Cold or Hot Lamp-Filaments in a CRASH !!?? - Page 1
Forum section: electronics / general

author: GlennSprigg
author_rank: Super Contributor
timestamp: June 05, 2019, 10:09:49 am
content: I've always believed, and seen in manyVehicle/Carcrash investigation shows, that discuss whathappens tolamp-filaments, like brake-lights in say a front end collision, to be like this........If the vehiclewas brakingat the time, then the filament is usually broken, due to theinertiaofthe white-hot (and now very soft) filament, still flying forward, and hence Breaking. (Not when cold).However, I just watched an Episode of "Air Crash Investigations", where they said theopposite(In this case, not 'brake' lights, but some other lamps after crashing at the end of a runway).They said... "During a crash, a cold filament is brittle, causing it to break, whereas a hot filamentis flexible, so it will just bend, and not break, in a collision"I call this bullshit ??

author: Psi
author_rank: Super Contributor
timestamp: June 05, 2019, 12:07:01 pm
content: It will obviously depend on the force of the crash. If it's large enough the filament will be broken regardless.I would expect that the state, broken or intact, is not that important.Placing whats left of the filament under a microscope is likely how you determine if it was on or off at the time of the crash.Maybe looking at the oxide coating level or the fracture patterns at the ends of the breaksThe broken vs intact thing sounds more like hollywood making up a simple story that will seem believable to the general public.

author: soldar
author_rank: Super Contributor
timestamp: June 05, 2019, 12:11:50 pm
content: I believe it is not whether it broke or not but the way it broke. A glowing filament will break in a different way. The ends will have melted as it broke.

author: Zero999
author_rank: Super Contributor
timestamp: June 05, 2019, 12:27:52 pm
content: Whether the break light was on or off does not conclusively prove whether the driver was breaking or not. There might be a fault with the switch causing the break light to be permanently on or not turn on. I suppose that could be investigated, but then it would be questionable whether the fault with the switch existed before the collision or not.The tyres and road surface will be better indicators of whether the driver was breaking or not.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 05, 2019, 02:57:10 pm
content: Depends where you need to look for the filaments and if they were mixed with other lamps that are not brake lights.Some lamps share brake light functionality with the rear lights anyway, so are always hot.So yeah, i think the tungsten looks different if it blew when hot (glass breaks, filament immediatly combusts in the middle) or broke cold (breaks off in pieces). Tungsten is quite brittle, even in helixes.But there are usually plenty of other signs, like amount of rubber on the road (ABS does this less when in use, but only > 8km/h; below 8 km/h it is practically off), weight distribution between front and rear axle, therefore how the cars body was angled, where it hit obstacles and how.And there is of course a log in airbag controllers to figure out which sensor caused a deployment and at which value, simply for liability reasons.Certain other controllers also save parameters like wheelspeed and I/O states when trouble codes are triggered, but those might be less useful because of debounce times.But CSI:Cyber will probably not come because someone needs to pay that fender.

author: Gregg
author_rank: Super Contributor
timestamp: June 05, 2019, 03:39:06 pm
content: A lot of differences in a relatively high current DC filament breaking under power from shock and an AC powered filament in similar situations. The Relative high current DC filament will draw a sizeable arc upon breaking and probably ball up the ends whereas the AC powered filament has zero crossings to extinguish the arc. The runway lights possibly have long service filaments that are thicker than the household incandescent lamps and may be far more flexible when powered than when cold.

author: amyk
author_rank: Super Contributor
timestamp: June 06, 2019, 12:30:40 am
content: A hot filament subjected to a shock will stretch considerably before breaking; a cold filament will tend to "shatter" into many pieces instead.

author: GlennSprigg
author_rank: Super Contributor
timestamp: June 07, 2019, 11:00:27 am
content: Quote from: Psi on June 05, 2019, 12:07:01 pmThanks Psi, (and to others). Yes of course the force of the collision would govern what if anything would happen,through to total annihilation. And your middle paragraph makes total scientific sense too !!Others were suggesting better/other methods of determining if say a car was breaking at collision time, andalthough that is certainly true, the main point of my post was regarding the 'norm' for a hot/cold filament.Police investigators here in Australia at least, generally believe that in a moderately severe collision, thatif the brake-light was blown, then their foot was more than likely on the brake at the time. Then again, inthe U.S. I believe, a cars turning-indicators just flash one of the brake-lights?? which could cause confusion.

author: CJay
author_rank: Super Contributor
timestamp: June 07, 2019, 02:38:07 pm
content: Quote from: Zero999 on June 05, 2019, 12:27:52 pmAgree but it's not the only factor used, depending on the prevailing road conditions it can be used in conjunction with other evidence, if you really make a mess of things, they can even use 'needle slap'.

author: Cyberdragon
author_rank: Super Contributor
timestamp: June 07, 2019, 08:13:48 pm
content: Quote from: Psi on June 05, 2019, 12:07:01 pmThat's strange, hollywood seems to like to keep car lights on even after being totally mutalated. Examples like Jurassic Park when the car gets rolled over, chewed on, stomped on, knocked over a massive ledge into a tree, falls down the tree onto the ground and flips and the lights are still on. My personal favorite is Super 8 when the truck gets hit by a train, explodes into a giant fireball and gets thrown but when they find the remains of the truck, which had been totally bifracated Mythbusters snowplow style, the driver was still alive and the remaining headlight still running.