Fabric Terminology
When buying fabrics for composite projects, you have many choices besides fiber type. The weight of the fabric, its thickness, type of weave, and drapeability are all important.
Most fabric suppliers, especially those who specialize in the home-built market, describe their materials by the areal density, or weight per unit area (usually just called weight), of the dry fabric. The most common unit of measure is ounces per square yard, often simply abbreviated as ounces. Thus, a fabric with a weight of 5.4 oz really has an areal density of 5.4 oz/sq yd.
Fabric weight is only one descriptor, and perhaps the least important. Thickness is primarily a function of weight and fiber type, but also depends on the weave. For example, Aircraft Spruce lists two glass fabrics of similar weight (5.79 oz and 5.85 oz); the heavier cloth has a thickness of 0.009 in, the lighter cloth a slightly larger thickness of 0.010 in.
To fully understand fabric terminology, we need to start with the fibers themselves. Most of the terms used to describe both fibers and fabrics come from the textile industry, predating composite materials. Thus, the same terms are used for any type of fiber, be it glass, Kevlar, graphite, or something else.
A single fiber is usually referred to as a filament. You can buy a single continuous filament rolled on a spool; this is called a monofilament. Mostly, however, you buy filaments bunches called a strand or an end. If the filaments are all parellel to each other, the end is called a roving (graphite rovings are also referred to as tows). Rovings are usually denoted by the number of ends they contain; tows by the number of filaments. The most common graphite tows are 3K, 6K, and 12K. If instead of being straight the filaments are twisted to hold the fibers together, the bundle is called a yarn.
Another term used to describe yarns is denier. This term isn't used very often for composites, and is most often applied to Kevlar. Denier is equal to the weight in grams of 9000 meters of yarn. The lower the number, the finer the yarn. For example, a 380 denier Kevlar 49 yarn has 267 filaments, and an 1140 denier yarn has 768 filaments.
Either roving (tow) or yarn can be woven into a fabric. If roving is used, the fabric is called woven roving; if yarn is used the fabric is called cloth. Many suppliers use the term cloth for woven roving, but refer to the weave as flat, not twisted. Cloth composites can have a slightly lower resin conent than woven rovings because the yarn bundles are more compact than rovings, but the twist weakens the fibers somewhat. In the example given above from the Aircraft Spruce catalog, the thicker but lighter fabric is a woven roving, and the other is a cloth.
If you look at a roll of fabric, some of the yarns run in the direction of the roll and are continuous for the entire length of the roll. These are the warp yarns. The short yarns which run crosswise to the roll direction are called the fill yarns. Warp yarns are usually called ends and fill yarns picks, but sometimes the fill yarns are also called ends. (The terms apply equally to rovings, but yarn will be used in the rest of the discussion for simplicity.)
Fabric count refers to the number of warp yarns (ends) and fill yarns (picks) per inch. For example, a 24 x 22 fabric has 24 ends in every inch of fill direction and 22 picks in every inch of warp direction Note that you count warp yarns in the fill direction and fill yarns in the warp direction.
If the end and pick counts are roughly equal, the fabric is considered bidirectional, often called BID. If the pick count is very small, most of the yarns run in the warp direction, and the fabric is nearly unidirectional. I have seen some unidirectional cloths which have no fill yarns; instead, the warp yarns are held together by a thin stream of glue (picture squeezing a bottle of glue over the fabric and moving it around randomly). Unidirectioanl fabrics are sometimes called tapes, though this term is also used for narrow rolls (less than a few inches wides) of woven fabric or unidirectional prepreg (which relies on the resin to hold the fibers together).
The last important fabric term is the weave. The weave describes how the warp and fill yarns are interlaced. The most popular weaves are plain, twill, harness satin, and crow-foot satin. Weave determines drapeability and isotropy of strength (some weaves are biased to the warp or fill direction). Because it's difficult to describe a weave without showing a picture of it, I'll be discussing weaves in more detail in a future column.
Finally, fabrics are often referred to with a style number. This number is a brief way of referring to a specific weight (or denier), count, and weave. For example, the three most common Kevlar styles are:
- 120: 1.8 oz, 34 x 34, plain
- 281: 5.0 oz, 17 x 17, plain
- 285: 5.0 oz, 17 x 17, crow-foot satin
For additional fabric-related terms, I recommend the Owens Corning Glossary. For other glossary pages, see my List of Glossaries.