Breaking straps to test their strength
GREENSBORO, N.C — Have you ever wondered just how strong those straps are that hold down loads on tractor-trailers? Or have you wondered how safe it is to drive on a highway behind a truck with a load tied down by these straps?
The researchers in the Materials Test Center at the University of North Carolina at Greensboro can answer those questions. The lab is unofficially called "The Fail Lab."
“Ten-thousand, 12,000,” Josh Tucker calls out the increasing pounds of force as he watches a yellow cloth strap pulled tighter and tighter by the machine in front of him. Tucker is a graduate student in mechanical engineering at UNC-Greensboro.
“We not only break things, we get to design the different ways to break things,” Tucker adds. “We have to come up with ways to test these certain products.”
As Tucker watches the strap, you start to hear faint, high pitched pops. Those are the individual fibers breaking. Suddently, the strap blows apart with a bang, blasting into the safety shield separating Tucker from the test. The strap was rated for 17,000 pounds of force. It broke at 20,000 pounds.
“You just really have to have some sort of standard to go by about how items are supposed to perform and we can help set the standard,” says Tucker. “That could be the difference between life and death in some situations.”
Fail Lab provides crucial information to companies
Companies hire the lab and its researchers to test their products and break them.
“I love breaking stuff because it goes 'boom,'” says Evan Kimbro, Ph.D, the lab director, as he laughs and looks at the broken strap. But then he turns serious.
“You actually do want it to fail and we need it to fail so that we can quantify its failure strength,” Kimbro says. “It’s especially important that products fail when the item will be used in a situation where human life is involved because we can quantify the strengths, so that when we do engineering and design and loading we know what the limits are so we don’t go near them or exceed them.”
Testing the flexibility of clothing
However the work isn’t always a matter of life and death. After breaking straps, Tucker places a square piece of material into a machine that looks a bit like a giant hole punch for paper on steroids. It’s called a textile tension test. It’s used to measure the strength and stretchiness of a new fiber for clothing.
Once the machine is turned on, a small round ball is slowly pushed into a hold that is covered by the swatch of material. Ever so slowly, the material stretches until it breaks. Sensors recorded the entire test. Textile firms will use the information to design material for new clothing lines that are strong yet flexible.
Researchers can test products in 13 different ways
But it’s not the breaking or tearing of products that makes the fail lab unique. It’s the research and feedback the lab provides to customers about why a product failed, as well as what can be done to prevent the failure, that makes the lab’s work so valuable. Essentially, the lab provides the information that engineers will use to design products.
Researchers can perform 13 different tests of mechanical and physical properties on products.
“Many times engineers will have a problem where they are trying to figure out what is wrong and we can test the product and provide feedback,” says Kimbro, as he sets up another test. “Using our data, we can give them guidance on where to look as to where the resolution might be.”
Kimbro flicks a switch and another machine begins slowly pulling on both sides of a large piece of balsa wood. It’s a composite; smaller pieces of balsa wood bonded together. It is ultra light but also very strong.
The test is called a sheering test. Braces attached to both sides of the balsa wood slowly pull the piece apart; but the force is applied sideways. Suddenly the wood slides apart. It failed when a force of 6,400 pounds was applied. That’s about the weight of one and half cars.
Seeing the straps In action
Which brings us back to where we started, on the highway. UNC-TV Science met Tony, a truck driver, at a rest stop along Interstate-40 near Greensboro. He was taking a break from the road, hauling sheet rock to Columbia, South Carolina. But he also took time to check the straps holding down his load of wallboard.
“You have to make sure the straps haven’t gotten loose because what happens as you drive along is that the load settles,” Tony says. “So you have to make sure there is a right tension but you can’t over tighten sheet rock, because if you do you will crack it.”
Nine straps secure the thousands of pounds and hundreds of sheets of wallboard to the flatbed. Straps now exposed to the elements and the wear and tear of daily use. “You’ve got to make sure the straps haven’t frayed before you pull out,” adds Tony as he climbed back into the cab.
The straps were all tightened down but none had to be replaced. “Every once in a while you might say this strap is frayed more than I thought it was and then you have to change it. But they hold up, they’re rated to hold a lot of weight.”
Thanks to researchers at The Fail Lab, drivers like Tony know exactly just much weight that is for safety and security while out on the job.
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