This section describes the different types of fabric used to make textile actuators; knits and wovens. For the purposes of textile actuators, knit fabrics are used for their anisotropic stretch properties while woven fabrics are used because they are effectively inextensible. Common examples of knit fabrics are t-shirts and socks; garments that stretch under tension. Common examples of woven fabrics are button down shirts or blouses, or any garment that does not stretch under tension.
| A woven fabric is characterized by the perpendicular crossing of two systems of threads (warp and weft yarn). The way in which warp and weft yarn is crossed is called the woven structure or weave pattern. Due to this structure, any tension applied in either direction will be transmitted directly to the yarn. The material will only stretch as much as the fibers themselves stretch under loading. In a textile actuator, the strain induced by air pressure will stretch the yarn of a woven material a small amount, but relative to a knit fabric this stretch is considered negligible. |
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| Knit fabrics contain a structure made up of interlocking loops of yarn known as courses and wales. Applying tension to a knit fabric stretches the material because the knit structure has room to open up before the yarn takes the load. The vertical direction in this example is the least stretchy direction of the knit and the horizontal direction is the most stretchy direction. |
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| Shown to the right is a magnified picture of a one-way stretch warp knit fabric at rest. The letters "a" and "b" represent the courses and wales respectively. Shown below to the left is the same fabric stretched in the horizontal direction. Note the elongation in wale spacing as well as how densely packed the courses become. Below to the right is the same fabric stretched in the vertical direction. Note the increase in course spacing and how tight the fibers look. Approximately the same tension was applied in both cases. The elongation of the wales, "b," in the horizontal stretch example is much larger than the elongation of the courses, "a," in the vertical stretch example. The predominant yarn direction is along the wales, therefore the portion of structural stretch in this direction is lower than along the courses. |
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A stress-strain plot of a knit fabric is shown below. There are three zones of stretch based on how much force is applied: structural stretch, material-stretch, and the non-linear transition between them. The structural stretch discussed above is based on the direction in which the fabric is under loading and has an effective linear stress-strain relationship. If the fabric is a one-way stretch knit, the structural stretch zone will be shorter if it is being pulled in the direction of the wales, and it will be longer if it is being pulled in the direction of the courses. Once the fabric reaches the material-stretch zone, it behaves effectively the same as a woven material with a linear stress-strain relationship.
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