Volume 22, Issue 3 (July 1977)
Interpretation of Vehicle Globe Failures: The Unlit Condition
There is little published information on the procedures which should be adopted in examining globes from a vehicle to determine whether they were lit or unlit at the time of an accident. The few which are available (for example, Ref 1) rely heavily on the chemical and physical changes of the components of a globe. None discusses, in terms of microstructure, the reaction of the components to the stress imposed by the accident. The technique of using chemical changes involves, inter alia, noting oxidation of the tungsten filament and adhesion of glass particles to the filament. These changes are sought as evidence for globes being lit at the time of the accident. These results used in isolation can lead to errors. This paper discusses the microstructure of components in new vehicle globes, how the microstructure changes with use, and the difference in reaction of the microstructure to stress as a function of the globe being lit or unlit at the time of the accident. It is suggested that this information used in conjunction with chemical and physical changes leads to a more reliable conclusion, especially when the globe is unlit.