Spall
Spall are fragments of a material that are broken off a larger solid body. It 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.
The terms spall, spalling, and spallation have been adopted by
Mechanical spalling
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.
One of the simplest forms of mechanical spalling is plate impact, in which two waves of compression are reflected on the free-surfaces of the plates and then interact to generate a region of high tensile stress inside one of the plates.
Spalling can also occur as an effect of cavitation, where fluids are subjected to localized low pressures that cause vapour bubbles to form, typically in pumps, water turbines, vessel propellers, and even piping under some conditions. When such bubbles collapse, a localized high pressure can cause spalling on adjacent surfaces.
Anti-tank warfare
In
A kinetic energy penetrator, if it can defeat the armour, generally causes spalling within the target as well, which helps to destroy or disable the vehicle and its crew.[1]
An early example of anti-tank weapon intentionally designed to cause spallation instead of penetration is the wz. 35 anti-tank rifle.
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
Unloading
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.
Freeze–thaw weathering
Some stone and masonry surfaces used as building surfaces will absorb moisture at their surface. If exposed to severe freezing conditions, the surface may flake off due to the expansion of the water. This effect can also be seen in terracotta surfaces (even if glazed) if there is an entrance for water at the edges.
Exfoliation
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 crystallizes 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 engineers[
Chimneys show spalling damage before other portions of buildings because they are more exposed to the elements.
Corrosion
In corrosion, spalling occurs when a substance (metal or concrete) sheds tiny particles of corrosion products as the corrosion reaction progresses. Although they are not soluble or permeable, these corrosion products do not adhere to the parent material's surface to form a barrier to further corrosion, as happens in passivation. Spallation happens as the result of a large volume change during the reaction.
In the case of
Spalling in refractory concrete
There are two drivers for spalling of concrete: thermal strain caused by rapid heating and internal pressures due to the removal of water. Being able to predict the outcome of different heating rates on thermal stresses and internal pressure during water removal is particularly important to industry and other concrete structures.
Explosive spalling events of
See also
- Behind-armour debris
- Frangibility
- Friability
- Misnay–Schardin effect
- Ore dressing
- Pyrophoricity
- Runit Island
- Whipple shield
References
- ^ "How M1 Tanks Work: Sabot". 7 May 2002.
- ^ The Accelerated Drying of Refractory Concrete – Parts I and II, Volume 6, Issues 2 and 4 /The Refractory Worldforum