Worldwide

Friction in fabrics

R. David Whitby | TLT Worldwide November 2018

How fit and stretch determine the feel of clothing.

It is well known that the feel of a woven fabric garment—such as a shirt, dress or jumper—depends on how it interacts with a person’s skin and other garments, particularly in terms of fit and stretch.

Fabric stretch is an important factor in determining pressure comfort, which largely depends on fabric elastic characteristics and elastic recovery properties. Whether a garment slips or stretches depends on the balance of the tensile forces in the fabric and the frictional forces between the skin and the fabric. A fabric that has a low resistance to stretch and high friction against the skin or another fabric tends to stretch rather than slip. Conversely, a fabric that has lower friction and high tensile resistance tends to slip rather than stretch. A fabric that has high friction resistance and high stretch resistance tends to exert higher pressure on a person’s body, possibly causing discomfort.

For tight-fitting sportswear, the garment’s response to body movements has been analysed by evaluating the dynamic elastic behavior of the fabric, such as dynamic work recovery and stress at specific extensions of the fabric. It has been found that the cotton/spandex fabrics have higher dynamic work recovery and lower stress values than those of cotton fabrics for both wale-wise and course-wise directions (1).

Also, many researchers have identified internal friction as the source of significant hysteresis in cloths, as indicated by force-deformation measurements. A group of researchers at the Massachusetts Institute of Technology showed that an augmented reparameterization of Dahl’s model provides a good match to several important features of cloth hysteresis, even with a minimal set of parameters (2).

Knitting, unlike weaving, can produce loose yet extremely stretchable fabrics with almost no rigidity. It also enables the engineering of arbitrarily shaped two- and three-dimensional objects with different mechanical responses. However, the interplay between the constitutive yarn properties and local topological features of the stitches cannot be easily described by standard continuum mechanics.

Recently researchers at Ėcole Normale Supérieure have found that the ability of a knitted jumper to stretch while it is being pulled on is due to the way in which its stitches slip (3). They made a piece of pullover-like fabric using nylon thread that doesn’t warp or twist. One edge of the fabric was clamped and the other edge pulled as far as it would stretch and then released before being stretched again. The fabric returned to its original shape. The researchers found that, when stretched, the threads experience stick-slip motion, in which one stitch sticks to its neighbor until the tension is sufficient to overcome the friction and slipping occurs. When the fabric is released, the stitches return to their original position.

The researchers reported experimental evidence that knitted fabrics “exhibit mechanical responses characterized by a global elasticity perturbed by plastic events.” A knit consists of a regular network of frictional contacts, linked by the elasticity of the yarn. When deformed, the fabric displays extended avalanche-like stick-slip events at inter-yarn contacts. They measured the size distribution of these avalanches, at the stitch level from the analysis of non-elastic displacement fields and globally from force fluctuations. The two measurements yield consistent power law distributions compatible with similar analysis in other soft amorphous materials. Additionally, the elastic response of the network to a single, local event allows it to retrieve the direction of propagation and intrinsic properties of the avalanches. The study introduces knitted textile as an original example of soft amorphous material that proves to be a handy experimental tool to explore the properties of these systems.

According to the researchers, friction in a knitted fabric proceeds through stick-slip events and that the strong elastic recall forces bring the system back to its minimum energy configuration in high frequency, collective relaxation events of small amplitude.

References
1. M. Senthilkumar and Anbumani, N. (2014), “Dynamic Elastic Behavior of Cotton and Cotton/Spandex Knitted Fabrics,” Journal of Engineered Fibers and Fabrics, pp. 93-100.
2. E. Miguel et al. (2013), “Modeling and Estimation of Internal Friction in Cloth,” ACM Trans. Graph. 32 (6), Article 212.
3. F. Lechenault et al. (2018), “Knits: An Archetype of Soft Amorphous Materials.” Available here.

David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.