We’ve been getting bombarded by reports over the last few months, at an ever-increasing rate: another reported incident of suddenly cracking windshields on 390, then 490 then even as far out as 104 in Ontario. Is there some lunatic waiting in the bushes, some wondered, waiting for their moment to do malevolence upon an unsuspecting car? Maybe someone with a near-silent air gun that might elude detection?
To test this particular hypothesis – that a pellet or BB gun might be able to shatter tempered glass – I started looking into the ultimate test case. That turns out to be a British-made pellet gun called the Daystate Air Ranger, a .22 calibre pellet rifle capable of firing at 1020 feet per second. Using the heaviest and therefore most destructive pellet I could find in that calibre, 32 grain, and an online impact force calculator, I came up with the following:
(2mv)/t or (2*0.00208kg*310mps)/1sec = ~1.29N
The tensile strength required by Federal Specification DD-G-1403B, which governs the definition of tempered glass, is 120 to 200N per square millimeter. In other words, while crazier things have happened, it is extremely unlikely that even the most powerful pellet gun will crack a windshield. And weighing in at $2000 for a single gun, it’s safe to say Daysiders are probably not that common in Rochester. Your kid’s Daisy plinker is woefully inadequate to the task, and not likely to be a suspect in this case, at all.
So, when all else fails, the best thing to do is look for the most obvious answer. And that answer is: weather.
Our winter was epic, as we all well know. Those of you who are on Twitter (and why aren’t the rest of you?) will be familiar with the great fun we all had with the #Hothchester hashtag:
— Rochester, NY (@rochester) February 28, 2014
Temperatures never seemed to go above zero for a month. Then, after an extraordinarily long, cold early spring, we suddenly shot up into the 80’s and 90’s, with barely a moment’s transition. This kind of weather plays havoc with all kinds of materials, but your car has probably borne the brunt of the abuse, spending all of it’s time out doors in the deep freeze.
And the thing is: tempered glass is very strong, but only because it is inherently unstable. The tempering process involves laying thin layer after thin layer of glass in an ever-thickening mat, in such a way that the tension lines in each layer oppose one another at odd angles. The result is that, since one layer wants to split in a direction directly opposite of another, the faults cancel each other out. In the event that something large enough hits the glass and shatters it, those tension lines will cause the glass to shatter into small, smooth-edged pellets that will only cause minor skin lacerations at best.
However, those tension lines get stressed by the shrinking that happens in extreme cold. Follow that up with the beating sun of an unexpectedly intense summer, and you’ve got a recipe for a whole lot of shattered windshields. Any small stone or nut from an overhanging tree could be enough to cause a crack. In fact, the window may spontaneously crack all on its own, as has been frequently reported in the past. Rest assured that when the crack happens, however it happens, it will be loud!
Considering the fact that no one has observed a man by the side of the road; a bullet-shaped hole in a windshield; anything more major than a small crack in the windshield. Considering the fact that we can safely rule out pellet guns as a non-lethal, whisper-quiet means to shatter glass. Considering the fact that cracked windshields rarely make headlines in any other circumstance; that a whole industry is built around repairing minor cracks (Safelite, anyone?). Considering all these facts, it really begins to look like a jumpy police force and an overeager media community are making a very obvious problem into a self-propelled mystery.