Spall are fragments of a material that are broken off a larger

solid Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...

body Body may refer to: In science * Physical body, an object in physics that represents a large amount, has mass or takes up space * Body (biology), the physical material of an organism * Body plan, the physical features shared by a group of anima ...

. It can be produced by a variety of mechanisms, including as a result of

projectile A projectile is an object that is propelled by the application of an external force and then moves freely under the influence of gravity and air resistance. Although any objects in motion through space are projectiles, they are commonly found in ...

impact,

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...

cavitation Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...

, or excessive rolling pressure (as in a

ball bearing A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races. The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this ...

). Spalling and spallation both describe the process of surface failure in which spall is shed. GVFKAStoneToolCraftsmanShowSpalls

The terms ''spall'', ''spalling'', and ''spallation'' have been adopted by

particle physicists Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) a ...

; in

neutron scattering Neutron scattering, the irregular dispersal of free neutrons by matter, can refer to either the naturally occurring physical process itself or to the man-made experimental techniques that use the natural process for investigating materials. Th ...

instruments,

neutrons The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave ...

are generated by bombarding a

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

(or other) target with a stream of

atoms Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, an ...

. 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

. 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

, where fluids are subjected to localized low pressures that cause vapor 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

anti-tank warfare Anti-tank warfare originated from the need to develop technology and tactics to destroy tanks during World War I. Since the Triple Entente deployed the first tanks in 1916, the German Empire developed the first anti-tank weapons. The first devel ...

, spalling through mechanical stress is an intended effect of

high-explosive squash head High explosive squash head (HESH) in British terminology, or high explosive plastic/plasticized (HEP) in American terminology, is a type of explosive projectile which uses a plastic explosive that conforms to the surface of a target before deton ...

(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

tank A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engin ...

s and other

armored fighting vehicle An armoured fighting vehicle (AFV) is an armed combat vehicle protected by armour, generally combining operational mobility with offensive and defensive capabilities. AFVs can be wheeled or tracked. Examples of AFVs are tanks, armoured cars, ...

s (AFVs) and explodes, creating a

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...

that travels through the armor as a compression wave and is reflected at the free surface as a tensile wave breaking (tensile stress/strain fracture) the 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 and/or its crew. Many AFVs are equipped with spall liners inside their

armor Armour (British English) or armor (American English; see spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat, or fr ...

for protection. A

kinetic energy penetrator A kinetic energy penetrator (KEP), also known as long-rod penetrator (LRP), is a type of ammunition designed to penetrate vehicle armour using a flechette-like, high-sectional density projectile. Like a bullet or kinetic energy weapon, this type ...

, if it can defeat the armor, generally causes spalling within the target as well, which helps to destroy or disable the vehicle and its crew. An early example of anti-tank weapon intentionally designed to cause spallation instead of penetration is the

wz. 35 anti-tank rifle The Model 35 antitank rifle (''Karabin przeciwpancerny wzór 35'', abbreviated "''kb ppanc wz. 35''") was a Polish 7.9 mm anti-tank rifle used by the Polish Army during the 1939 Invasion of Poland. It was designated model 35 for its desi ...

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.

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

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. 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 terra-cotta surfaces (even if glazed) if there is an entrance for water at the edges.

Exfoliation

200px, Granite dome exfoliation 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 outermost layer becomes much hotter than the rock underneath causing differential

thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...

. 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 material Building material is material used for construction. Many naturally occurring substances, such as clay, rock (geology), rocks, sand, wood, and even twigs and leaves, have been used to construct buildings. Apart from naturally occurring materia ...

s, 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 believe that porous building materials can be protected against salt spalling by treatment with penetrating sealants which are

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

(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 consulted, however, 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. 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 A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...

) 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

actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...

metals (most notably the

depleted uranium Depleted uranium (DU; also referred to in the past as Q-metal, depletalloy or D-38) is uranium with a lower content of the fissile isotope than natural uranium.: "Depleted uranium possesses only 60% of the radioactivity of natural uranium, hav ...

used in some types of

ammunition Ammunition (informally ammo) is the material fired, scattered, dropped, or detonated from any weapon or weapon system. Ammunition is both expendable weapons (e.g., bombs, missiles, grenades, land mines) and the component parts of other weap ...

), the material expands so strongly upon exposure to air that a fine layer of oxide is forcibly expelled from the surface. A slowly

oxidised Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

plug of metallic uranium can sometimes resemble an

onion An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus ''Allium''. The shallot is a botanical variety of the onion ...

subjected to

desquamation Desquamation occurs when the outermost layer of a tissue, such as the skin, is shed. The term is . Physiologic desquamation Keratinocytes are the predominant cells of the epidermis, the outermost layer of the skin. Living keratinocytes reside in ...

. The main hazard however arises from the

pyrophoric A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...

character of actinide metals which can spontaneously ignite when their specific area is high. This property, along with the inherent

toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

and (for some to a lesser extent) radioactivity of these elements, make them dangerous to handle in metallic form under air. Therefore, they are often handled under an inert atmosphere (

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

) inside an

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

glovebox A glovebox (or glove box) is a sealed container that is designed to allow one to manipulate objects where a separate atmosphere is desired. Built into the sides of the glovebox are gloves arranged in such a way that the user can place their han ...

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 refractory concrete can result in serious problems. If an explosive spalling occurs, projectiles of reasonable mass (1–10 kg) can be thrust violently over many metres, which will have safety implications and render the refractory structure unfit for service. Repairs will then be required resulting in significant costs to industry.''The Accelerated Drying of Refractory Concrete – Parts I and II'', Volume 6, Issues 2 and 4
The Refractory Worldforum
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References

External links

Midwest Chimney Safety Council
{{Webarchive, url=https://web.archive.org/web/20210608224214/http://www.mcsc-net.org/ , date=2021-06-08
Masonry Heater Association of North America
Ammunition Concrete Corrosion Vehicle armour Materials degradation