Term: Spall
**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 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 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".