Term: Spall

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**Mechanical Spalling:**
– Occurs at high-stress contact points
– Can happen in ball bearings
– Plate impact is a simple form
– Cavitation can lead to spalling
– Bubbles collapsing cause localized high pressure

**Spalling in Anti-tank Warfare:**
– Spalling is intended with HESH anti-tank shells
– Soft warheads flatten against armor
– Shock waves travel through armor
– Spall liners protect against spalling
– Kinetic energy penetrators cause spalling

**Spalling in Mechanical Weathering:**
– Common mechanism of rock weathering
– Happens due to large shear stresses
– Can be caused by freezing and thawing
– Unloading leads to spalling
– Salt deposition can cause spalling

**Factors Leading to Spalling:**
– Unloading:
– Pressure release due to overburden removal
– Rapid reduction in pressure causes spalling
– Rapid expansion of rock leads to high surface stress
– Freeze-Thaw Weathering:
– Moisture freezing in rock cracks causes spalling
– Volume expansion from freezing creates large forces
– Outer surface repeatedly undergoes spalling
– Severe freezing can cause surface flaking
– Exfoliation:
– Gradual removal of spall due to temperature changes
– Extreme heat causes differential thermal expansion
– Differential expansion leads to sub-surface shear stress
– Extreme temperature changes can cause spalling

**Other Types of Spalling:**
– Salt Spalling:
– Occurs in porous building materials
– Dissolved salt crystallizes near the surface
– Salt crystals expand and build up shear stresses
– Treatment with penetrating sealants can protect against spalling
– Chimneys show spalling damage due to exposure
– Corrosion:
– Spalling happens as a substance sheds tiny particles
– Corrosion products do not form a barrier
– Volume change during reaction leads to spallation
– Actinide metals can forcibly expel oxide layer
– Actinide metals are handled under inert atmosphere
– Spalling in Refractory Concrete:
– Thermal strain from rapid heating causes spalling
– Internal pressures due to water removal lead to spalling
– Predicting outcome of heating rates is crucial

Spall (Wikipedia)

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.

Very high-speed photography of a small projectile striking a thin aluminium plate at 7,000 m/s. The impact causes the projectile to disintegrate, and generates a large number of small fragments from the aluminium (spallation). This can occur without penetration of the plate.
Spall from knapping obsidian arrowheads and other tools. These unique obsidians are found at Glass Buttes, Oregon.

The terms spall, spalling, and spallation have been adopted by particle physicists; in neutron scattering instruments, neutrons are generated by bombarding a uranium (or other) target with a stream of atoms. The neutrons that are ejected from the target are known as "spall".

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