A material is said to be frangible if through deformation it tends to break up into fragments, rather than deforming elastically and retaining its cohesion as a single object. Common crackers are examples of frangible materials, while fresh bread, which deforms plastically, is not frangible. A structure is frangible if it breaks, distorts, or yields on impact so as to present a minimum hazard. A frangible structure is usually designed to be frangible and to be of minimum mass. Light poles A frangible light pole base is designed to break away when a vehicle strikes it. This lessens the risk of injury to occupants of the vehicle. Frangible supports are also used for airport approach structures. Bullets A frangible bullet is one that is designed to disintegrate into tiny particles upon impact to minimize their penetration for reasons of range safety, to limit environmental impact, or to limit the danger behind the intended target. Examples are the Glaser Safety Slug and the breaching round. Frangible bullets will disintegrate upon contact with a surface harder than the bullet itself. Frangible bullets are often used by shooters engaging in close quarter combat training to avoid ricochets; targets are placed on steel backing plates that serve to completely fragment the bullet. Frangible bullets are typically made of non-toxic metals, and are frequently used on "green" ranges and outdoor ranges where lead abatement is a concern. Airport structures Following a serious incident where an aircraft hit a donut lighting structure at San Francisco International airport, the FAA instigated frangible design rules for such structures. A frangible object was defined as "an object of low mass, designed to break, distort or yield on impact, so as to present the minimum hazard to aircraft". This characteristic is seemingly contradictory to the operational requirements for stiffness and rigidity imposed on this type of equipment. In order to develop international regulation for the frangibility of equipment or installations at airports, required for air navigation purposes (e.g., approach lighting towers, meteorological equipment, radio navigational aids) and their support structures, ICAO initiated the "Frangible Aids Study Group" in 1981, with the task to define design requirements, design guidelines and test procedures. This work has resulted in part 6 of the Aerodrome Design Manual, dedicated to frangibility. An overview of the activities carried out to achieve these results is given in "Frangibility of Approach Lighting Structures at Airports". The missing reference