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.
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.
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.
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 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.
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