July 1, 2008
Written by Cory Chandler
Shannon Hutchison, senior technician for Texas Tech University’s Debris Impact Facility, tests storm shelter products from across the nation using a cannon that fires 2x4’s at speeds up to 100 mph. He explains why a .44 Magnum’s got nothing on his wooden missiles.
Before we begin, a lesson in physics – kinetic energy specifically:
Kinetic energy, or the energy of motion, is directly proportional to the square of an object’s speed. This may not mean anything to you at this point, but trust me, it’s important.
Let me explain.
You’re driving a car. The car’s speed doubles as it moves from 10 to 20 mph, but its kinetic energy will actually quadruple, meaning that while the car is moving only twice as fast, it will hit an object – another car, say – four times harder. In other words, the same car that merely smashes your fender doing 30 mph is going to total your ride if it hits you at 60.
It also means that a lowly 15-pound 2x4 will rip a hole through a sturdy brick house when moving at 100 mph. Like I said, important.
We have people who call us up and say they have a product they need tested. They say they’ve built a storm door they think will stand up to a tornado. They’ve shot at it with slugs from a .44 Magnum, they say, and it held up just fine.
Then they send it to us for testing. When they get it back, they call and ask what we did to it. Sometimes they have to come out to Lubbock and see our cannon in action before they believe us. Once people come in and see the cannon fire, feel the ground shake, hear the concussion, they get it.
The issue is mass.
A .44 Magnum slug may move much faster than our missiles, but the slug is very small. A 15-pound 2x4 will hit much harder even though it’s not moving nearly as fast.
Bullet-resistant glass? We’ll blow right through it. The glass will stop a slug. It won’t stop a 2x4. Nor will typical residential construction.
I’ve been doing this for seven years. The initial blast of the cannon? The impact?
After a while you get used to it. When I was starting out things would happen that
would make me jump. I’d hear something break and think ‘what was that?’
Yet as much as we think we can predict what’s going to happen, we learn something new with every test. Even a material we’ve tested time and again, you just never know. Texas Tech’s been doing some form of impact testing for about 30 years, and it never fails to amaze me how many ways the tests can break away from the expected.
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