Spall
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Spall are flakes of a material that are broken off a larger solid body, usually as a result of projectile impact. Spalling describes surface failure in which chips are shed from a contact point, for example, in a ball bearing. This is due to the maximal shear stress being not at the surface but just below it leading an overloaded bearing to lose flakes rather than brinelling. Another common form of spalling occurs due to the mechanical weathering of rocks. Mechanical weathering can be caused by freezing and thawing, unloading, and thermal expansion and contraction. Freezing and thawing is caused by moisture freezing inside cracks in rock, its volume expanses, and cracks off the outer surfaces. Unloading is the release of pressure due to the removal of an overburden. When the pressure is reduced rapidly, expansion occurs causing spalling. Gradual removing of these spalls by weathering is called exfoliation (geology). Also extreme temperature change, such as forest fires, can cause spalling of rock. Rocks do not conduct heat well, so when they are exposed to extreme heat the outer most layer becomes much hotter than the rock underneath causing it to expand more rapidly. This rapid expansion causes the outer surface of the rock to fall off in thin fragments, sheets or flakes. Spalling can occur on a concrete surface if exposed to salt or if improperly finished. Spallation is a process that physicists take advantage of under special circumstances. In neutron-scattering instruments, neutrons are generated by striking a uranium target with atoms. The neutrons that are ejected from the target are spall.
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[edit] Salt spalling
Occurs in porous building materials, such as brick, natural stone, tiles and concrete, when dissolved salt is carried through the material in water and the salt crystalises inside the material near the surface, breaking away shards of the surface as the salt crystals expand.
Porous building materials can be protected against salt spalling, by treatment with impregnating sealers which are hydro-phobic (water repellent) and will penetrate deeply enough to keep water with dissolved salts well away from the surface.
[edit] Corrosion
In corrosion, spalling occurs when a substance (metal or concrete) sheds tiny particles of corrosion products as the reaction progresses. These corrosion products are not soluble or permeable, but, as opposed to passivation, they fail to adhere to the material's surface or form a barrier to further corrosion. This happens as the result of a large volume change during the reaction. In the case of actinide metals (most notably the depleted uranium used in some types of ammunition), the material expands so violently upon exposure to air that a fine powder of oxide is forcibly expelled from the surface. This property, along with these elements' inherent toxicity and (often to a lesser extent) radioactivity, make them very dangerous to handle in metallic form.
[edit] Antitank warfare
In antitank warfare, spalling is an intended effect of the high explosive squash head (HESH) anti-tank shells and of many other munitions which may not be powerful enough to pierce the armor of a target. The relatively soft warhead flattens against the armor plating on tanks and other armored fighting vehicles and explodes, creating a shock wave that travels through the armor and breaks the softer metal on the inside. The resulting spall is dangerous to crew and equipment, and may result in a partial or complete kill of a vehicle. Many AFVs are equipped with spall liners inside their armour for protection.
