Spall 303690 215974910 2008-05-30T14:45:13Z 12.41.208.210 /* Mechanical spalling */ {{For|the town in Germany|Spall, Germany}} {{unreferenced|date=May 2008}} [[Image:Aluminium plate spalling.gif|right|frame|Very high speed photography of a small projectile impacting a thin aluminium plate at 7000 m/s. The impact disintegrates the projectile and generates a large number of small fragments. This can occur without penetration of the plate]] '''Spall''' are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of [[projectile]] impact, [[corrosion]], [[weathering]], [[cavitation]], or excessive rolling pressure (as in a ball bearing). '''Spalling''' and [[spallation]] both describe the process of surface failure in which spall is shed. '''Spall''' and '''spalling''' have been adopted by [[particle physics|particle physicists]]; in [[neutron scattering]] instruments, [[neutrons]] are generated by bombarding a [[uranium]] target with a stream of [[atoms]]. The neutrons that are ejected from the target are known as spall. ==Mechanical spalling== Mechanical spalling occurs at high stress contact points, for example, in a [[ball bearing]]. Spalling occurs in preference to [[Brinell scale|brinelling]] where the maximal shear stress occurs not at the surface, but just below, shearing the spall off. Spalling can also occur as an effect of [[cavitation]], where fluids are subjected to localized low pressures that cause vapor bubbles to form, typically in pumps, water turbines, vessel propellors, and even piping under some conditions. When such bubbles collapse, a localized high pressure can cause spalling on adjacent surfaces. ===Antitank warfare=== In [[anti-tank warfare]], spalling through mechanical stress is an intended effect of [[high explosive squash head]] (HESH) anti-tank [[Shell (projectile)|shells]] and many other munitions which may not be powerful enough to pierce the armor of a target. The relatively soft warhead, containing or made of plastic explosive, flattens against the armor plating on [[tank]]s and other [[armoured fighting vehicle|armored fighting vehicle]]s (AFVs) 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 disablement of a vehicle. Many AFVs are equipped with spall liners inside their [[vehicle armour|armor]] for protection. ==Spalling in mechanical weathering== Spalling is a common mechanism of rock weathering, and occurs at the surface of a rock when there are large shear stresses under the surface. This form of [[Mechanical weathering]] can be caused by freezing and thawing, unloading, thermal expansion and contraction or salt deposition. [[Freeze thaw weathering]] is caused by moisture freezing inside cracks in rock. Upon freezing its volume expands, causing large forces which cracks spall off the outer surface. As this cycle repeats the outer surface repeatedly undergoes spalling, resulting in weathering. Unloading is the release of pressure due to the removal of an overburden. When the pressure is reduced rapidly, the rapid expansion of the rock causes high surface stress and spalling. ===Exfoliation=== [[exfoliation (geology)|Exfoliation]] (or onion skin weathering) is the gradual removing of spall due to the cyclic increase and decrease in the temperature of the surface layers of the 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 different [[thermal expansion]]. This differential expansion causes sub-surface shear stress, in turn causing spalling. Extreme temperature change, such as forest fires, can also cause spalling of rock. This mechanism of weathering causes the outer surface of the rock to fall off in thin fragments, sheets or flakes, hence the name exfoliation or onion skin weathering. ===Salt spalling=== Salt spalling is a specific type of weathering which occurs in porous building materials, such as brick, natural stone, tiles and concrete. Dissolved salt is carried through the material in water and crystalises inside the material near the surface as the water evaporates. As the salt crystals expand this builds up shear stresses which break away spall from the surface. Porous building materials can be protected against salt spalling by treatment with penetrating sealants which are [[hydrophobic]] (water repellent) and will penetrate deeply enough to keep water with dissolved salts well away from the surface. ==Corrosion== In [[corrosion]], spalling occurs when a substance (metal or concrete) sheds tiny particles of corrosion products as the corrosion reaction progresses. These corrosion products are not soluble or permeable, but, unlike [[passivation]], they do not adhere to the parent material's surface to 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 [[heavy metals|toxicity]] and (often to a lesser extent) [[radioactivity]], make them very dangerous to handle in metallic form. == See also == * [[Misznay-Schardin effect]] * [[Pyrophoric]] * [[Ore dressing]] [[Category:Ammunition]] [[Category:Vehicle armour]] [[Category:Materials science]] [[et:Helvesväsimus]]