
Windshield Cracks Hold Secrets of Impact - youngerdryas
http://physics.aps.org/articles/v6/48?referer=rss
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jacquesm
What cracks me up about whindshield damage is that _every_ time I get a new
car within 2 days or so I get a stone on the windshield chipping my otherwise
perfect view. I always get it fixed but you can't help but notice the repair
(especially at night they can be quite annoying).

Of course, after the initial maiden impact nothing else will ever happen for
the duration of the ownership of the car.

This is probably somehow related to Murphy's law. The secret to these impacts
seems to be that I'm behind the wheel and on any highway at all. I've
contemplated not driving the car for 2 weeks after buying but I'd hate to
admit that I'm getting superstitious about this.

~~~
Nick_C
I feel your pain. I had a new-ish car I was taking on its first road trip,
about 4000 kms into the middle of Australia and back. Ten kilometres out of
Melbourne, and a car coming the other way threw up a small stone which hit the
windscreen and caused a six inch crack almost right in the centre of my field
of view. I had to put up with that crack for the next 3990 kms and three
weeks. Aargh!

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chiph
They have videos showing the regular structure of cracks in Plexiglas (4
cracks and 8 cracks), and a photo of many cracks appearing in what may be
automotive glass.

The thing that struck me (no pun intended) is that Plexiglas is a polymer and
thus has a more-or-less regular repeating internal structure, while glass is
amorphous, with little internal structure. So comparing the crack structure
between them after similar impacts probably wouldn't give you the same
information about the energy input. Which might be the point, but it appears
they're comparing apples & oranges, so lessons learned in glass vs. lessons
learned with polymers wouldn't be transferable.

Also - automobile windshield glass is laminated (to prevent shards of broken
glass flying around the inside of your car), and there are different materials
used for the laminating plastic & adhesive, depending on the "recipe" chosen
by the manufacturer. So the crack structure would be different between them,
and again, I doubt the lessons are transferable.

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btilly
If you read the article, you'd know that they tested both Plexiglas and
automotive glass. The scaling relationship that they found experimentally
between speed of impact and number of cracks they then verified theoretically.

Different specific windshield designs will, of course, vary. But this is a
good starting point across designs.

~~~
chiph
My point is that it _isn't_ a good starting point across designs, because the
designs vary so much as to make their formula inapplicable in the general
case.

Now, if they said that their formula was consistent within a certain class of
materials & construction, I'd buy it. But even then with the Plexiglas, their
formula predicted 7 cracks, while the experiment produced 8. They need to
explain that, and I'm saying it's because of the near-crystalline internal
structure of Plexiglas. If they had altered their hypothesis & and prediction,
and then retested, and did some analysis to explain it (perhaps I'm wrong -
maybe its because of some other factor and not the internal structure) I'd buy
it.

~~~
btilly
It is a scaling rule for a statistical phenomena, not an exact formula. For
different kinds of surfaces, you'll have a different constant.

Getting 7 cracks versus 8 in the experiment is actually a pretty good fit.
Particularly since repeating the experiment multiple times would be unlikely
to give the exact same number of cracks. (The cracking pattern does have a lot
of randomness in it.)

What is more important is that if the sqrt scaling rule proves reasonably
consistent across a wide variety of types of materials, then testing the
cracking pattern for different kinds of impacts across different kinds of
windshields becomes a much more tractable problem.

The obvious followup becomes how angle of impact matters. My hope would be
that it is something reasonably simple - such as very little is deposited by
friction, so it is only the component of the velocity normal to the surface
that matters.

Establish those two things reasonably well, and now you're in a good position
to measure every kind of windshield a handful of times, and you have some
useful information for forensics on all kinds of windshield cracks.

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300bps
Public service announcement: if you have a crack in your windshield that is
smaller than a U.S. dollar then it likely can be repaired. If it can be
repaired, most insurance companies in the U.S. will waive your comprehensive
deductible. That's right - the glass company will either direct bill the
insurance company or the insurance company will reimburse you. Either way, if
your insurance company is like most it will not cost you anything to have it
repaired.

Another fun fact is that any day you have a small chip or crack, it can
instantly turn into a huge crack that is unrepairable. So if you have a small
chip or crack get it repaired right away. Check with your insurance company -
it will probably cost you nothing.

~~~
bostonpete
Also, if you live in Florida, Kentucky, Massachusetts, or South Carolina and
have comprehensive insurance, there is no deductible for a windshield
replacement.

