4 ms·

Windshield Cracks Hold Secrets of Impact

31 points by youngerdryas 13y ago

chiph 13y ago
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.
- btilly 13y ago
  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 13y ago
    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 13y ago
      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.
- claudius 13y ago
  They found a general law (a square root relation) and tested that law on two types of materials. Now of course they can’t go around saying that material X will have n cracks at speed v because material Y had m cracks at this speed. However, given a glass which produced n cracks at an unknown speed, they can take an – ideally – identical glass, hit it at a known speed, count the number of cracks and infer the previously-unknown speed. If the square root law is indeed as simple as I imagine (I didn’t read the paper), they will only need one data point to estimate the unknown speed, though more are obviously helpful. Over time, one could easily build up a database of materials, since there are only so many different cars, that shouldn’t be too difficult.
jacquesm 13y ago
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 13y ago
  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!
300bps 13y ago
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 13y ago
  Also, if you live in Florida, Kentucky, Massachusetts, or South Carolina and have comprehensive insurance, there is no deductible for a windshield replacement.

31 points by youngerdryas 13y ago

chiph 13y ago

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.

btilly 13y ago
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 13y ago
  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 13y ago
    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.
claudius 13y ago
They found a general law (a square root relation) and tested that law on two types of materials. Now of course they can’t go around saying that material X will have n cracks at speed v because material Y had m cracks at this speed. However, given a glass which produced n cracks at an unknown speed, they can take an – ideally – identical glass, hit it at a known speed, count the number of cracks and infer the previously-unknown speed. If the square root law is indeed as simple as I imagine (I didn’t read the paper), they will only need one data point to estimate the unknown speed, though more are obviously helpful. Over time, one could easily build up a database of materials, since there are only so many different cars, that shouldn’t be too difficult.

jacquesm 13y ago

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 13y ago
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!

300bps 13y ago

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 13y ago
Also, if you live in Florida, Kentucky, Massachusetts, or South Carolina and have comprehensive insurance, there is no deductible for a windshield replacement.