Spall 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 . The terms 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 . 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 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 . Antitank warfare . 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 tank s and other armored fighting vehicle s ( AFVs ) and explodes , creating a shock wave 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 for protection . A kinetic energy penetrator , if it can defeat the armor , generally causes spalling within the target as well , which helps to destroy / disable the vehicle and / or its crew . 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 . 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 . 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 ( 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 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 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 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 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 strongly upon exposure to air that a fine layer of oxide is forcibly expelled from the surface . A slowly oxidised plug of metallic uranium can sometimes resemble an onion subjected to desquamation . The main hazard however arises from the pyrophoric character of actinide metals which can spontaneously ignite when their specific area is high . This property , along with these elements inherent toxicity and ( for some to a lesser extent ) radioactivity , make them dangerous to handle in metallic form under air . Therefore , they are often handled under an inert atmosphere ( nitrogen or argon ) inside an anaerobic glovebox . References . Midwest Chimney Safety Council Masonry Heater Association of North America