Mechanical Properties: Choosing the Perfect Fiber

In 2016, I received a lovely skein of yarn for Christmas. It was a small skein of a wool, alpaca, and silk blend that seemed perfect for a hat. The hat looked great and was warm, but it had one major issue: it would constantly slip down and cover my eyes. Was there something wrong with the pattern or my knitting? No, it’s that I didn’t understand the mechanical properties of the yarn.

Mechanical properties are how a material reacts when a force is applied to it. If you pull it, does it break? If it stretches, does it go back to its original shape? If it hangs, does it drape well? If you rub it, does it pill? By understanding the mechanical properties of fibers, you can better choose the yarn for your projects and knit garments that turn out how you imagined.

There are many mechanical properties and even more fibers, so this article focuses on the ones most relevant to knitting: tenacity, elongation/elasticity, drape, and abrasion resistance, all in keratin fibers. Keratin is a protein that makes up animal fibers such as wool, alpaca, angora, and mohair. Silk is another animal fiber, but it is made of the protein fibroin, giving silk different properties.

Tenacity

Tenacity is the tensile strength of a fiber—how much force it takes to break the material in relation to its fineness. The tensile strength of a knitting yarn can vary depending on the structure of the yarn, how it was spun, and how the fiber was produced. One of the largest influences is the type of fiber.

In the world of textile fibers, keratin fibers are relatively weak, but they are strong enough for most knitting projects. You need to consider the strength required of your knitted garment. For something like a scarf, tenacity isn't very important. For a pair of socks to survive many hiking trips, the fibers making them up need to be strong. This is why wool socks often have other fibers, such as nylon, blended in to give them strength.

This sock shows signs of wear on the heel.

The tenacity of a fiber can also change depending on whether it is dry or wet. Keratin fibers become weaker in water. Tenacity in water is very important when thinking about how to launder something. Wool objects can be washed in water but should be cleaned gently. Consider how often and how intensely your knit garment will need to be cleaned when choosing your yarn. (You also need to think of felting when considering wool, but that's another topic!)

The easiest way to measure tenacity at home is to try to break the yarn apart with your hands. If it’s really easy to do, it has low tenacity. If it’s impossible, it has high tenacity. Compare your yarn to similar weight yarns in your stash to see how the different fibers change the tenacity. However, it is also important to consider other fiber properties to create your dream knit garment.

Elongation and Elasticity

Elongation and elasticity are two similar properties related to the stretchiness of a fiber. Elongation is how much a fiber can stretch before breaking. Elasticity is the ability of a fiber to return to its original length after stretching. Wool has high elongation and elasticity, meaning that it can stretch quite a lot and return to its original shape. On the other hand, angora rabbit has low elongation and elasticity and will break rather than stretch when pulled.

When choosing yarn for knitwear,