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 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 occurs at high stress contact points, for example, in a ball bearing
. Spalling occurs in preference to 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.
In anti-tank warfare
, spalling through mechanical stress is an intended effect of high explosive squash head
(HESH) anti-tank 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 tanks
and other armored fighting vehicles
(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 armor
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.
(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 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 crystallises 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.
Some believe that 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. Great care and expert advice must be taken, though, to ensure that any coating is compatible with the substrate in terms of breathability (ability to allow the release of vapors from inside while preventing water intrusion), or other serious problems can be created.
It must always be assumed that water, — possibly even arriving in vapor form from the interior, — will collect behind the wall surface, and it must be allowed to both drain and evaporate. Many a brick and stone has been damaged beyond repair by the well-intentioned application of the wrong coating, once the coated masonry has passed through a few freeze-thaw cycles, pipe leaks, etc.
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 toxicity and (often to a lesser extent) radioactivity, make them very dangerous to handle in metallic form.