Hot Filament Failure
Definition
Fracture of a tungsten bulb filament while at operational temperature (2,500-3,000 K); characterised by ductile deformation at the break, coiling, and tungsten trioxide (WO3) oxidation, confirming the headlight was on at impact.
- Operating temperature
- 2,500-3,000 K
- Filament break appearance
- Ductile deformation, coiling
- Oxidation product
- Tungsten trioxide, WO3
- Investigative conclusion
- Confirms headlight was on at impact
Common questions
Why does the filament's condition at fracture indicate whether the light was on?+
A hot, energised filament is soft and ductile at operating temperature, so impact stretches and coils it before it breaks, while a cold filament is brittle and shatters cleanly, giving distinctly different fracture appearances that reveal the light's on-off state at the moment of impact.
Why does the presence of tungsten trioxide specifically support an on-state finding?+
WO3 forms when the exposed hot tungsten filament reacts with oxygen after the bulb envelope breaks during impact. A filament that was already off and cold does not reach the temperature needed to oxidise this way, so its presence points to the filament being hot at the moment of breakage.
Related terms
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- Cold Filament Failure
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- EUCAP
- European Collection of Automotive Paints, maintained at the Bundeskriminalamt (BKA) in Wiesbaden; the European counterpart to PDQ for automotive hit-and-run paint comparison.
- FCIN
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- LA-ICP-MS
- Laser ablation inductively coupled plasma mass spectrometry; a technique that fires a laser pulse at a sample surface, vaporises a tiny amount,...
- PDQ (Paint Data Query)
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- Tungsten Trioxide (WO3)
- Pale yellow oxide of tungsten formed rapidly at temperatures above approximately 800 K when tungsten is exposed to atmospheric oxygen; its presence...