Motorsport Composite Fundamentals: Woven Cloth

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Woven Cloth

09.35

00:00	When getting started with composites, all the different types of reinforcements, from the tow and non-woven mat, unidirectionals and NCFs we've just covered, to plain weaves, V-weaves, 2x2 twills and spread tow, with the added complexity within these groups, can be overwhelming to say the least.
00:18	Thankfully it's not as confusing as it first seems, so in this module we'll make sure we have a clear understanding of the different woven cloths or fabrics, as well as their uses, properties and considerations.
00:31	Regardless of the differences, at first glance, the purpose of woven cloth seems about creating reinforcement fabric with relatively uniform strength and stiffness in all directions.
00:42	However, we can achieve this with our multi-axial cloth, and recalling our discussions in the last module, these non-crimp fabrics also provide better stiffness than woven fabrics for high performance laminates, so why use woven fabrics? One reason is the integrity of the dry reinforcement, before it's wet out with a resin.
01:05	A woven fabric holds itself together without the need for an additional stitch or binder, meaning it's easier to handle during construction.
01:13	Another reason is aesthetics, which of course depends on personal taste, but if the reinforcement is visible in the finished product, it's fair to say a weave looks better in most cases than unidirectional fibres.
01:27	With that said, it would be easy to assume that this is the reason the woven fabric is used on the outer layer of high performance parts, but that isn't necessarily true.
01:37	For example, a composite panel is damaged and there is a unidirectional fibre on the surface, then it's more likely to strip the exposed fibres down their entire length.
01:47	A woven fabric's integrity will help prevent the stripping, limiting the damage to a smaller area.
01:53	Before getting into the different weaves, we'll first need to understand a few textile fabric specific terms, being warp and weft.
02:02	For the purposes of this course, it's not critical we understand the manufacturing process of creating woven fabrics, we just need to understand the warp is the toe that runs vertically down the fabric.
02:13	The warp is stretched out on a loom or frame, and then the weft is the toe run horizontally through it.
02:21	We'll start with the fundamental plain weave, also called a one-by-one weave.
02:26	This is about as simple as it gets, with the warp and the weft running perpendicular to each other, where each toe runs under one toe and then over the next toe in both the warp and the weft directions.
02:37	Alongside plain weaves, the other most common reinforcement is what's called a two-by-two twill weave.
02:44	As you might expect, this is where each toe runs under two perpendicular toes and then over the next two perpendicular toes, and again it does this in both the warp and the weft directions.
02:56	There's a few key differences in the properties between these most common weaves, and that's simply because the plain weave has twice the number of intersections of toe compared to the twill.
03:06	This means that the twill is more susceptible to fraying and coming apart, so it needs to be handled with more care than the plainer weaves, which simply hold itself together better and is less likely to distort when being laid out on something like a large body panel.
03:24	It also means however that the twill follows contours more easily when being laid up, which generally speaking makes it easier to work with, especially with moulds that have tighter and more complex curves.
03:35	Recalling our discussions on NCFs in the previous modules, crimps are where the toe has small folds and ridges as it weaves under and over perpendicular toe.
03:45	When placed in tension, these will act as a spring and the toe will extend, which is a form of deformation.
03:52	Since the twill has half as many crimps, it will be slightly stiffer than a plain weave when initially placed under tension.
04:00	Once the load is taken up however and the crimps have straightened, there shouldn't be any difference.
04:05	In all practicality, this is a very small difference that's likely not going to be noticeable, but it's worth keeping in mind if we're ever working on something critical like a suspension arm.
04:16	4x4 twills are also available, although rare as a dry reinforcement.
04:22	As you'd expect, these are much more compliant for working into and around tight curves, but they're also more fragile, which is why they're more common as a prepreg, which naturally helps to hold the fibres together.
04:35	Before moving on, we need to mention V-weaves.
04:38	This is where the pattern of the weave meets in a V shape down the middle of the cloth.
