A knitted scarf may seem to stretch when you pull it 鈥� but does it really? A close analysis of knitwear as it鈥檚 pulled and tugged has shown that it is the way the fabric鈥檚 stitches 鈥榮lip鈥� that makes it seem like the material it鈥檚 made of聽is stretching.
鈥淵ou can take your grandmother鈥檚 scarf and you can stretch it double the length, but the yarn itself doesn鈥檛 stretch. This is the paradox,鈥� says Frederic Lechenault at Ecole Normale Superieure in Paris, France.
He and his team used a knitting machine to make a patch of fabric 51 stitches square out of a special kind of nylon thread that doesn鈥檛 warp or twist. They used a basic knitting technique, the stockinette stitch, which produces loosely knitted fabrics like those used for a winter pullover.
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Knit one, purl one
One edge of the fabric was clamped down and the other repeatedly pulled as far as it would stretch, releasing it in between. The knit had an elasticity a bit like rubber, eventually pinging back to its original shape even after repeated stretching.
They found the reason knitwear does that is due to the way the interlocked stitches spread friction throughout the material. When yanked, the yarn experiences friction in a stick-slip motion: one stitch will stick a bit to its neighbour and get pulled along until the tension is high enough that it slips away. This happens in a cascade through the fabric. As the speed of the stretching increases, friction goes up.
When released, the force lessens, allowing the stitches to return to their original position. The team confirmed this by tracking the geometric centre of each stitch with a camera.
They also found that the nylon thread isn鈥檛 permanently deformed from knitting or stretching. After five cycles of stretch-and-release, the yarn remained straight when they un-knitted the fabric. The researchers built a model based on their experiments and say the same stretchability should apply to other textiles stitched in the same way, such as wool.
