Frangibility

A material is said to be frangible if through deformation it tends to break up into fragments, rather than deforming plastically and retaining its cohesion as a single object. Common cookies or crackers are examples of frangible materials, while fresh bread, which deforms elastically, is not frangible.

A structure is frangible if it breaks, distorts or yields on impact so as to present a minimum hazard to the vehicle. A frangible structure is usually designed to be frangible, and to be of minimum mass.

Smoke outlets
Frangible pavement lights or smoke outlet panels may be placed in the pavement in front of a building, to allow these areas to be broken and smoke to escape from the basement in case of fire. These are commonly found in London, for example, where the frangible area may be outlined by a metal demarcation strip, generally of brass or stainless steel.

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, in which a Boeing 747 hit a portion of the approach lighting structure at San Francisco International airport, the FAA instigated the development of a concept for the frangible design of 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.

Design requirements were defined as follows: the structure should break/distort/yield when subjected to the sudden collision forces of a 3000 kg airborne aircraft travelling at 140 km/h or on ground at 50 km/h. With this requirement formulated, impact experiments were carried out (1976–2000) on commercial and experimental approach light masts in the US, Sweden, The Netherlands, Finland, Norway and Canada, with light wing structures impacting the masts at velocities of up to 140 km/h. Based on the results of these tests, design criteria were formulated. The most important ones are: the support structure should not impose a force on the aircraft in excess of 45 kN and the maximum energy imparted to the aircraft as a result of the collision should not exceed 55 kJ, and: high speed, full scale testing is a proven method for verification of 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 (17) in this article is in "Impact simulation of a frangible approach light structure by an aircraft wing section". With the evolvement of numerical methods suitable for impact analysis, a Chapter 6 was added to the Aerodrome Design Manual part 6, dedicated to "numerical simulation methods for evaluating frangibility". It states that numerical methods can be used to evaluate the frangibility of structures, but that the analytical models should still be verified through a series of representative field tests (an example is given in ref. 9).

Of all equipment or installations at airports required for air navigation purposes, ICAO has not yet formulated frangibility criteria for the tower structure supporting the ILS glide path antenna, "considering its unique nature", basically: its size. A first publication on this subject is given in "Frangible design of instrument landing system/glide slope towers".