-acceleration play in the propagation of overpowering pulses of energy migrating through the medium – be they cracks or pockets of phase state change material, or any other pattern of successively overwhelmed molecular bonds. In Figure 1 (below) , a heavy chisel and hammer is placed on the smooth surface near the edge of a piece of homogenous stone material. When struck, this stone will cleave off more or less in the direction of the chisel’s angle of impact thereby producing the desired effect of a naturally cleaved rock surface. If the distance of the cleaved surface is greater, the desired effect is most reliably achieved with the heaviest available chisel and hammer.
In Figure 2, a light chisel and hammer is used with the same or greater amount of force – the result being either no fissure or a premature migration of the shock wave to the surface. If the hammer is used with several times a greater force, the shockwave will travel only slightly deeper into the face. Then, that shock wave will then reflect off of the face of the stone. Migrating backwards into the stone, the shock wave will blow out the stone’s bottom section – an undesired effect , especially near the end surface of the stone where a doubly reinforced reflection will occur. The sharper pulse of energy delivered by the lighter media is diffused more rapidly and has less directional cohesion than the more massive impactor. Sedimentary rock was formed in layers. Such rock struck at angles perpendicular to the layers requires a distinctly greater force to produce an adequate depth of fissure, since these layers partially reflect the shock wave laterally and attenuate its forward progression. Irregular surfaces at the contact and reflection points also significantly reduce the cohesion of the energy or shock wave. The more massive the object being struck is, relative to the impactor, the greater is its effect at diffusing the propagation of the shock wave through its inherent inertia and diffusive effects. All materials in the armour system must be capable of absorbing and transmitting the impact energy within their individual limits. The maximum threshold of resistance of each material dictates where it can be employed, and in what quantities and form. < Part 2 — “... in every fibre of his being ...” the Fibre And Ceramic Armours |
