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.

Spall from knapping obsidian arrowheads and other tools. These unique obsidians are found at Glass Buttes, Oregon.
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 aluminum (spallation). This can occur without penetration of the plate.

The terms spall, spalling, and spallation 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".