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Fabric Terminology
When buying fabrics for composite projects, you have manychoices 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 thehome-built market, describe their materials by the arealdensity, or weight per unit area (usually just called
weight), of the dry fabric. The most common unit of measure isounces per square yard, often simply abbreviated as ounces. Thus, afabric with a weight of 5.4 oz really has an areal density of 5.4oz/sq yd.
Fabric weight is only one descriptor, and perhaps the leastimportant. Thickness is primarily a function of weight and fiber type,but also depends on the weave. For example,
AircraftSpruce lists two glass fabrics of similar weight (5.79 oz and 5.85oz); the heavier cloth has a thickness of 0.009 in, the lighter clotha slightly larger thickness of 0.010 in.
To fully understand fabric terminology, we need to start with thefibers themselves. Most of the terms used to describe both fibers andfabrics come from the textile industry, predating compositematerials. Thus, the same terms are used for any type of fiber, be itglass, Kevlar, graphite, or something else.
A single fiber is usually referred to as a
filament. You canbuy a single continuous filament rolled on a spool; this is called a
monofilament. Mostly, however, you buy filaments bunches calleda
strand or an
end. If the filaments are all parellel toeach other, the end is called a
roving (graphite rovings arealso referred to as
tows). Rovings are usually denoted by thenumber of ends they contain; tows by the number of filaments. The mostcommon graphite tows are 3K, 6K, and 12K. If instead of being straightthe filaments are twisted to hold the fibers together, the bundle iscalled a
yarn.
Another term used to describe yarns is
denier. This term isn'tused very often for composites, and is most often applied toKevlar. Denier is equal to the weight in grams of 9000 meters ofyarn. The lower the number, the finer the yarn. For example, a 380denier Kevlar 49 yarn has 267 filaments, and an 1140 denier yarn has768 filaments.
Either roving (tow) or yarn can be woven into a fabric. If rovingis used, the fabric is called
woven roving; if yarn is used thefabric is called
cloth. Many suppliers use the term cloth forwoven roving, but refer to the weave as flat, not twisted. Clothcomposites can have a slightly lower resin conent than woven rovingsbecause the yarn bundles are more compact than rovings, but the twistweakens the fibers somewhat. In the example given above from theAircraft Spruce catalog, the thicker but lighter fabric is a wovenroving, and the other is a cloth.
If you look at a roll of fabric, some of the yarns run in thedirection of the roll and are continuous for the entire length of theroll. These are the
warp yarns. The short yarns which runcrosswise to the roll direction are called the
fill yarns. Warpyarns are usually called
ends and fill yarns
picks, butsometimes the fill yarns are also called ends. (The terms applyequally to rovings, but yarn will be used in the rest of thediscussion for simplicity.)
Fabric
count refers to the number of warp yarns (ends) andfill yarns (picks) per inch. For example, a 24 x 22 fabrichas 24 ends in every inch of fill direction and 22 picks in every inchof warp direction Note that you count warp yarns in the filldirection and fill yarns in the warp direction.
If the end and pick counts are roughly equal, the fabric isconsidered
bidirectional, often called
BID. If the pickcount is very small, most of the yarns run in the warp direction, andthe fabric is nearly
unidirectional. I have seen someunidirectional cloths which have no fill yarns; instead, the warpyarns are held together by a thin stream of glue (picture squeezing abottle of glue over the fabric and moving it aroundrandomly). Unidirectioanl fabrics are sometimes called
tapes,though this term is also used for narrow rolls (less than a few incheswides) of woven fabric or unidirectional prepreg (which relies on theresin to hold the fibers together).
The last important fabric term is the
weave. The weavedescribes how the warp and fill yarns are interlaced. The most popularweaves are
plain,
twill,
harness satin, and
crow-foot satin. Weave determines drapeability and isotropy ofstrength (some weaves are biased to the warp or filldirection). Because it's difficult to describe a weave without showinga picture of it, I'll be discussing weaves in more detail in a futurecolumn.
Finally, fabrics are often referred to with a
stylenumber. This number is a brief way of referring to a specific weight(or denier), count, and weave. For example, the three most commonKevlar 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
The most common glass styles include 7533, 7520, 7781, and7500. Graphite fabrics, although they also have a style, are usuallyreferred to by their weave and tow size (or ply thickness).For additional fabric-related terms, I recommend the
OwensCorning Glossary. For other glossary pages, see my
List of Glossaries.
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