04:42	This is primarily used for cosmetic purposes and most often used down the centre of the vehicle to create a line of symmetry.
04:50	Only the first layer in the mould, which will be the outer layer of the part, needs to have this, especially as the fabric is significantly more expensive.
04:59	Moving on, another common weave we're likely to encounter is the use of a spread toe.
05:04	This is really just the same as a plain weave or twill weave that we've discussed, but the individual toes are spread very flat and wide.
05:13	For reference, in a conventional weave, each toe would be between one to five millimetres wide, depending on the K count, with larger K counts naturally being wider.
05:24	In a spread toe weave, each toe is spread significantly wider, usually over 15 millimetres or so.
05:30	The result is a smoother and flatter fabric with less crimp, meaning a slight increase in and also a larger, bolder weave pattern.
05:40	Harness weave on the other hand has a significantly different weave pattern.
05:44	For example, a five harness weave sees the toe go over four perpendicular toes in one direction, then under one toe, and then back over four more toes and so on.
05:55	This happens in both the warp and weft directions, meaning there will essentially be a layer of toe in the warp direction, and another layer in the weft direction, with the single overlapping toes that link the layers together.
06:08	Because of this imbalanced ply, if we were to construct a composite using only one sheet of this weave, it would have different stiffness when flexed in each direction.
06:18	This of course can be used to our advantage for specific designs that require such properties, although it's more of a rarity.
06:25	The final thing to discuss here is hybrid plain weaves or twills.
06:29	You've likely heard of carbon kevlar or carbon glass, and that's where these fit in.
06:34	Other aramid fibers are also available in these fabrics, like twarin for example.
06:39	The purpose of hybrids is to combine the properties of each reinforcement to find some middle ground.
06:45	Carbon glass woven cloths are about half the price of pure carbon cloths, but also significantly stiffer than fiberglass cloths.
06:53	Compared to 100% carbon fiber, this is a great way of reducing the material cost for a part, while only sacrificing a small amount of performance, or adding stiffness to what would be a fiberglass part.
07:05	For this reason, it's also commonly used in backing layers, where the outer layer of the part remains solely carbon fiber for aesthetic purposes.
07:13	A similar idea applies to carbon kevlar cloths.
07:16	Compared to 100% carbon fiber cloths, this adds impact and abrasion resistance, with a slight compromise in strength and stiffness.
07:25	This makes it ideal for safety critical parts, like race seat shelves or body panels, that are expected to see some impact.
07:33	Hybrids are available as plain weaves and twill weaves, with the same differences that we just discussed.
07:39	But what's more important to understand is how the different reinforcing fibers are woven together.
07:45	For example, a hybrid weave, or a twill weave, with the alternating reinforcements in the warp and weft directions, will have uniform properties in each direction.
07:54	However, a plain weave with only carbon warp and only kevlar weft will naturally be significantly stiffer in one direction.
08:02	Carbon kevlar weaves commonly follow a two carbon one kevlar warp formula and vice versa in the weft direction, which results in somewhat of a middle ground.
08:11	In the end though, carbon kevlar is usually used as an outer layer for impact and abrasion resistance, and stiffness is achieved through the backing layers of 100% carbon fiber reinforcement.
08:23	With that, let's recap the main points on woven fabrics.
08:27	The most common weaves are plain weaves and twill weaves.
08:30	Plain or 1x1 weaves are where the warp and weft direction overlap every toe, whereas with a 2x2 twill weave, the toe goes over and under pairs of perpendicular toes in both the warp and weft directions.
08:43	The reduction in crimps makes the twill slightly stiffer in tension, while also conforming to curves better, making it easier to work with in molds.
08:52	The downside is it's more fragile when handled and prone to warpage compared to a plain weave.
08:58	Spread toe weave has very flat and wide toes, and the result provides a slight increase in stiffness, a flatter and smoother fabric, and a visually larger embolder pattern.
09:09	Harness weaves essentially have a separate warp and weft layer held together by harnesses of toe, giving them significantly different stiffnesses in each direction.
09:20	Finally, hybrid weaves combine the properties of each reinforcement fibre, we just need to consider how the different reinforcements are woven together and the resulting directional properties of the materials.