The science of the strongest stitch

A teen shows that some kinds of sewing are tougher than others

Holly Jackson Broadcom

Holly Jackson shows off her winning seam at the Broadcom MASTERS Public Day.

B. Brookshire/SSP

WASHINGTON — Most people may not give much thought to the thread that binds their clothes together, that keeps teddy bear’s stuffing from falling out and that holds a parachute intact. But Holly Jackson, 14, has always loved to sew. She decided to find out which type of sewing stitch was strongest. The teen’s results could help make fabric products from seatbelts to spacesuits stronger.

Holly presented the results of her eighth grade science fair project at a competition called Broadcom MASTERS (Math, Applied Science, Technology and Engineering for Rising Stars). This annual science program was created by the Society for Science & the Public. It’s sponsored by Broadcom, a company that builds devices to help computers connect to the Internet. Broadcom MASTERS brings together students who conducted research in middle school from all over the United States. Finalists share their science projects with each other and the public in Washington, D.C.

The teen, now a freshman at Notre Dame High School in San Jose, Calif., says that sewing is far more important than most people realize. “Any time you want to connect two piece of fabric together you have to sew it,” she explains. “I think that sewing is a really fundamentally important thing in the world.” Holly decided that she wanted to know whether nylon or polyester thread was stronger. She also tested which stitches were stronger, seams stitched in a straight line or those sewn with a zigzag stitch.

Holly brought some of her fabric swatches to show how fabric tears along seams. P. Thornton/SSP
Holly sewed samples of denim or nylon fabric together using either polyester or nylon thread. Some seams were sewn in straight lines. Others used the zigzag stitch.  She then built a machine to apply weight that pulled heavily on the sewn seams. Seams were pulled until they ripped apart. Her system also recorded the force needed to break the seam.

“I had the seam being pulled apart by two pipes,” she explains. “The pipes were being pulled apart by an electric winch, which I had at the bottom the pipes.” The pipes pulled down on a bathroom scale. A slow motion camera recorded the maximum force (or weight) exerted before the seam broke. Afterward, Holly could play back the footage and read the exact weight at which each seam gave out.

At first, Holly thought that she could just weigh the sample down until it tore. But she soon realized that strong samples needed far more weight than she had expected. Then she ran across a video on the Internet. It showed a machine “with a winch that pulled apart a sewn sample,” she notes. “I had a winch from a dancing bear toy, and I used that. It worked really well!”

The nylon thread proved stronger. Similarly, straight seams held up better than zigzagged ones. A zigzag seam concentrates force at the points of the zigs and zags, while a straight seam spreads force across a long line, Holly says. It turned out that a strong seam can be very tough to tear. Her strongest sample, with polyester thread in a straight seam, tore at 136 kilograms (300 pounds).

The teen hopes that her findings will help people create strong seams on more than just blue jeans. “What about going to Mars?” she says. “How are we going to get the right space suit? And when rovers go to Mars, they have parachutes [to slow them down as they land on the planet].” Those could rip if their seams are not iron strong, she notes. As scientists explore space, Holly says, the fabrics, threads and stitches they use to hold their equipment together could make a big difference. 

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Power Words

fabric  Any flexible material that is woven, knitted or can be fused into a sheet by heat.

force  Some outside influence that can change the motion of a body or produce motion or stress in a stationary body.

nylon   A silky material that is made from long, manufactured molecules called polymers. These are long chains of atoms linked together.

polyester  A synthetic material used chiefly to make fabrics.

polymer  Substances whose molecules are made of long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).

rover  A carlike vehicle, such as those designed by NASA to travel across the surface of the moon or some planet without a human driver. Some rovers also can perform computer-driven science experiments.

seam  The site where two or more fabrics are held together with stitches or are fused together by heat or glue. For non-fabrics, such as metals, the seams may be crimped together or folded over several times and then locked in place.

stitch  A length of thread that binds two or more fabrics together.

synthetic (as in materials) Materials created by people. Many have been developed to stand in for natural materials, such as synthetic rubber, synthetic diamond or a synthetic hormone. Some may even have the same chemical structure as the original.

winch    A mechanical device used to wind up or let out rope or wire. Increasing tension with a winch increases the force applied to the rope or wire. Among possible uses: A winch can pull a sail up the side of a mast on a ship or increase the force applied to a material to test its strength.

Bethany Brookshire was a longtime staff writer at Science News Explores and is the author of the book Pests: How Humans Create Animal Villains. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.